Decreased Incidence of Infections Caused by Pathogens Transmitted Commonly Through Food During the COVID-19 Pandemic — Foodborne Diseases Active Surveillance Network, 10 U.S. Sites, 2017–2020

On March 13, 2020, the United States declared a national emergency to combat coronavirus disease 2019 (COVID-19). As the number of persons hospitalized with COVID-19 increased, early reports from Austria ( 1 ), Hong Kong ( 2 ), Italy ( 3 ), and California ( 4 ) suggested sharp drops in the numbers of persons seeking emergency medical care for other reasons. To quantify the effect of COVID-19 on U.S. emergency department (ED) visits, CDC compared the volume of ED visits during four weeks early in the pandemic March 29–April 25, 2020 (weeks 14 to 17; the early pandemic period) to that during March 31–April 27, 2019 (the comparison period). During the early pandemic period, the total number of U.S. ED visits was 42% lower than during the same period a year earlier, with the largest declines in visits in persons aged ≤14 years, females, and the Northeast region. Health messages that reinforce the importance of immediately seeking care for symptoms of serious conditions, such as myocardial infarction, are needed. To minimize SARS-CoV-2, the virus that causes COVID-19, transmission risk and address public concerns about visiting the ED during the pandemic, CDC recommends continued use of virtual visits and triage help lines and adherence to CDC infection control guidance. To assess trends in ED visits during the pandemic, CDC analyzed data from the National Syndromic Surveillance Program (NSSP), a collaborative network developed and maintained by CDC, state and local health departments, and academic and private sector health partners to collect electronic health data in real time. The national data in NSSP includes ED visits from a subset of hospitals in 47 states (all but Hawaii, South Dakota, and Wyoming), capturing approximately 73% of ED visits in the United States able to be analyzed at the national level. During the most recent week, 3,552 EDs reported data. Total ED visit volume, as well as patient age, sex, region, and reason for visit were analyzed. Weekly number of ED visits were examined during January 1, 2019–May 30, 2020. In addition, ED visits during two 4-week periods were compared using mean differences and ratios. The change in mean visits per week during the early pandemic period and the comparison period was calculated as the mean difference in total visits in a diagnostic category between the two periods, divided by 4 weeks ([visits in diagnostic category {early pandemic period} – visits in diagnostic category {comparison period}]/4). The visit prevalence ratio (PR) was calculated for each diagnostic category as the proportion of ED visits during the early pandemic period divided by the proportion of visits during the comparison period ([visits in category {early pandemic period}/all visits {early pandemic period}]/[visits in category {comparison period}/all visits {comparison period}]). All analyses were conducted using R software (version 3.6.0; R Foundation). Reason for visit was analyzed using a subset of records that had at least one specific, billable International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) code. In addition to Hawaii, South Dakota, and Wyoming, four states (Florida, Louisiana, New York outside New York City, and Oklahoma), two California counties reporting to the NSSP (Santa Cruz and Solano), and the District of Columbia were also excluded from the diagnostic code analysis because they did not report diagnostic codes during both periods or had differences in completeness of codes between 2019 and 2020. Among eligible visits for the diagnostic code analysis, 20.3% without a valid ICD-10-CM code were excluded. ED visits were categorized using the Clinical Classifications Software Refined tool (version 2020.2; Healthcare Cost and Utilization Project), which combines ICD-10-CM codes into clinically meaningful groups ( 5 ). A visit with multiple ICD-10-CM codes could be included in multiple categories; for example, a visit by a patient with diabetes and hypertension would be included in the category for diabetes and the category for hypertension. Because COVID-19 is not yet classified in this tool, a custom category, defined as any visit with the ICD-10-CM code for confirmed COVID-19 diagnosis (U07.1), was created ( 6 ). The analysis was limited to the top 200 diagnostic categories during each period. The lowest number of visits reported to NSSP occurred during April 12–18, 2020 (week 16). Although visits have increased since the nadir, the most recent complete week (May 24–30, week 22) remained 26% below the corresponding week in 2019 (Figure 1). The number of ED visits decreased 42%, from a mean of 2,099,734 per week during March 31–April 27, 2019, to a mean of 1,220,211 per week during the early pandemic period of March 29–April 25, 2020. Visits declined for every age group (Figure 2), with the largest proportional declines in visits by children aged ≤10 years (72%) and 11–14 years (71%). Declines in ED visits varied by U.S. Department of Health and Human Services region,* with the largest declines in the Northeast (Region 1, 49%) and in the region that includes New Jersey and New York (Region 2, 48%) (Figure 2). Visits declined 37% among males and 45% among females across all NSSP EDs between the comparison and early pandemic periods. FIGURE 1 Weekly number of emergency department (ED) visits — National Syndromic Surveillance Program, United States,* January 1, 2019– May 30, 2020† * Hawaii, South Dakota, and Wyoming are not included. † Vertical lines indicate the beginning and end of the 4-week coronavirus disease 2019 (COVID-19) early pandemic period (March 29–April 25, 2020) and the comparison period (March 31–April 27, 2019). The figure is a line graph showing the weekly number of emergency department visits, using data from the National Syndromic Surveillance Program, in the United States, during January 1, 2019–May 30, 2020. FIGURE 2 Emergency department (ED) visits, by age group (A) and U.S. Department of Health and Human Services (HHS) region* (B) — National Syndromic Surveillance Program, United States,† March 31–April 27, 2019 (comparison period) and March 29–April 25, 2020 (early pandemic period) * Region 1: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont; Region 2: New Jersey and New York; Region 3: Delaware, District of Columbia, Maryland, Pennsylvania, Virginia, and West Virginia; Region 4: Alabama, Florida, Georgia, Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee; Region 5: Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin; Region 6: Arkansas, Louisiana, New Mexico, Oklahoma, and Texas; Region 7: Iowa, Kansas, Missouri, and Nebraska; Region 8: Colorado, Montana, North Dakota, and Utah; Region 9: Arizona, California, and Nevada; Region 10: Alaska, Idaho, Oregon, and Washington. † Hawaii, South Dakota, and Wyoming are not included. The figure is a bar chart showing the emergency department visits, by age group and U.S. Department of Health and Human Services region, using data from the National Syndromic Surveillance Program, in the United States, during March 31–April 27, 2019 (comparison period) and March 29–April 25, 2020 (pandemic period). Among all ages, an increase of >100 mean visits per week from the comparison period to the early pandemic period occurred in eight of the top 200 diagnostic categories (Table). These included 1) exposure, encounters, screening, or contact with infectious disease (mean increase 18,834 visits per week); 2) COVID-19 (17,774); 3) other general signs and symptoms (4,532); 4) pneumonia not caused by tuberculosis (3,911); 5) other specified and unspecified lower respiratory disease (1,506); 6) respiratory failure, insufficiency, or arrest (776); 7) cardiac arrest and ventricular fibrillation (472); and 8) socioeconomic or psychosocial factors (354). The largest declines were in visits for abdominal pain and other digestive or abdomen signs and symptoms (–66,456), musculoskeletal pain excluding low back pain (–52,150), essential hypertension (–45,184), nausea and vomiting (–38,536), other specified upper respiratory infections (–36,189), sprains and strains (–33,709), and superficial injuries (–30,918). Visits for nonspecific chest pain were also among the top 20 diagnostic categories for which visits decreased (–24,258). Although not in the top 20 declining diagnoses, visits for acute myocardial infarction also declined (–1,156). TABLE Differences in mean weekly numbers of emergency department (ED) visits* for diagnostic categories with the largest increases or decreases† and prevalence ratios§ comparing the proportion of ED visits in each diagnostic category, for categories with the highest and lowest ratios — National Syndromic Surveillance Program, United States,¶ March 31–April 27, 2019 (comparison period) and March 29–April 25, 2020 (early pandemic period) Diagnostic category Change in mean no. of weekly ED visits* Prevalence ratio (95% CI)§ All categories with higher visit counts during the early pandemic period Exposure, encounters, screening, or contact with infectious disease** 18,834 3.79 (3.76–3.83) COVID-19 17,774 — Other general signs and symptoms** 4,532 1.87 (1.86–1.89) Pneumonia (except that caused by tuberculosis)** 3,911 1.91 (1.90–1.93) Other specified and unspecified lower respiratory disease** 1,506 1.99 (1.96–2.02) Respiratory failure, insufficiency, arrest** 776 1.76 (1.74–1.78) Cardiac arrest and ventricular fibrillation** 472 1.98 (1.93–2.03) Socioeconomic or psychosocial factors** 354 1.78 (1.75–1.81) Other top 10 highest prevalence ratios Mental and substance use disorders, in remission** 6 1.69 (1.64–1.75) Other specified encounters and counseling** 22 1.69 (1.67–1.72) Stimulant-related disorders** −189 1.65 (1.62–1.67) Top 20 categories with lower visit counts during the early pandemic period Abdominal pain and other digestive or abdomen signs and symptoms −66,456 0.93 (0.93–0.93) Musculoskeletal pain, not low back pain −52,150 0.81 (0.81–0.82) Essential hypertension −45,184 1.11 (1.10–1.11) Nausea and vomiting −38,536 0.85 (0.84–0.85) Other specified upper respiratory infections −36,189 0.82 (0.81–0.82) Sprains and strains, initial encounter †† −33,709 0.61 (0.61–0.62) Superficial injury; contusion, initial encounter −30,918 0.85 (0.84–0.85) Personal or family history of disease −28,734 1.21 (1.20–1.22) Headache, including migraine −27,458 0.85 (0.84–0.85) Other unspecified injury −25,974 0.84 (0.83–0.84) Nonspecific chest pain −24,258 1.20 (1.20–1.21) Tobacco-related disorders −23,657 1.19 (1.18–1.19) Urinary tract infections −23,346 1.02 (1.02–1.03) Asthma −20,660 0.91 (0.90–0.91) Disorders of lipid metabolism −20,145 1.12 (1.11–1.13) Spondylopathies/Spondyloarthropathy (including infective) −19,441 0.78 (0.77–0.79) Otitis media †† −17,852 0.35 (0.34–0.36) Diabetes mellitus without complication −15,893 1.10 (1.10–1.11) Skin and subcutaneous tissue infections −15,598 1.01 (1.00–1.02) Chronic obstructive pulmonary disease and bronchiectasis −15,520 1.05 (1.04–1.06) Other top 10 lowest prevalence ratios Influenza †† −12,094 0.16 (0.15–0.16) No immunization or underimmunization †† −1,895 0.28 (0.27–0.30) Neoplasm-related encounters †† −1,926 0.40 (0.39–0.42) Intestinal infection †† −5,310 0.52 (0.51–0.54) Cornea and external disease †† −9,096 0.54 (0.53–0.55) Sinusitis †† −7,283 0.55 (0.54–0.56) Acute bronchitis †† −15,470 0.59 (0.58–0.60) Noninfectious gastroenteritis †† −11,572 0.63 (0.62–0.64) Abbreviations: CI = confidence interval; COVID-19 = coronavirus disease 2019. * The change in visits per week during the early pandemic and comparison periods was calculated as the difference in total visits between the two periods, divided by 4 weeks ([visits in diagnostic category, {early pandemic period} – visits in diagnostic category, {comparison period}] / 4). † Analysis is limited to the 200 most common diagnostic categories. All eight diagnostic categories with an increase of >100 in the mean number of visits nationwide in the early pandemic period are shown. The top 20 categories with decreasing visit counts are shown. § Ratio calculated as the proportion of all ED visits in each diagnostic category during the early pandemic period, divided by the proportion of all ED visits in that category during the comparison period ([visits in category {early pandemic period}/all visits {early pandemic period})/(visits in category {comparison period}/all visits {comparison period}]). Ratios >1 indicate a higher proportion of visits in that category during the early pandemic period than the comparison period; ratios <1 indicate a lower proportion during the early pandemic than during the comparison period. Analysis is limited to the 200 most common diagnostic categories. The 10 categories with the highest and lowest ratios are shown. ¶ Florida, Hawaii, Louisiana, New York outside of New York City, Oklahoma, South Dakota, Wyoming, Santa Cruz and Solano counties in California, and the District of Columbia are not included. ** Top 10 highest prevalence ratios; higher proportion of visits in the early pandemic period than the comparison period. †† Top 10 lowest prevalence ratios; lower proportion of visits in the early pandemic period than the comparison period. During the early pandemic period, the proportion of ED visits for exposure, encounters, screening, or contact with infectious disease compared with total visits was nearly four times as large as during the comparison period (Table) (prevalence ratio [PR] = 3.79, 95% confidence interval [CI] = 3.76–3.83). The other diagnostic categories with the highest proportions of visits during the early pandemic compared with the comparison period were other specified and unspecified lower respiratory disease, which did not include influenza, pneumonia, asthma, or bronchitis (PR = 1.99; 95% CI = 1.96–2.02), cardiac arrest and ventricular fibrillation (PR = 1.98; 95% CI = 1.93–2.03), and pneumonia not caused by tuberculosis (PR = 1.91; 95% CI = 1.90–1.93). Diagnostic categories that were recorded less commonly during the early pandemic period included influenza (PR = 0.16; 95% CI = 0.15–0.16), no immunization or underimmunization (PR = 0.28; 95% CI = 0.27–0.30), otitis media (PR = 0.35; 95% CI = 0.34–0.36), and neoplasm-related encounters (PR = 0.40; 95% CI = 0.39–0.42). In the 2019 comparison period, 12% of all ED visits were in children aged ≤10 years old, compared with 6% during the early pandemic period. Among children aged ≤10 years, the largest declines were in visits for influenza (97% decrease), otitis media (85%), other specified upper respiratory conditions (84%), nausea and vomiting (84%), asthma (84%), viral infection (79%), respiratory signs and symptoms (78%), abdominal pain and other digestive or abdomen symptoms (78%), and fever (72%). Mean weekly visits with confirmed COVID-19 diagnoses and screening for infectious disease during the early pandemic period were lower among children than among adults. Among all ages, the diagnostic categories with the largest changes (abdominal pain and other digestive or abdomen signs and symptoms, musculoskeletal pain, and essential hypertension) were the same in males and females, but declines in those categories were larger in females than males. Females also had large declines in visits for urinary tract infections (–19,833 mean weekly visits). Discussion During an early 4-week interval in the COVID-19 pandemic, ED visits were substantially lower than during the same 4-week period during the previous year; these decreases were especially pronounced for children and females and in the Northeast. In addition to diagnoses associated with lower respiratory disease, pneumonia, and difficulty breathing, the number and ratio of visits (early pandemic period versus comparison period) for cardiac arrest and ventricular fibrillation increased. The number of visits for conditions including nonspecific chest pain and acute myocardial infarction decreased, suggesting that some persons could be delaying care for conditions that might result in additional mortality if left untreated. Some declines were in categories including otitis media, superficial injuries, and sprains and strains that can often be managed through primary or urgent care. Future analyses will help clarify the proportion of the decline in ED visits that were not preventable or avoidable such as those for life-threatening conditions, those that were manageable through primary care, and those that represented actual reductions in injuries or illness attributable to changing activity patterns during the pandemic (such as lower risks for occupational and motor vehicle injuries or other infectious diseases). The striking decline in ED visits nationwide, with the highest declines in regions where the pandemic was most severe in April 2020, suggests that the pandemic has altered the use of the ED by the public. Persons who use the ED as a safety net because they lack access to primary care and telemedicine might be disproportionately affected if they avoid seeking care because of concerns about the infection risk in the ED. Syndromic surveillance has important strengths, including automated electronic reporting and the ability to track outbreaks in real time ( 7 ). Among all visits, 74% are reported within 24 hours, with 75% of discharge diagnoses typically added to the record within 1 week. The findings in this report are subject to at least four limitations. First, hospitals reporting to NSSP change over time as facilities are added, and more rarely, as they close ( 8 ). An average of 3,173 hospitals reported to NSSP nationally in April 2019, representing an estimated 66% of U.S. ED visits, and an average of 3,467 reported in April 2020, representing 73% of ED visits. Second, diagnostic categories rely on the use of specific codes, which were missing in 20% of visits and might be used inconsistently across hospitals and providers, which could result in misclassification. The COVID-19 diagnosis code was introduced recently (April 1, 2020) and timing of uptake might have differed across hospitals ( 6 ). Third, NSSP coverage is not uniform across or within all states; in some states nearly all hospitals report, whereas in others, a lower proportion statewide or only those in certain counties report. Finally, because this analysis is limited to ED visit data, the proportion of persons who did not visit EDs but received treatment elsewhere is not captured. Health care systems should continue to address public concern about exposure to SARS-CoV-2 in the ED through adherence to CDC infection control recommendations, such as immediately screening every person for fever and symptoms of COVID-19, and maintaining separate, well-ventilated triage areas for patients with and without signs and symptoms of COVID-19 ( 9 ). Wider access is needed to health messages that reinforce the importance of immediately seeking care for serious conditions for which ED visits cannot be avoided, such as symptoms of myocardial infarction. Expanded access to triage telephone lines that help persons rapidly decide whether they need to go to an ED for symptoms of possible COVID-19 infection and other urgent conditions is also needed. For conditions that do not require immediate care or in-person treatment, health care systems should continue to expand the use of virtual visits during the pandemic ( 10 ). Summary What is already known about this topic? The National Syndromic Surveillance Program (NSSP) collects electronic health data in real time. What is added by this report? NSSP found that emergency department (ED) visits declined 42% during the early COVID-19 pandemic, from a mean of 2.1 million per week (March 31–April 27, 2019) to 1.2 million (March 29–April 25, 2020), with the steepest decreases in persons aged ≤14 years, females, and the Northeast. The proportion of infectious disease–related visits was four times higher during the early pandemic period. What are the implications for public health practice? To minimize SARS-CoV-2 transmission risk and address public concerns about visiting the ED during the pandemic, CDC recommends continued use of virtual visits and triage help lines and adherence to CDC infection control guidance.

Salmonella enterica infections are transmitted not only by animal-derived foods but also by vegetables, fruits, and other plant products. To clarify links between Salmonella serotypes and specific foods, we examined the diversity and predominance of food commodities implicated in outbreaks of salmonellosis during 1998–2008. More than 80% of outbreaks caused by serotypes Enteritidis, Heidelberg, and Hadar were attributed to eggs or poultry, whereas >50% of outbreaks caused by serotypes Javiana, Litchfield, Mbandaka, Muenchen, Poona, and Senftenberg were attributed to plant commodities. Serotypes Typhimurium and Newport were associated with a wide variety of food commodities. Knowledge about these associations can help guide outbreak investigations and control measures.

From January 21 through February 23, 2020, a total of 14 cases of coronavirus disease 2019 (COVID-19) were diagnosed in six U.S. states, including 12 cases in travelers arriving from China and two in household contacts of persons with confirmed infections. An additional 39 cases were identified in persons repatriated from affected areas outside the United States ( 1 ). Starting in late February, reports of cases with no recent travel to affected areas or links to known cases signaled the initiation of pandemic spread in the United States ( 2 ). By mid-March, transmission of SARS-CoV-2, the virus that causes COVID-19, had accelerated, with rapidly increasing case counts indicating established transmission in the United States. Ongoing traveler importation of SARS-CoV-2, attendance at professional and social events, introduction into facilities or settings prone to amplification, and challenges in virus detection all contributed to rapid acceleration of transmission during March. Public health responses included intensive efforts to detect cases and trace contacts, and implementation of multiple community mitigation strategies. Because most of the population remains susceptible to infection, recognition of factors associated with amplified spread during the early acceleration period will help inform future decisions as locations in the United States scale back some components of mitigation and strengthen systems to detect a potential transmission resurgence. U.S. circulation of SARS-CoV-2 continues, and sustained efforts will be needed to prevent future spread within the United States. The first cases of COVID-19 in the United States occurred in January and February 2020 in travelers from China’s Hubei Province, where the virus was first recognized, and their household contacts ( 1 ). Beginning in late February, cases with no history of international travel and no contact with infected persons were recognized ( 1 ). By mid-March, transmission had become widespread, and by April 21, a total of 793,669 confirmed COVID-19 cases had been reported in the United States, the majority resulting from widespread community transmission (Figure 1). Factors that contributed to the acceleration of dissemination in March included 1) continued importation of the virus by travelers infected elsewhere (e.g., on cruise ships or in countries experiencing outbreaks); 2) attendance at professional and social events, resulting in amplification in the host locations and multistate spread; 3) introduction of the virus into facilities or settings prone to amplification (e.g., long-term care facilities and high-density urban areas) with the potential for seeding the broader community; and 4) challenges in virus detection, including limited testing, emergence during the peak months of influenza circulation and influenza and pneumonia hospitalizations, and other cryptic transmission including from persons who were asymptomatic or presymptomatic. During March 2020, national, state, and local public health responses also intensified and adapted, augmenting case detection, contact tracing, and quarantine with targeted layered community mitigation measures. Because SARS-CoV-2, the virus that causes COVID-19, remains in circulation and a large proportion of the population remains susceptible, the potential for future acceleration remains. FIGURE 1 Number of confirmed COVID-19 cases, by date of report, in the United States during February 20–April 21, 2020,* with initiation and early acceleration periods highlighted in Louisiana, Massachusetts, and Georgia Abbreviation: COVID-19 = coronavirus disease 2019. *Cumulative case count was 13 before February 20, 2020. The figure consists of four histograms, epidemiologic curves that show the number of confirmed COVID-19 cases, by date of report, in the United States during February 20–April 21, 2020, with initiation and early acceleration periods highlighted in Louisiana, Massachusetts, and Georgia. Travel and COVID-19 Spread Continued introductions of SARS-CoV-2 from outside the United States contributed to the initiation and acceleration of domestic COVID-19 cases in March. After Chinese authorities halted travel from Wuhan and other cities in Hubei Province on January 23, followed by U.S. restrictions on non-U.S. travelers from China issued on January 31 (effective February 2), air passenger journeys from China decreased 86%, from 505,560 in January to 70,072 in February. However, during February, 139,305 travelers arrived from Italy and 1.74 million from all Schengen countries,* where the outbreak was spreading widely and rapidly. Travelers from Italy and all Schengen countries decreased 74% to 35,877 and 50% to 862,432, respectively, in March. † Genomic analysis of outbreak strains suggested an introduction from China to the state of Washington around February 1. § However, examination of strains collected from northern California during early February to mid-March indicated multiple introductions resulting from international travel (from China and Europe) as well as from interstate travel. ¶ Sequencing of strains collected in the New York metropolitan area in March also suggested origins in Europe and other U.S. regions.** Returning cruise ship travelers also contributed to amplification during this time ( 3 ). Persons from many countries are in close contact on cruises, and crew members continue to work on ships for multiple voyages. As a result, passengers returning from cruises contributed to the early acceleration phase. For example, 101 persons who had been on nine separate Nile River cruises during February 11–March 5 returned to 18 states and had a positive test result for SARS-CoV-2, nearly doubling the total number of known COVID-19 cases in the United States at that time (Figure 2). FIGURE 2 Number of confirmed COVID-19 cases (N = 101) linked to nine Nile River cruises held during February 11–March 5, 2020, by patient state of residence — 18 states Abbreviations: COVID-19 = coronavirus disease 2019; DC = District of Columbia; DE = Delaware; RI = Rhode Island. The figure is a map of the United States showing the number of confirmed COVID-19 cases (N = 101) linked to nine Nile River cruises held during February 11–March 5, 2020, by the patients’ 18 states of residence. Public health steps to mitigate continued importations of the virus included travel restrictions for non-U.S. citizens or permanent residents arriving from China beginning in early February and later expanded to include other countries with widespread sustained transmission (Table). Travel health notices were issued for countries with known outbreaks as the pandemic evolved, and ultimately warnings were issued to avoid nonessential international travel as well as all cruise ship travel ( 1 , 4 ). Quarantine measures were implemented for arriving international travelers with known exposure to locations and settings of concern, such as Hubei Province and the Diamond Princess cruise ship docked off the coast of Yokohama, Japan. Screening and public health risk assessment of travelers in selected U.S. airports, initiated on January 17, were also expanded. As of April 21, 2020, CDC staff members and U.S. Customs and Border Protection officers had screened approximately 268,000 returning travelers, among whom testing confirmed 14 COVID-19 cases. State and local health departments were advised to supervise self-monitoring of travelers who had been directed to stay home after returning from countries with widespread sustained transmission. On March 14, 2020, the CDC Director issued a No Sail Order for cruise ships, suspending operation in U.S. waters; the order was renewed April 9, effective April 15. TABLE Factors contributing to COVID-19 acceleration and corresponding public health actions — United States, January–April 2020 Factor contributing to acceleration Examples Public health actions Continued travel-associated importations of the virus Travelers arriving from countries or cruise ships with ongoing transmission Travel health notices, traveler screening (including risk assessment, public health management and monitoring), travel restrictions, federal isolation and quarantine orders, educating travelers and clinicians regarding symptoms and evaluation Large gatherings Social, cultural, and professional gatherings where persons convene and then disperse over broad areas Restricting mass gatherings; global travel restrictions and domestic travel recommendations, recommending transition to virtual events Introductions into high-risk workplaces/settings Long-term care facilities, hospitals, correctional facilities, and homeless shelters Restricting visitor access, establishing cohort units or facilities for residential settings, vigorous contact tracing around persons with confirmed cases, increased infection control, environmental surface cleaning, use of recommended personal protective equipment Crowding and high population density Densely populated areas, crowded workplaces, schools, and public spaces Stay-at-home orders, recommendations for hand washing and social distancing, cloth face covering guidance, school dismissals, extended telework, environmental surface cleaning Cryptic transmission Presymptomatic or asymptomatic spread, limited testing, co-occurrence with circulation of other respiratory viruses Increased testing, COVID-19–specific surveillance, cloth face covering guidance, aggressive contact tracing accompanied by quarantine and/or testing of asymptomatic contacts, stay-at-home orders Abbreviation: COVID-19 = coronavirus disease 2019. Events and Gatherings Various gatherings of persons from different locations, followed by return to their home communities, played a notable role in the early U.S. spread of COVID-19. During February 2020, the number of confirmed cases originating in the United States was low and appeared contained; thus, federal and local jurisdictions did not recommend restrictions on gatherings. However, during the last week of February, several large events led to further spread of the disease. These included Mardi Gras celebrations in Louisiana with more than 1 million attendees, an international professional conference held in Boston, Massachusetts, with approximately 175 attendees, and a funeral in Albany, Georgia, with more than 100 attendees (Figure 1). In the weeks after these events, amplifications in the host locations contributed to increasing U.S. case counts ( 5 ). Dougherty County, Georgia, a small rural county that includes Albany, had one of the highest cumulative incidences of COVID-19 (1,630/100,000 population) in the country. The substantial transmissibility of the virus and severity of COVID-19 triggered a series of recommendations, beginning in mid-March, to limit mass gatherings and travel (Table). Workplaces and Settings Contributing to Accelerated Spread Skilled nursing and long-term care facilities ( 6 ) and hospitals ( 7 ) are settings in which persons at higher risk for severe COVID-19 illness are in close contact with staff members, many of whom work at multiple facilities. Other workplaces also facilitated amplification of virus transmission, including critical infrastructure sectors, such as multiple meat packing facilities in rural areas. Clusters of cases related to religious service attendance have been reported within the United States and worldwide ( 8 ). Congregate, high-density settings also might contribute to the spread of COVID-19 ( 9 ). For example, population density might account for the very high numbers of COVID-19 cases in the New York metropolitan area (Box). Public health actions aimed at reducing COVID-19 spread in high-risk settings have focused on infection control measures, including identifying and isolating ill persons, cleaning and disinfection, restricting visitors, physical distancing through shift work, and appropriate use of personal protective equipment (Table). To protect health care capacity and slow community spread of COVID-19, local, state, and federal authorities issued stay-at-home orders, and closed schools and nonessential workplaces. On April 3, CDC issued guidance for use of cloth face coverings in public areas to reduce spread, based on increasing evidence of transmission in the absence of symptoms. †† BOX Critical factors contributing to COVID-19 spread in New York Multiple interrelated factors that complicated identification and isolation of cases and tracing of contacts contributed to the COVID-19 outbreak in New York. Population density New York City’s boroughs represent the top four population-dense U.S. counties. Reliance on mass transit (subways, buses, and ferries) results in frequent, prolonged close contact. High prevalence of apartment living contributed to household spread. Domestic and global destination Three major airports serve as domestic and global hubs, serving >1 million air passengers per week. Approximately 1.6 million persons commute into Manhattan daily during the work week, primarily using mass transit. Large number of crowded settings housing vulnerable populations Long-term care facilities, skilled nursing facilities: At least 80 facilities in the state have reported five or more cases as of April 21; initial infections were noted in early March. Correctional institutions: As of April 21, incidence in Department of Corrections and Community Supervision facilities was approximately seven times that in the state overall. Homeless shelters: As of the week of April 21, approximately 600 cases were confirmed among shelter residents and other persons experiencing homelessness. Large gatherings Initial cases in Westchester County were associated with attendance at large gatherings in late February. All types of large work and social gatherings accelerated transmission across jurisdictional boundaries. Cryptic Transmission Unrecognized transmission played a key role in the initiation and acceleration phases of the U.S. outbreak. Cases were not detected during this time for various reasons. First, introduction of the virus into the United States occurred during the annual influenza season. Although syndromic surveillance systems tracked respiratory illness in outpatient settings and emergency departments in many U.S. jurisdictions, including areas where early COVID-19 clusters were detected, such as Seattle, Washington, none of these systems detected unusual trends during the early part of the acceleration period because of the preponderance of seasonal influenza illness. After the first community case in Santa Clara, California, was confirmed on February 27, the county conducted COVID-19 surveillance with polymerase chain reaction–based virus testing during March 5–14 at four urgent care centers. Influenza accounted for 23% of respiratory illnesses; among those who had a negative test result for influenza, 11% had a positive test result for SARS-CoV-2, representing approximately 8% of patients with respiratory symptoms ( 10 ). Seroprevalence data from Seattle during March 2020, a period when transmission of the virus was rapidly accelerating, suggested that there were limited undetected infections in healthy adults without respiratory illness (1 of 221 remnant clinical sera representing a convenience sample tested seropositive [Helen Chu, University of Washington School of Public Health, personal communication, April 2020]); at the population level, this still translates into substantial numbers of unrecognized community infections. No samples from 59 children with acute respiratory infections during January–March were seropositive (Janet Englund, Seattle Children’s Hospital and University of Washington, personal communication, April 2020). Because the incidence of SARS-CoV-2 infections was still relatively low during the initiation and early acceleration periods, as evidenced by seroprevalence data, widespread testing would have been needed to detect all cases. The contribution of spread from persons without symptoms also complicated detection and containment ( 11 ). Public health actions included expanded surveillance and testing capacity and community measures, such as enhanced teleworking and stay-at-home orders, school closures, social distancing, and use of cloth face coverings (Table). Discussion The acceleration phase of a pandemic is complex and requires a multifaceted and rapidly adapting public health response. During a 3-week period in late February to early March, the number of U.S. COVID-19 cases increased more than 1,000-fold. Various community mitigation interventions were implemented with the aim of reducing further spread and controlling the impact on health care capacity. Recognition of factors associated with amplified spread during this early acceleration period will help inform future decisions as locations in the United States scale back some components of mitigation and strengthen systems to detect transmission resurgence. The findings in this report are subject to at least five limitations. First, the various factors facilitating viral spread described in this report occurred simultaneously; therefore, it is not possible to quantify the relative contribution of each to the outbreak trajectory in the United States. Second, the examples of factors contributing to amplification are illustrative and not meant to be comprehensive. Third, because the mitigation strategies highlighted here were implemented concurrently, the ability to estimate the relative impact of each intervention is limited. Fourth, the epidemic curve presented was likely affected by limited testing, particularly in the early phases of the outbreak. Finally, the case counts presented are an underestimate of the actual number of COVID-19 cases in the United States. As the pandemic evolves, control efforts must be continuously refined. Certain interventions that were critical in the early stages, such as quarantine and airport screening, might have less impact when transmission is widespread in the community. However, many elements of the mitigation strategies used during the acceleration phase will still be needed in later stages of the outbreak. Preliminary results from serologic surveys suggest that even in the U.S. regions with the largest numbers of recognized cases, most persons have not been infected and remain susceptible. §§ , ¶¶ Therefore, sustained and concerted efforts will be needed to prevent future spread of SARS-CoV-2 within the United States. Summary What is already known about this topic? The first confirmed coronavirus disease 2019 (COVID-19) case in the United States was reported on January 21, 2020. The outbreak appeared contained through February, and then accelerated rapidly. What is added by this report? Various factors contributed to accelerated spread during February–March 2020, including continued travel-associated importations, large gatherings, introductions into high-risk workplaces and densely populated areas, and cryptic transmission resulting from limited testing and asymptomatic and presymptomatic spread. Targeted and communitywide mitigation efforts were needed to slow transmission. What are the implications for public health practice? Factors that amplified the March acceleration and associated mitigation strategies that were implemented can inform public health decisions as the United States prepares for potential re-emergences.