Effectiveness of COVID-19 Vaccination on Transmission: A Systematic Review

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Abstract

Vaccination against infectious disease affords direct protection from vaccine-induced immunity and additional indirect protection for unvaccinated persons. A systematic review was conducted to estimate the indirect effect of COVID-19 vaccination. From PubMed and Embase, 31 studies were included describing the impact of original wild-type COVID-19 vaccines on disease transmission or viral load. Overall, study results showed the effectiveness of COVID-19 vaccination against SARS-CoV-2 transmission (range 16–95%), regardless of vaccine type or number of doses. The effect was apparent, but less pronounced against omicron (range 24–95% for pre-omicron variants versus 16–31% for omicron). Results from viral load studies were supportive, showing SARS-CoV-2 infections in vaccinated individuals had higher Ct values, suggesting lower viral load, compared to infections among the unvaccinated. Based on these findings, well-timed vaccination programs may help reduce SARS-CoV-2 transmission—even in the omicron era. Whether better-matched vaccines can improve effectiveness against transmission in the omicron era needs further study.

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A guide to vaccinology: from basic principles to new developments

Andrew Pollard, Else M. Bijker (2020)

Immunization is a cornerstone of public health policy and is demonstrably highly cost-effective when used to protect child health. Although it could be argued that immunology has not thus far contributed much to vaccine development, in that most of the vaccines we use today were developed and tested empirically, it is clear that there are major challenges ahead to develop new vaccines for difficult-to-target pathogens, for which we urgently need a better understanding of protective immunity. Moreover, recognition of the huge potential and challenges for vaccines to control disease outbreaks and protect the older population, together with the availability of an array of new technologies, make it the perfect time for immunologists to be involved in designing the next generation of powerful immunogens. This Review provides an introductory overview of vaccines, immunization and related issues and thereby aims to inform a broad scientific audience about the underlying immunological concepts.

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What defines an efficacious COVID-19 vaccine? A review of the challenges assessing the clinical efficacy of vaccines against SARS-CoV-2

Susanne Hodgson, Kushal Mansatta, Garry Mallett … (2020)

The novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has caused more than 1 million deaths in the first 6 months of the pandemic and huge economic and social upheaval internationally. An efficacious vaccine is essential to prevent further morbidity and mortality. Although some countries might deploy COVID-19 vaccines on the strength of safety and immunogenicity data alone, the goal of vaccine development is to gain direct evidence of vaccine efficacy in protecting humans against SARS-CoV-2 infection and COVID-19 so that manufacture of efficacious vaccines can be selectively upscaled. A candidate vaccine against SARS-CoV-2 might act against infection, disease, or transmission, and a vaccine capable of reducing any of these elements could contribute to disease control. However, the most important efficacy endpoint, protection against severe disease and death, is difficult to assess in phase 3 clinical trials. In this Review, we explore the challenges in assessing the efficacy of candidate SARS-CoV-2 vaccines, discuss the caveats needed to interpret reported efficacy endpoints, and provide insight into answering the seemingly simple question, “Does this COVID-19 vaccine work?”

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Effect of Delta variant on viral burden and vaccine effectiveness against new SARS-CoV-2 infections in the UK

Koen Pouwels, Emma Pritchard, Philippa C. Matthews … (2021)

The effectiveness of the BNT162b2 and ChAdOx1 vaccines against new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections requires continuous re-evaluation, given the increasingly dominant B.1.617.2 (Delta) variant. In this study, we investigated the effectiveness of these vaccines in a large, community-based survey of randomly selected households across the United Kingdom. We found that the effectiveness of BNT162b2 and ChAdOx1 against infections (new polymerase chain reaction (PCR)-positive cases) with symptoms or high viral burden is reduced with the B.1.617.2 variant (absolute difference of 10–13% for BNT162b2 and 16% for ChAdOx1) compared to the B.1.1.7 (Alpha) variant. The effectiveness of two doses remains at least as great as protection afforded by prior natural infection. The dynamics of immunity after second doses differed significantly between BNT162b2 and ChAdOx1, with greater initial effectiveness against new PCR-positive cases but faster declines in protection against high viral burden and symptomatic infection with BNT162b2. There was no evidence that effectiveness varied by dosing interval, but protection was higher in vaccinated individuals after a prior infection and in younger adults. With B.1.617.2, infections occurring after two vaccinations had similar peak viral burden as those in unvaccinated individuals. SARS-CoV-2 vaccination still reduces new infections, but effectiveness and attenuation of peak viral burden are reduced with B.1.617.2. A large, community-based study in the United Kingdom indicates that the effectiveness of BNT162b2 and ChAdOx1 vaccines against SARS-CoV-2 infections with symptoms or high viral burden is reduced with the Delta variant compared to the Alpha variant.