Protection Conferred by COVID-19 Vaccination, Prior SARS-CoV-2 Infection, or Hybrid Immunity Against Omicron-Associated Severe Outcomes Among Community-Dwelling Adults

Abstract

Introduction

We assessed protection from coronavirus disease 2019 (COVID-19) vaccines and/or prior severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection against Omicron-associated severe outcomes during successive sublineage-predominant periods.

Methods

We used a test-negative design to estimate protection by vaccines and/or prior infection against hospitalization/death among community-dwelling, polymerase chain reaction (PCR)-tested adults aged ≥50 years in Ontario, Canada, between 2 January 2022 and 30 June 2023. Multivariable logistic regression was used to estimate the relative change in the odds of hospitalization/death with each vaccine dose (2–5) and/or prior PCR-confirmed SARS-CoV-2 infection (compared with unvaccinated, uninfected subjects) up to 15 months since the last vaccination or infection.

Results

We included 18 526 cases with Omicron-associated severe outcomes and 90 778 test-negative controls. Vaccine protection was high during BA.1/BA.2 predominance but was generally <50% during periods of BA.4/BA.5 and BQ/XBB predominance without boosters. A third/fourth dose transiently increased protection during BA.4/BA.5 predominance ( third-dose, 6-month: 68%, 95% confidence interval [CI] 63%–72%; fourth-dose, 6-month: 80%, 95% CI 77%–83%) but was lower and waned quickly during BQ/XBB predominance ( third-dose, 6-month: 59%, 95% CI 48%–67%; 12-month: 49%, 95% CI 41%–56%; fourth-dose, 6-month: 62%, 95% CI 56%–68%, 12-months: 51%, 95% CI 41%–56%). Hybrid immunity conferred nearly 90% protection throughout BA.1/BA.2 and BA.4/BA.5 predominance but was reduced during BQ/XBB predominance ( third-dose, 6-month: 60%, 95% CI 36%–75%; fourth-dose, 6-month: 63%, 95% CI 42%–76%). Protection was restored with a fifth dose (bivalent; 6-month: 91%, 95% CI 79%–96%). Prior infection alone did not confer lasting protection.

Conclusions

Protection from COVID-19 vaccines and/or prior SARS-CoV-2 infections against severe outcomes is reduced when immune-evasive variants/subvariants emerge and may also wane over time. Our findings support a variant-adapted booster vaccination strategy with periodic review.

Abstract

Protection from coronavirus disease 2019 (COVID-19) vaccines, prior COVID-19 infections or hybrid immunity against severe outcomes is reduced when immune-evasive variants/subvariants emerge and may also wane over time. Our findings support a variant-adapted booster vaccination strategy with periodic review.