The following is a summary of “Population-Level Relative Effectiveness of the COVID-19 Vaccines and the Contribution of Naturally Acquired Immunity,” published in the March 2023 issue of Infectious Diseases by Shioda, et al.

The interaction between vaccine-induced immunity and naturally acquired immunity against SARS-CoV-2 has yet to be extensively studied. Therefore, this study aimed to determine whether the impact of COVID-19 vaccines differed across states with different levels of naturally acquired immunity in the United States.

Regression models were used to evaluate the impact of COVID-19 vaccines across states with varying levels of naturally acquired immunity from March 2021 to April 2022 in the United States. The study analyzed three evaluation periods separately – Alpha, Delta, and Omicron waves. The proportion of the population with naturally acquired immunity was estimated using either the reported seroprevalence or the estimated proportion of the population ever infected in each state.

The study found that COVID-19 mortality decreased as vaccine coverage of ≥1 dose increased among people ≥65 years of age, and this effect did not vary by seroprevalence or the proportion of the total population ever infected. Furthermore, seroprevalence and proportion ever infected were not associated with COVID-19 mortality after controlling for vaccine coverage. These findings were consistent in all evaluation periods.

The study concludes that COVID-19 vaccination was associated with a sustained reduction in mortality at the state level during the Alpha, Delta, and Omicron periods. The effect did not vary by naturally acquired immunity.

Reference: academic.oup.com/jid/article/227/6/773/6880998