Course: SARS-CoV-2 Attack Rate and Population Immunity in Southern New England, March 2020 to May 2021
CME Credits: 1.00
Released: 2022-05-26
Key Points
Question What proportion of individuals living in southern New England had immunity to SARS-CoV-2, either through past infection or vaccination, by May 31, 2021?Findings This case series analysis for Rhode Island, Massachusetts, and Connecticut revealed that two-thirds of residents were immune to SARS-CoV-2 by May 31, 2021. The population immune fraction was lower than desired because 27% of vaccines during the winter to spring 2021 vaccination campaign were administered to individuals who were already seropositive.
Meaning These findings suggest that SARS-CoV-2 population immunity was overestimated in summer 2021 and that future emergency-setting vaccination campaigns may need to exceed traditional coverage goals.
Abstract
Importance In emergency epidemic and pandemic settings, public health agencies need to be able to measure the population-level attack rate, defined as the total percentage of the population infected thus far. During vaccination campaigns in such settings, public health agencies need to be able to assess how much the vaccination campaign is contributing to population immunity; specifically, the proportion of vaccines being administered to individuals who are already seropositive must be estimated.Objective To estimate population-level immunity to SARS-CoV-2 through May 31, 2021, in Rhode Island, Massachusetts, and Connecticut.
Design, Setting, and Participants This observational case series assessed cases, hospitalizations, intensive care unit occupancy, ventilator occupancy, and deaths from March 1, 2020, to May 31, 2021, in Rhode Island, Massachusetts, and Connecticut. Data were analyzed from July 2021 to November 2021.
Exposures COVID-19–positive test result reported to state department of health.
Main Outcomes and Measures The main outcomes were statistical estimates, from a bayesian inference framework, of the percentage of individuals as of May 31, 2021, who were (1) previously infected and vaccinated, (2) previously uninfected and vaccinated, and (3) previously infected but not vaccinated.
Results At the state level, there were a total of 1,160,435 confirmed COVID-19 cases in Rhode Island, Massachusetts, and Connecticut. The median age among individuals with confirmed COVID-19 was 38 years. In autumn 2020, SARS-CoV-2 population immunity (equal to the attack rate at that point) in these states was less than 15%, setting the stage for a large epidemic wave during winter 2020 to 2021. Population immunity estimates for May 31, 2021, were 73.4% (95% credible interval [CrI], 72.9%-74.1%) for Rhode Island, 64.1% (95% CrI, 64.0%-64.4%) for Connecticut, and 66.3% (95% CrI, 65.9%-66.9%) for Massachusetts, indicating that more than 33% of residents in these states were fully susceptible to infection when the Delta variant began spreading in July 2021. Despite high vaccine coverage in these states, population immunity in summer 2021 was lower than planned owing to an estimated 34.1% (95% CrI, 32.9%-35.2%) of vaccines in Rhode Island, 24.6% (95% CrI, 24.3%-25.1%) of vaccines in Connecticut, and 27.6% (95% CrI, 26.8%-28.6%) of vaccines in Massachusetts being distributed to individuals who were already seropositive.
Conclusions and Relevance These findings suggest that future emergency-setting vaccination planning may have to prioritize high vaccine coverage over optimized vaccine distribution to ensure that sufficient levels of population immunity are reached during the course of an ongoing epidemic or pandemic.
Educational Objective
To identify the key insights or developments described in this article
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