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SARS-CoV-2 Attack Rate and Population Immunity in Southern New England, March 2020 to May 2021

Educational Objective
To identify the key insights or developments described in this article
1 Credit CME
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.

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Article Information

Accepted for Publication: April 9, 2022.

Published: May 26, 2022. doi:10.1001/jamanetworkopen.2022.14171

Open Access: This is an open access article distributed under the terms of the CC-BY License. © 2022 Tran TNA et al. JAMA Network Open.

Corresponding Author: Maciej F. Boni, PhD, Center for Infectious Disease Dynamics, Department of Biology, Pennsylvania State University, University Park, PA 16802 (mfb9@psu.edu).

Author Contributions: Ms Tran and Dr Boni had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Ms Tran and Dr Wikle contributed equally.

Concept and design: Tran, Gentilesco, Hanage, Boni.

Acquisition, analysis, or interpretation of data: Tran, Wikle, Yang, Inam, Leighow, Chan, Albert, Strong, Pritchard, Hanks, Crawford, Boni.

Drafting of the manuscript: Tran, Yang, Albert, Strong, Hanage, Boni.

Critical revision of the manuscript for important intellectual content: Tran, Wikle, Inam, Leighow, Gentilesco, Chan, Pritchard, Hanage, Hanks, Crawford, Boni.

Statistical analysis: Tran, Wikle, Inam, Leighow, Albert, Strong, Hanks, Crawford.

Obtained funding: Pritchard, Crawford.

Administrative, technical, or material support: Leighow, Gentilesco, Pritchard, Crawford, Boni.

Supervision: Pritchard, Hanage, Hanks, Boni.

Conflict of Interest Disclosures: Dr Pritchard reported receiving personal fees from Theseus Pharmaceuticals, Moma Therapeutics, and Third Rock Ventures; grants from Theseus Pharmaceuticals; and owning stock in Theseus Pharmaceuticals and Moma Therapeutics outside the submitted work. Dr Hanage reported receiving personal fees from Biobot Analytics outside the submitted work. Dr Crawford reported receiving personal fees from Global Diagnostic Systems, Revelar Biotherapeutics, and Whitespace outside the submitted work. Dr Boni reported receiving personal fees from a financial services company outside the submitted work. No other disclosures were reported.

Funding/Support: Dr Boni and Ms Tran are funded by grant No. INV-005517 from the Bill and Melinda Gates Foundation. Ms Yang is supported by contract No. HHS N272201400007C from the National Institutes of Health (NIH) National Institute of Allergy and Infectious Diseases Center of Excellence in Influenza Research and Surveillance. Dr Hanage is funded by award No. U54 GM088558 from the National Institute of General Medical Sciences. Ms Albert is funded by grant No. NSF DMR-1420620 from the Penn State Materials Research Science and Engineering Center, Center for Nanoscale Science. Dr Hanks was partially supported by grant No. DMS-2015273 from the National Science Foundation. Dr Crawford is supported by Cooperative Agreement No. 6NU50CK000524-01 from the Centers for Disease Control and Prevention, funds from the COVID-19 Paycheck Protection Program and Health Care Enhancement Act, NIH Eunice Kennedy Shriver National Institute of Child Health and Human Development grant No. 1DP2HD091799-01, and the Pershing Square Foundation.

Role of the Funder/Sponsor: The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Additional Contributions: Larry Madoff, MD, and Catherine Brown, DVM, MSc, MPH (Massachusetts Department of Public Health), helped in interpretation of the COVID-19 epidemic in Massachusetts. David Kennedy, PhD (Department of Biology, Pennsylvania State University), provided information on behavioral heterogeneity and the correlation between vaccination and past infection. They did not received any compensation for these contributions.

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