December 5, 2021

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Effect of Vaccination on Transmission of SARS-CoV-2

To the Editor:

Whether vaccination of individual persons for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) protects members of their households is unclear. We investigated the effect of vaccination of health care workers in Scotland (who were among the earliest groups to be vaccinated worldwide) on the risk of coronavirus disease 2019 (Covid-19) among members of their households.

We evaluated data from 194,362 household members (which represented 92,470 households of 2 to 14 persons per household) of 144,525 health care workers who had been employed during the period from March 2020 through November 2020. The mean ages of the household members and the health care workers were 31 and 44 years, respectively; a majority (>96%) were White. A total of 113,253 health care workers (78.4%) had received at least one dose of either the BNT162b2 (Pfizer–BioNTech) mRNA vaccine or the ChAdOx1 nCoV-19 (Oxford–AstraZeneca) vaccine, and 36,227 (25.1%) had received a second dose.

The primary outcome was any confirmed case of Covid-19 that occurred between December 8, 2020, and March 3, 2021. We also report results for Covid-19–associated hospitalization. The primary time periods we compared were the unvaccinated period before the first dose and the period beginning 14 days after the health care worker received the first dose. No adjustment was made for multiplicity. Events that occurred after any household member was vaccinated were censored. Detailed methods and results, strengths and limitations, and the protocol are provided in the Supplementary Appendix, which is available with the full text of this letter at NEJM.org. This study was approved by the Public Benefit and Privacy Panel (2021-0013), and the scientific officer of the West of Scotland Research Ethics Committee provided written confirmation that formal ethics review was not required.

Effect of Vaccination of Health Care Workers on Documented Covid-19 Cases and Hospitalizations in Health Care Workers and Their Households.

Cases of Covid-19 were less common among household members of vaccinated health care workers during the period beginning 14 days after the first dose than during the unvaccinated period before the first dose (event rate per 100 person-years, 9.40 before the first dose and 5.93 beginning 14 days after the first dose). After the health care worker’s second dose, the rate in household members was lower still (2.98 cases per 100 person-years). These differences persisted after fitting extended Cox models that were adjusted for calendar time, geographic region, age, sex, occupational and socioeconomic factors, and underlying conditions. Relative to the period before each health care worker was vaccinated, the hazard ratio for a household member to become infected was 0.70 (95% confidence interval [CI], 0.63 to 0.78) for the period beginning 14 days after the first dose and 0.46 (95% CI, 0.30 to 0.70) for the period beginning 14 days after the second dose (Table 1 and the Supplementary Appendix). Not all the cases of Covid-19 in the household members were transmitted from the health care worker; therefore, the effect of vaccination may be larger.1 For example, if half the cases in the household members were transmitted from the health care worker, a 60% decrease in cases transmitted from health care workers would need to occur to elicit the association we observed (see the Supplementary Appendix). Vaccination was associated with a reduction in both the number of cases and the number of Covid-19–related hospitalizations in health care workers between the unvaccinated period and the period beginning 14 days after the first dose.

Given that vaccination reduces asymptomatic infection with SARS-CoV-2,2,3 it is plausible that vaccination reduces transmission; however, data from clinical trials and observational studies are lacking.4,5 We provide empirical evidence suggesting that vaccination may reduce transmission by showing that vaccination of health care workers is associated with a decrease in documented cases of Covid-19 among members of their households. This finding is reassuring for health care workers and their families.

Anoop S.V. Shah, M.D.
London School of Hygiene and Tropical Medicine, London, United Kingdom

Ciara Gribben, M.Sc.
Jennifer Bishop, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom

Peter Hanlon, M.D.
University of Glasgow, Glasgow, United Kingdom

David Caldwell, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom

Rachael Wood, Ph.D.
University of Edinburgh, Edinburgh, United Kingdom

Martin Reid, B.Sc.
Jim McMenamin, M.D.
David Goldberg, M.D.
Diane Stockton, M.Sc.
Public Health Scotland, Edinburgh, United Kingdom

Sharon Hutchinson, Ph.D.
Glasgow Caledonian University, Glasgow, United Kingdom

Chris Robertson, Ph.D.
University of Strathclyde, Glasgow, United Kingdom

Paul M. McKeigue, Ph.D.
Helen M. Colhoun, Ph.D.
University of Edinburgh, Edinburgh, United Kingdom

David A. McAllister, M.D.
University of Glasgow, Glasgow, United Kingdom
[email protected]

Supported by the British Heart Foundation and Wellcome.

Disclosure forms provided by the authors are available with the full text of this letter at NEJM.org.

This letter was published on September 8, 2021, at NEJM.org.

  1. 1. Shah ASV, Wood R, Gribben C, et al. Risk of hospital admission with coronavirus disease 2019 in healthcare workers and their households: nationwide linkage cohort study. BMJ 2020;371:m3582m3582.

  2. 2. Voysey M, Costa Clemens SA, Madhi SA, et al. Single-dose administration and the influence of the timing of the booster dose on immunogenicity and efficacy of ChAdOx1 nCoV-19 (AZD1222) vaccine: a pooled analysis of four randomised trials. Lancet 2021;397:881891.

  3. 3. Hall VJ, Foulkes S, Saei A, et al. COVID-19 vaccine coverage in health-care workers in England and effectiveness of BNT162b2 mRNA vaccine against infection (SIREN): a prospective, multicentre, cohort study. Lancet 2021;397:17251735.

  4. 4. Dagan N, Barda N, Kepten E, et al. BNT162b2 mRNA Covid-19 vaccine in a nationwide mass vaccination setting. N Engl J Med 2021;384:14121423.

  5. 5. Chodick G, Tene L, Patalon T, et al. Assessment of effectiveness of 1 dose of BNT162b2 vaccine for SARS-CoV-2 infection 13 to 24 days after immunization. JAMA Netw Open 2021;4(6):e2115985e2115985.

https://www.nejm.org/doi/full/10.1056/NEJMc2106757