Herd immunity provides indirect protection to people who are susceptible to an infectious disease
The herd immunity threshold is the minimum proportion of the population that must be immune to an infectious disease, usually due to vaccination, for the incidence of the disease to remain stable or decrease.1
The herd immunity threshold correlates with the infectiousness of the pathogen
The herd immunity threshold for measles, one of the most infectious diseases, is about 94%.1 For SARS-CoV-2 variants of concern, such as B.1.1.7 (Alpha), the threshold is about 80%2 and it may be higher for newly emerging variants like B.1.617.2 (Delta).3
Effective vaccines with lasting protection are essential for achieving herd immunity
Vaccines are not perfect, and some vaccinated people can still become infected and transmit SARS-CoV-2.4 People who are immunosuppressed may not mount an adequate immune response. Outbreaks may occur despite high vaccine coverage in the population. Effectiveness varies by type of vaccine, number of doses administered and the particular SARS-CoV-2 variant in circulation. Based on estimates of the effectiveness of 2 vaccine doses against symptomatic infection with B.1.1.7 (Alpha) in Ontario (90% effectiveness), nearly 90% vaccine coverage in the population may be required for herd immunity.5 Increases in the prevalence of more infectious variants, and reduced vaccine effectiveness against them, would require even higher coverage. The duration of immunity from infection or vaccination is currently unknown.4
Vaccine hesitancy and structural barriers to vaccination may threaten herd immunity
High and even vaccine coverage is needed to achieve herd immunity. Achieving sufficient vaccine coverage depends on addressing vaccine hesitancy and barriers to vaccination that may result in pockets of susceptibility.
Maintaining herd immunity requires sustained effort
Maintaining herd immunity will depend on vaccines being effective against variants, addressing barriers to vaccination, and sustaining coverage if repeat vaccination is required and as the population changes (e.g., due to births, immigration). Even if herd immunity is not achieved, high vaccine coverage will substantially reduce rates of morbidity and mortality, and lessen the burden of COVID-19 on Canada’s health care systems.
Footnotes
Competing interests: Shelly Bolotin reports funding from the Canadian Institutes of Health Research, the Canadian Immunization Research Network, the COVID-19 Immunity Task Force and the Public Health Agency of Canada, outside the submitted work. She is a member of the Canadian Immunization Research Network Management Committee, COVID-19 Immunity Task Force Leadership Group. The authors are employees of Public Health Ontario; this organization paid for the article processing fees. No other competing interests were declared.
This article has been peer reviewed.
This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY-NC-ND 4.0) licence, which permits use, distribution and reproduction in any medium, provided that the original publication is properly cited, the use is noncommercial (i.e., research or educational use), and no modifications or adaptations are made. See: https://creativecommons.org/licenses/by-nc-nd/4.0/
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- Herd immunity provides indirect protection to people who are susceptible to an infectious disease
- The herd immunity threshold correlates with the infectiousness of the pathogen
- Effective vaccines with lasting protection are essential for achieving herd immunity
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- Maintaining herd immunity requires sustained effort
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