Thresholds, evidence, and the limits of community protection
Herd immunity (also called community immunity) occurs when a sufficient proportion of a population becomes immune to an infectious disease, thereby reducing its spread and protecting even those who are not immune.
This concept is central to vaccination strategies. When vaccination rates fall below the herd immunity threshold, outbreaks can occur — the unvaccinated become vulnerable and diseases can spread through communities.
However, herd immunity is more complex than simple percentages suggest. This page explains how thresholds are calculated, what factors affect them, and why achieving herd immunity in practice is often more challenging than the math suggests.
The herd immunity threshold depends on a disease's basic reproduction number (R₀) — the average number of secondary infections caused by a single infected person in a fully susceptible population.
The formula:
Herd Immunity Threshold = (R₀ - 1) / R₀
For example, measles has an R₀ of 12–18, meaning one infected person typically spreads it to 12–18 others, giving a herd immunity threshold of 92–94% (Fine et al., 2011). Even with 90% vaccination coverage, measles can spread.
| Disease | R₀ (typical range) | Herd Immunity Threshold | Vaccine Efficacy |
|---|---|---|---|
| Measles | 12–18 | 92–94% | 97% (2 doses) |
| Pertussis (whooping cough) | 12–17 | 92–94% | 80–90% (varies) |
| Polio | 5–7 | 80–86% | 99%+ (3 doses) |
| Rubella | 5–7 | 80–86% | 97% (1 dose) |
| Seasonal Flu | 1–3 | 33–50% | 40–60% (varies) |
| COVID-19 (original strain) | 2–3 | 50–67% | 90%+ (original vaccines) |
| COVID-19 (Omicron) | 8–15 | 87–93% | Lower (variants) |
Sources: R₀ estimates from Fine, Eames & Heymann (2011); Randolph & Barreiro (2020); WHO (2021); CDC Pink Book (2021). All values are estimates — R₀ varies by setting, population density, and study methodology. Vaccine efficacy figures reflect published clinical trial and post-licensure data; effectiveness in real-world settings may differ. See Efficacy vs. Effectiveness for detail.
The simple formula for herd immunity assumes ideal conditions that rarely exist in practice. Several factors complicate the picture:
People don't mix randomly. Schools, households, and workplaces create clusters of susceptibility. Even with high overall vaccination rates, pockets of low coverage can sustain transmission.
Vaccine-induced immunity can decline over time, requiring boosters. If boosters aren't maintained, previously protected populations can become susceptible again.
Viruses and bacteria evolve. New variants can escape immune protection from both vaccination and prior infection, effectively resetting herd immunity calculations.
Some vaccines prevent disease but don't fully prevent infection or transmission. Even vaccinated individuals can carry and spread pathogens, though usually at lower rates.
A common question is whether "natural infection" provides better or longer-lasting immunity than vaccination. The evidence varies by disease:
For many diseases, peer-reviewed evidence shows that vaccination produces comparable or superior immunity to natural infection without the associated risks. However, the relative durability of vaccine-induced vs. infection-induced immunity varies by pathogen and is an active area of research.
Herd immunity thresholds are population-level models with important caveats that are frequently misunderstood:
Fine P, Eames K, Heymann DL. "Herd immunity: A rough guide." Clinical Infectious Diseases. 2011;52(7):911-916. https://doi.org/10.1093/cid/cir007
Randolph HE, Barreiro LB. "Herd Immunity: Understanding COVID-19." Immunity. 2020;52(5):737-741. https://doi.org/10.1016/j.immuni.2020.04.012
WHO. "Herd Immunity, Lockdowns and COVID-19." World Health Organization. https://www.who.int/news-room/questions-and-answers/item/herd-immunity-lockdowns-and-covid-19
Metcalf CJE, et al. "Use of serological surveys to generate key insights into the changing global landscape of infectious disease." The Lancet. 2016;388(10045):728-730. https://doi.org/10.1016/S0140-6736(16)30164-7
CDC. "Epidemiology and Prevention of Vaccine-Preventable Diseases" (The Pink Book). 14th edition. 2021. https://www.cdc.gov/vaccines/pubs/pinkbook/index.html
Liu Y, et al. "The effective reproductive number of the Omicron variant of SARS-CoV-2 is several times relative to Delta." Journal of Travel Medicine. 2022;29(3). https://doi.org/10.1093/jtm/taac037