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 — even among those who are vaccinated may be protected, but 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. This gives a herd immunity threshold of 92-94%. 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) |
The simple formula for herd immunity assumes ideal conditions that rarely exist in the real world. Several factors complicate the picture:
People don't mix randomly. Schools, households, and workplaces create clusters of susceptibility. Even with high overall vaccination, 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 people can carry and spread pathogens (though usually at lower rates).
A common misconception is that "natural infection" provides better immunity than vaccination. The reality is more nuanced:
For most diseases, vaccination provides comparable or superior immunity to natural infection without the risks of contracting the disease. The phrase "natural immunity" is often misleading — it implies infection is the "natural" way to immunity, ignoring the substantial risks.
• Fine P, Eames K, Heymann DL. "Herd immunity": A rough guide. Clin Infect Dis. 2011;52(7):911-916.
• WHO. "Herd Immunity." World Health Organization.
• Randolph HE, Barreiro LB. Herd Immunity: Understanding COVID-19. Immunity. 2020;52(5):737-741.