Why Herd Immunity Matters and How Vaccines Help
, by Andrew Odgers, 7 min reading time
Public Health
Why Herd Immunity Matters and How Vaccines Help
Herd immunity describes the indirect protection that unvaccinated individuals receive when a sufficient proportion of the surrounding population is immune to a disease. It is one of the most important public health concepts in vaccination, and understanding it explains why vaccination rates across a community matter — not just protection for individuals.
95%MMR coverage needed to maintain measles herd immunity
80–85%Coverage needed for polio herd immunity
70%Coverage needed for flu (seasonal estimate)
~60MUK population protected by NHS immunisation programme
The mathematics of herd immunity
Why the threshold varies between diseases
The basic reproduction number R0
Every transmissible infectious disease has a basic reproduction number, R0, which represents the average number of people one infected individual will transmit the disease to in a fully susceptible population with no immunity. R0 is determined by how contagious the pathogen is, how long it is transmissible, and how it spreads. Measles has one of the highest R0 values of any known infectious disease, at 12 to 18. This means that in an entirely unvaccinated population, one measles case would produce 12 to 18 secondary cases, each of which would produce a further 12 to 18, rapidly producing exponential spread.
Calculating the herd immunity threshold
The herd immunity threshold is the proportion of a population that must be immune to interrupt sustained transmission. It is calculated from R0 using the formula: threshold = 1 − (1/R0). For measles with an R0 of 15, the threshold is 1 − (1/15) = approximately 93 to 95 percent. For polio with an R0 of 5 to 7, the threshold is approximately 80 to 85 percent. These thresholds assume uniform mixing of the population; real populations have clustered communities where local coverage below threshold enables outbreaks even when national coverage is adequate.
What happens when a vaccinated person is in an immune population
When a vaccinated person encounters the pathogen, they are protected. But they also contribute to herd immunity by not becoming infected and therefore not transmitting. In an immune population, transmission chains are broken because most contacts of any infected individual are immune. The probability that an infected person encounters a susceptible individual falls as coverage rises, and at the herd immunity threshold, each infection produces on average less than one secondary case, causing the outbreak to die out.
Herd immunity thresholds by disease
TrueCoverage needed to interrupt transmission
MMR vaccine coverage in England over time
Who depends on herd immunity
TrueThe individuals who cannot be vaccinated
Frequently asked
Questions answered
Can herd immunity be achieved through natural infection instead of vaccination?
Theoretically yes, but at an unacceptable cost. To reach the 95 percent immunity threshold for measles through natural infection rather than vaccination requires an enormous number of measles infections, each carrying a 1 to 2 per 1,000 risk of death and higher risk of complications including encephalitis, pneumonia, and lasting disability. Vaccination achieves the same or better immunity threshold without this disease burden.
If I am vaccinated, does it matter if others in my community are not?
Your personal protection from vaccination is not affected by others' vaccination status — you remain protected. However, vaccination contributes to herd immunity, reducing the probability that susceptible individuals in your community will be exposed. If coverage in your community drops below the threshold, unvaccinated people including those who cannot be vaccinated face increased risk.
Why is measles coming back when we have a vaccine?
Measles is returning in many European countries, including the UK, specifically because vaccination rates have declined in some communities below the 95 percent threshold needed to maintain herd immunity. The measles virus has not changed; human vaccination behaviour has. The vaccine remains highly effective at over 97 percent protection with two doses.
Does herd immunity apply to COVID-19?
Herd immunity to COVID-19 through vaccination has been complicated by the evolution of new variants that partially escape immunity from earlier vaccines or infection. The concept applies but the target threshold shifts as the virus evolves. Unlike measles, where the virus is stable and the vaccine provides very long-term protection, COVID-19 appears to require ongoing booster programmes targeting current variants for high-risk groups.
Vaccination supplies
Clinical vaccination consumables from Charles Medical
Charles Medical supplies hypodermic needles, syringes, and all consumables used in vaccination practice. Next-day UK delivery, no minimum order.