Herd Immunity: How a Community Protects the People Who Cannot Be Protected Themselves

A newborn baby is too young to be vaccinated. A child on chemotherapy cannot safely receive live vaccines. A person with HIV may not mount a strong response to one. These people cannot be protected directly, yet in a well-vaccinated community they rarely catch the diseases that would once have killed them. Something is protecting them, and it is not their own immune system. It is everyone else's. When enough people around them are immune, a microbe that enters the community runs out of people to infect and dies out before it reaches the vulnerable. This indirect protection, provided by the immunity of the crowd, is herd immunity.

The one idea, and the formula that captures it

Herd immunity rests on a single mechanical idea: a microbe spreads only if each infected person passes it to at least one new susceptible person. Break that chain, and the outbreak dies out.

When many people in a community are immune, an infected person meets mostly immune people, who do not pass the microbe on. The chain keeps hitting dead ends. Even the people who are not immune are protected, because the microbe cannot reach them through the wall of immune people around them. This is why herd immunity protects the vulnerable without vaccinating them directly.

The powerful part is that this can be captured in one simple formula. The threshold, the fraction of the population that must be immune to stop spread, depends only on how contagious the microbe is, measured by R0 (the average number of people one infected person would infect in a fully susceptible population):

Herd immunity threshold = 1 − (1 / R0)

Figure: Herd Immunity (Image courtesy: The National Institute of Allergy and Infectious Disease )

That formula is worth more than any memorized table, because it lets you work out the threshold for any disease from its R0. Measles has an R0 of about 12 to 18. Using R0 = 15, the threshold is 1 − (1/15) = 0.93, so about 93 to 95 percent must be immune. Polio has an R0 of about 5 to 7. Using R0 = 6, the threshold is 1 − (1/6) = 0.83, so about 80 to 85 percent. The more contagious the microbe (higher R0), the higher the wall of immunity has to be.

So the whole topic reduces to one chain-breaking idea and one formula. Everything else is a consequence.

How herd immunity works

Every immune person in a community does two things: they cannot catch the microbe, and they cannot pass it on. So each immune person removes a link from the chain of transmission.

When only a few people are immune, the microbe moves easily from person to person, and an outbreak grows. As the immune fraction rises, the microbe increasingly meets immune people and fails to spread. Once the immune fraction crosses the threshold, each infected person on average infects fewer than one new person, and the outbreak shrinks instead of growing. The disease cannot sustain itself.

Immunity in the community can come from two sources: vaccination, or recovery from natural infection (including mild or symptomless infection). Vaccination is by far the safer route to herd immunity, because it builds the immune fraction without people having to suffer, and possibly die from, the disease itself.

R0 and the herd immunity threshold

The threshold rises with R0. The values below use the formula 1 − (1/R0), rounded to the ranges commonly taught.

The pattern is the point: the more contagious the disease, the higher the threshold. Measles, one of the most contagious diseases known, needs one of the highest thresholds, around 95 percent, which is why even small drops in measles vaccination coverage lead quickly to outbreaks.

When herd immunity does not apply

Herd immunity only works against diseases that spread from person to person. It cannot protect against diseases you catch from a source other than other people.

The classic example is tetanus. Tetanus is caused by a bacterium (Clostridium tetani) whose spores live in soil, not in other people. You catch it from a contaminated wound, not from a contagious person. So no matter how many people around you are vaccinated against tetanus, their immunity cannot protect you. This is why tetanus vaccination protects only the individual vaccinated, and why everyone needs their own tetanus protection regardless of community coverage.

The rule: herd immunity protects against contagious (person-to-person) diseases. It does not protect against diseases acquired from the environment.

Why herd immunity matters

Vaccination is the best way to protect an individual, but not everyone can be vaccinated. Newborns are too young. People on chemotherapy, organ transplant recipients on immunosuppressants, and people with certain immune conditions may not be able to receive some vaccines or may not respond fully to them. These people depend on the immunity of those around them.

This is the deepest reason herd immunity matters: it is how a community protects its most vulnerable members. When enough healthy people are immune, the microbe cannot travel through the population to reach those who cannot protect themselves. High vaccination coverage is not only self-protection. It is protection of the people who have no other defense.

How to remember herd immunity

One formula beats one table: 1 minus 1 over R0. If you learn one thing, learn the formula for the threshold: 1 − (1/R0). From it you can derive the threshold for any disease. The table is just the formula applied.

Higher R0, higher wall. The more contagious the disease (bigger R0), the more of the population must be immune. Measles is extremely contagious, so it needs the highest wall (about 95 percent).

Herd immunity is for people-to-people diseases only. If you catch it from soil, water, or a wound rather than from another person, herd immunity does not help. Tetanus is the standard example: it comes from soil, so a vaccinated crowd cannot protect you.

The herd protects the ones who cannot protect themselves. The whole point in one line. Herd immunity exists to shield the newborn, the immunocompromised, and the person who cannot be vaccinated, using the immunity of everyone else.

Key exam facts in one table

Where students get confused

"Herd immunity means everyone is immune." No. It means enough people are immune to stop the disease spreading, not all of them. The threshold is usually well below 100 percent, and the whole value of herd immunity is that it protects the remaining non-immune people.

"A higher R0 means a lower threshold." The opposite. A higher R0 means the disease is more contagious, so a larger fraction of the population must be immune to stop it. Measles (very high R0) needs about 95 percent; less contagious diseases need less.

"Herd immunity protects against every disease." Only person-to-person diseases. It cannot protect against diseases caught from the environment, such as tetanus from soil. If the microbe does not spread between people, breaking the person-to-person chain achieves nothing.

"Reaching herd immunity by letting the disease spread is a good strategy." Building immunity through natural infection means people must actually get the disease, with all its risks of severe illness and death. Vaccination reaches the same threshold without that harm, which is why it is the safe route to herd immunity.

"Herd immunity is permanent once reached." It can be lost. If immunity wanes over time, or vaccination coverage falls, the immune fraction can drop back below the threshold and outbreaks return. This is why maintaining high coverage matters, not just reaching it once.

References

• Fine P, Eames K, Heymann DL. "Herd immunity": a rough guide. Clinical Infectious Diseases. 2011;52(7):911-916.
• Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Philadelphia: Elsevier; 2020.
• World Health Organization. Immunization coverage. Geneva: WHO; 2024.