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Herd Immunity

Herd immunity gives protection to vulnerable people such as newborn babies, elderly people and those who are too sick to be vaccinated.

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Key Summary

  • What herd immunity is
    • When enough people are immune to a disease, its spread slows or stops.
    • Protects vulnerable groups who can’t be vaccinated, such as newborns or people with weak immune systems.
  • How herd immunity is achieved
    • Vaccination: safest and most effective way.
    • Infection and recovery: also creates immunity but comes with risks of serious illness, complications, and death.
  • Herd immunity thresholds
    • The proportion of people who need immunity varies by disease:
      • Measles: ~95% of the population.
      • Polio: ~80%.
      • COVID-19: originally thought to be ~60–70%, but new variants have pushed the threshold much higher (>90%), making herd immunity unlikely with current vaccines.
  • COVID-19 and herd immunity
    • More transmissible variants and waning immunity mean herd immunity is not a realistic goal.
    • Focus has shifted to preventing severe illness, hospitalisation, and death with vaccines, boosters, and public health measures.
  • Limits of herd immunity
    • Not all diseases are prevented this way (e.g., tetanus is infectious but not contagious).
    • Still highly effective for diseases like whooping cough, measles, mumps, rubella, and pneumococcal disease.
  • Why vaccination matters
    • Vaccination is the safest path to herd immunity.
    • Protects individuals and the broader community by reducing illness, complications, and deaths.

What Is Herd Immunity?

Herd immunity is when a significant portion of the population gains immunity to an infectious disease, thereby limiting its spread. It helps to protect vulnerable people like babies, the elderly and those who are immunocompromised.

How Is Herd Immunity Achieved?

Herd immunity can be achieved either by vaccination, or by developing the disease and recovering from it.

Herd immunity through vaccination

When a high percentage of the population is vaccinated, it is difficult for infectious diseases to spread, because there are not many people who can be infected. For example, if someone with measles is surrounded by people who are vaccinated against measles, the disease cannot easily be passed on to anyone, and it will quickly disappear again.

Herd immunity can protect those who cannot be vaccinated, like newborns and those who are too sick to be vaccinated.

Herd immunity by developing the disease

Enough people need to develop the infection and recover from it in order to protect the population against developing it in the future. People who develop the infection will develop antibodies that will protect them and the community in the future.

Herd immunity threshold

Often, a percentage of the population must be capable of getting a disease for it to spread. This is called a threshold proportion. If the proportion of the population that is immune to the disease is greater than this threshold, the spread of the disease will decline. This is known as the herd immunity threshold.

What Percentage Of The Community Needs To Be Immune To Achieve Herd Immunity?

It depends on the disease. For more infectious diseases, the percentage of the community that will need to be immune will be higher. Measles is highly infectious and around 95% of the community will need to be immune to stop the chain of infection.

For polio, the threshold is about 80%.

COVID-19

Achieving herd immunity for COVID-19 through vaccination has proven to be complex and likely unfeasible with current vaccines. While early estimates suggested that 60–70% of the population would need to be immune to reduce transmission, the emergence of more transmissible variants such as Delta and Omicron pushed that threshold much higher—possibly above 90%1-2.

Additional challenges include waning immunity over time and the ability of new variants to partially evade immune protection3-4. These factors mean that long-term herd immunity through vaccination or prior infection is unlikely. Instead, public health efforts have shifted toward reducing severe illness, hospitalisation, and death through a combination of vaccines, booster doses, and non-pharmaceutical interventions1,3,5.

Vaccines remain a critical tool in managing COVID-19, particularly for protecting vulnerable populations and preventing healthcare system overload5.

Does Herd Immunity Protect Against All Vaccine Preventable Diseases?

Herd immunity does not protect against all vaccine-preventable diseases. The best example of this is tetanus, which is infectious but not contagious. It is caught from bacteria in the environment, not from other people who have the disease. No matter how many people around you are vaccinated against tetanus, it will not protect you from tetanus.

Herd immunity does protect against many serious illnesses like whooping cough, measles, mumps, rubella and pneumococcal disease. Vaccination is a safe way to achieve herd immunity and protect the community against serious illnesses, their complications, admissions to hospital and deaths.

Click here to read more about Herd Immunity

Credit: The History of Vaccines is an award-winning informational, educational website created by The College of Physicians of Philadelphia, one of the oldest medical societies in the United States. A group of prominent Philadelphia physicians, including Declaration of Independence signer Benjamin Rush, established the College in 1787 “to advance the science of medicine and to thereby lessen human misery.”

References

  1. World Health Organization. COVID-19 vaccines [Internet]. 2024 [cited 2025 May 28]. Available from: https://www.who.int/news-room/questions-and-answers/item/coronavirus-disease-(covid-19)-vaccines
  2. Randolph HE, Barreiro LB. Herd Immunity: Understanding COVID-19. Immunity. 2020;52(5):737–41.
  3. Fontanet A, Cauchemez S. COVID-19 herd immunity: where are we? Nat Rev Immunol. 2020;20(10):583–4.
  4. Aschwanden C. Five reasons why COVID herd immunity is probably impossible. Nature. 2021;591(7851):520–2.
  5. Khoury DS, Cromer D, Reynaldi A, et al. Neutralizing antibody levels are highly predictive of immune protection from symptomatic SARS-CoV-2 infection. Nat Med. 2021;27(7):1205–11.