Herd Immunity: Types, Threshold, Usefulness

There are basically three ways to stop infectious diseases like covid19, one involves the interruption of transmission by using extraordinary restrictions including social distancing, another is vaccination (vaccine is in developing phase for Covid19), and the third is waiting until enough people get it and become immune to it (if they survive).

In Covid19 pandemic, government of UK was criticized for its ‘Herd Immunity’ debacle. After growing pressure, British Prime Minister Boris Johnson later reverted their strategy of allowing the virus to spread and build up immunity

2019 Novel coronavirus will keep on spreading until it finds susceptible individuals but when many individuals become immune to this virus, the chain of transmission will break and the incidence of new cases will decrease. This phenomenon is known as herd immunity (also known as ‘community immunity’ or ‘herd protection’.

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

This type of immunity refers to the collective resistance to the disease displayed by the community in its environmental setting. The ratio of resistance to the susceptible person in any community is crucial in the transmission of infectious diseases. If a community has more number of resistant individuals to a particular infection, they may provide indirect protection to remaining susceptible individuals.  For example, after a successful vaccination program, there will be a decrease in the percentage of the susceptible persons within a population which will result in a reduction of the possibility of transmission of infection to others in the community, because each immune individuals plays a role to interrupt the disease transmission chain. 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.

If a suitable pathogen is introduced in a community where herd immunity is low, it may result in epidemics or pandemics, as there are large numbers of susceptible persons in the community. But if the number of resistant individuals in that community increased either by vaccination or by recovery after clinical or subclinical infection, the immune individuals act as a shield to prevent transmission of the infections to other susceptible individuals. Every immune person adds to the effectiveness of herd immunity.

Complete vs Incomplete Herd Immunity

Herd immunity only works if a sufficient number of people in a population are immune to this infection. This number depends on the transmissibility and severity of the infections. To set a threshold, epidemiologists use a value called “basic reproduction number i.e. number of people one infected person could pass the disease in an unprotected population,” often referred to as “R0.”

Disease R0 Threshold
Mumps 4-7 75-86
Polio 5-7 80-86
Smallpox 5-7 80-86
Diphtheria 6-7 85
Rubella 6-7 83-85
Pertusis 12-17 92-94
Measles 12-18 83-94

For highly contagious infections such as measles, the immunity threshold needed to protect a community is high, at 95% but for diseases like polio, which are a little less contagious, immunity threshold of 80% to 85% can provide community protection.

Herd immunity doesn’t protect against all diseases. For example, no matter how many people around you are vaccinated against tetanus, it won’t protect you from getting tetanus.

Usefulness of Herd Immunity

Vaccination is the best way to protect individuals from infection but all individuals in a community may not be a suitable candidate for vaccination such as people with impaired immunity, people with HIV/AIDS, organ transplant recipients, people under chemotherapy, etc. Such individuals are protected by herd immunity.

References and further reading

  1. The power of herd immunity, a TED talk by Romina Libster
  2. Herd immunity (Herd protection), University of Oxford, Oxford Vaccine group.
  3. Relationship between R0 and threshold level needed for herd immunity© Tangled Bank Studios; data from Epidemiologic Reviews 1993.
  4. Herd Immunity, Public Broadcasting Service (PBS)

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

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