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Herd immunity and its limitations

Herd immunity, also called community immunity, represents the threshold at which enough individuals in the population are immunized against a certain disease to prevent outbreaks and indirectly protect those who can’t be vaccinated.  In theory, this is a simple concept, but it is much more complicated in practice.

Herd immunity depends on how easily transmitted the disease is and how long immunity lasts. For example, measles and stomach flu are both highly contagious and require around 95% of the population to have immunity to prevent outbreaks. However, while measle immunity via vaccine or infection lasts for life, immunity from stomach flu infection only lasts a couple of months. Hence, reaching herd immunity for stomach flu is practically impossible.

Herd immunity is amazing. But there is a problem. If too many people start thinking “well, everyone else is vaccinated, so I don’t need to be,” then achieving community immunity becomes much more difficult.

Even when herd immunity is reached among a population, it does not immunize the unvaccinated. Pockets of unvaccinated individuals can still lead to outbreaks (i.e., the U.S. polio outbreak of 2022).

Herd immunity can theoretically be achieved through natural or vaccine immunity. However, vaccination remains the safest and most direct way to protect yourself, and the most vulnerable, against severe outcomes.

Vaccinated individuals can still pass the pathogen, but the likelihood/risk of transmission is much less probable (i.e. shorter time sick, less severe symptoms, less contact with care workers all of which reduce overall transmission).

Remember…herd immunity only provides indirect protection. As long as the disease exists, transmission is possible. Vaccination offers safer protection against disease. Herd immunity only works if enough people get vaccinated.

This information was made possible by ScienceUpFirst, a national initiative that works with a collective of independent scientists, researchers, health care experts and science communicators.


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