herd immunity

What is Herd Immunity?

Herd immunity happens when a population is immune to an infectious contagious disease either through vaccination or immunity developed through previous infection.The immunity acquired in this case is indirect.

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This means that even people who aren’t vaccinated are protected because people around them who are immune can act as buffers between them and an infected person.The vaccinated/immune individuals are not infected by the diseased ones and hence, the chain of transmission is broken.This protects even the unvaccinated individuals with compromised immune conditions or chronic ailments.

Once herd immunity has been established and the ability of the disease to spread is blocked(as the chain of transmission is broken, explained above), the disease can eventually be eliminated. This is how the world eradicated smallpox.

Red-Infected; Orange-Unvaccinated; Green-Vaccinated

‘Herd Immunity’ can be achieved  in two ways:

  1. Many people contract the disease and in time build up an immune response to it (natural immunity).
  2. Many people are vaccinated against the disease to achieve immunity.


The more infectious a disease, the greater the population immunity needed to ensure herd immunity. For example, measles is highly contagious and one person with measles can infect up to 18 other people. This means that around 95% of people need to be immune in order for the wider group to have herd immunity.

The new coronavirus has a lower infection rate than measles, with each infected person passing it on to two or three new people, on average. This means that herd immunity should be achieved when around 60% of the population becomes immune to COVID-19.

However, natural herd immunity – achieved through infection rather than vaccination – can be challenging to induce through unchecked infection as there would be a very high rate of serious illness and death, with health systems overwhelmed well beyond their surge capacity, even in high-income countries. This is why herd immunity is generally pursued through vaccination programmes.

Even when vaccines are available, it is not always possible to achieve herd immunity for very long. Some viruses, such as seasonal flu, mutate frequently, evading the body’s immune response. So immunity doesn’t always last forever, which is why the flu shot is necessary every single year.

How It Works?

When a large percentage of the population becomes immune to a disease, the spread of that disease slows down or stops.

Many viral and bacterial infections spread from person to person. This chain is broken when most people don’t get or transmit the infection.

This helps protect people who aren’t vaccinated or who have low functioning immune systems and may develop an infection more easily, such as:

  • older adults
  • babies
  • young children
  • pregnant women
  • people with weakened immune systems
  • people with certain health conditions


Mass vaccination has been highly successful in inducing herd immunity for many diseases, protecting those that are unable to build up immunity, such as people with immune deficiencies or whose immune systems are being suppressed for medical reasons.

When herd immunity is well established, however, some people choose to behave as ‘free riders’, essentially benefitting from everyone else getting vaccinated, while abstaining from vaccination either because they choose not to or are actively anti-vaccination.

When a population has too many of these free riders, the overall immunity level is compromised and herd immunity can be lost, putting everyone at risk.

There are several reasons why herd immunity isn’t the answer to stopping the spread of the new coronavirus:

  1. There isn’t yet a vaccine for SARS-CoV-2. Vaccinations are the safest way to practice herd immunity in a population.
  2. The research for antivirals and other medications to treat COVID-19 is ongoing.
  3. Scientists don’t know if you can contract SARS-CoV-2 and develop COVID-19 more than once.
  4. People who contract SARS-CoV-2 and develop COVID-19 can experience serious side effects. Severe cases can lead to death.
  5. Doctors don’t yet know exactly why some people who contract SARS-CoV-2 develop severe COVID-19, while others do not.
  6. Vulnerable members of society, such as older adults and people with some chronic health conditions, could get very sick if they’re exposed to this virus.
  7. Hospitals and healthcare systems may be overburdened if many people develop COVID-19 at the same time.
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