Herd immunity

Is when enough people in a community become immune to an infectious disease so that it slows down or stops the disease from spreading. Vaccination is regarded by scientists as the safest way to achieve herd immunity in a population.

If 80% of a population are immune, for example, it means that the 20% who are not immune have a one in five chance of meeting somebody who can infect them. If only 20% are immune, however, susceptible people run a four in five chance of meeting someone who can infect them. The more people become immune, the less chance there is of the infection spreading to others.

There are two ways to achieve herd immunity:

  • natural immunity which is when many people get the disease and in time build up an  immune  response to it or when
  • many people are vaccinated against the disease to achieve immunity.

In the absence of a vaccine against SARS-CoV-2, natural herd immunity was one of the controversial health strategies some countries, most notably Sweden, adopted early on in the COVID-19 pandemic. Sweden, however,  had one of the highest numbers of deaths in Europe, and natural herd immunity appears to be taking longer to build up than the country’s epidemiologists were expecting.

More recently, the United States controversially favoured the natural immunity approach as it allows economies to stay open for longer and means schools and businesses don’t need to be shut down. However, this has raised ethical and moral concerns because many more people are likely to die with this approach as it takes longer for the population to build immunity.


Immune System Infectious Disease Transmission


EpidemiologyImmune systemCOVID-19PandemicSARS-CoV-2Vaccines