What is herd immunity?

Vaccination has two roles. It protects individuals against infections and it provides herd immunity. Herd or population immunity is the resistance of a group of people to infection.

Individuals that have been vaccinated are less likely to be a source of infection. This protects the small number of people who are unable to be vaccinated (because they are too young, have health problems or are pregnant), since there are not enough susceptible people to allow the disease to spread.

Herd immunity thresholds (the percentage of the population that needs to be immune) are quite high. Polio is 80–86%, diphtheria is 85%; and for the most infectious diseases like measles, it is around 95%. Herd immunity will only work for diseases that are transmitted between people and not for diseases that can be caught from animals. 

Why does herd immunity matter to microbiology? 

Herd immunity is very important, as some people within a community cannot be immunized, e.g those who are immunocompromised. Herd immunity only applies to diseases that are caught from other people. 

You might think that you need to vaccinate the whole population, however this is not the case. Herd immunity means that as long as a certain proportion of the population is vaccinated, the disease won't be able to transmit in that population.

Herd immunity is especially useful for members of the population that cannot be vaccinated. This can be those that are too old, or too young, or those who have weak immune systems. If they are surrounded by people who have the vaccine, this can protect them from infection. If however, people forgo vaccination, the herd can no longer protect these people.

Vaccines are an incredibly powerful public health tool and the best way to protect populations against disease is to make sure as many people are vaccinated as possible, so we don't run the risk of slipping below the vaccine threshold for herd immunity.