◎ The top box shows an outbreak in a community in which a few people are infected (red) and the rest are healthy but not immunized (blue); the illness spreads freely through the population.
◎ The middle box shows a population where a small number have been immunized (yellow); those not immunized become infected while those immunized do not.
◎ In the bottom box, a large proportion of the population have been immunized; this prevents the illness from spreading significantly, including to non-immunized people. In the first two examples, most healthy non-immunized people become infected, whereas in the bottom example, only a small fraction of the healthy non-immunized people become infected.
Natural herd immunity – achieved through infection rather than vaccination – can be challenging to acquire, especially for infections that are highly contagious. As we have seen with Covid-19, there would be a sudden and overwhelming occurrence of illness that could potentially overload health care systems beyond their capacities. This is why herd immunity is now usually achieved through organized vaccination programs.
Do Vaccinations Work?
Then and Now: Herd immunity relies on the many getting vaccinated. If too many people stopped getting vaccinated, herd immunity would weaken, putting the population at risk for outbreaks of infection.
Here is the current Canadian vaccination schedule: