The Delta variant, the highly transmissible version of the virus that causes COVID-19, currently makes up almost all new cases in the United States. Some of those infections have been reported in fully vaccinated people as breakthrough cases. However, this does not mean that the vaccine does not work. Unvaccinated people are still much more likely to get COVID and more likely to have symptoms. Most importantly, vaccines greatly reduce the risk of hospitalization and death from the virus.

Comparing vaccinated with unvaccinated people

Think of two similar groups of people: in Group A, only 20% are vaccinated, and in Group B, 95% have received both shots. If every person in both groups is exposed to the Delta variant, what is likely to happen?

In Group B (vaccinated people), the vaccine protects most people against becoming infected with the Delta variant. But some breakthrough infections would be expected, although most would be mild or without symptoms. In Group A, most of the unvaccinated people would become infected with the virus, apart from some people who previously had COVID. However, infections among the vaccinated people in Group A would outnumber those among the unvaccinated in Group B simply because there were many more vaccinated people to start with.

Vaccines reduce the impact on hospitals

In addition, the outcomes are far worse for people in Group A, with more of them becoming seriously unwell. More people in Group A will catch the virus, and more of them will become visibly ill needing to be admitted to hospital or dying. In contrast, in Group B (highly vaccinated group), some people will get the infection, but it will be much milder. This is because Delta may cause some breakthrough infections, but the vaccines are very protective against hospitalization. Therefore, because of Group A, more severely ill people will cause an overcrowded hospital system.

A recent study also showed that those who are fully vaccinated may carry the virus, and therefore be contagious. However, this is for fewer days than those who are unvaccinated. That suggests an even bigger difference in transmission between places with high and low vaccination rates.


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