Breakthrough cases occur when a person who has been vaccinated against an illness contracts that illness.
Breakthrough cases are not unique to the COVID-19 vaccine and are inevitable, given that no vaccine is 100 percent effective. Given the scope of the pandemic, researchers and public health experts are following these cases extremely closely. COVID vaccines remain highly effective and vaccinated individuals are far less likely than unvaccinated individuals to contract COVID-19.
As more people are vaccinated, chances are greater that we will hear about more breakthrough infections. These cases still represent a very small fraction of the total number of people who are fully vaccinated.
The purpose of a vaccine is to reduce both the chances of getting a disease and, among those who experience breakthrough cases, to reduce the negative outcomes of the disease. In the case of COVID-19, the goals are as follows: 1) reduce transmission all together and 2) if transmission does happen, reduce severity (less severe symptoms, fewer hospitalizations and deaths).
Current evidence suggests that the vaccines authorized by the FDA accomplish these goals. The risk of transmission in fully vaccinated people is very low. The CDC and FDA are monitoring this constantly. If they notice an increase in breakthrough cases above what is statistically expected, it may mean that booster shots need to be considered. At present, the data suggests this is not a concern and booster shots are not needed.
Most important right now is ensuring that anyone who is unvaccinated gets vaccinated as soon as possible to slow transmission in the unvaccinated population. This will also have the effect of reducing the amount of virus circulating among all people, further reducing the chances anyone is exposed to the virus.