The CDC published new information Friday about the performance of COVID-19 booster shots.
In a new study, researchers found that after receiving a third dose of the vaccine, protection wanes over time.
When the omicron variant was dominant, effectiveness decreased as soon as four months after the third dose.
However, protection from serious illness was still high after the third dose.
In the study, effectiveness was higher among people who had received the booster shot, than among people who only received two doses of the vaccine.
Researchers also noted that effectiveness after a booster was higher when delta was the dominant strain.
Health experts expect protection from the vaccines to wane.
Previous evidence had already suggested that vaccines offered less protection against the omicron variant in comparison to earlier versions of the virus.
The CDC still recommends people to remain up to date with COVID-19 vaccinations to protect themselves against the virus.