According to a study released by US health officials on Friday, the booster dose effect of vaccines from Pfizer and Moderna will diminish over time, but will be greater than hospitalization four months after the injection.
This effect is now well known after two doses of vaccine, but very little has been studied after the third injection.
The study examined 93,000 hospital admissions and more than 240,000 COVID-19 related visits to emergency departments in 10 US states.
It was held between the end of August 2021 and January 2022, covering two waves related to the Delta and Omicron variants.
In both periods, efficacy measured after the third dose is always higher than after two doses, according to the Centers for Disease Prevention and Control (CDC), which publishes this study.
Once Omicron became the dominant variant, efficacy was 91% for those who received their third dose within two months and those who were hospitalized, but dropped to 78% for those who received it four months or more ago.
According to the CDC, the “excess left” percentage.
After the arrival of Omicron, the effectiveness of the booster dose against emergency department visits was only 87% within two months, 66% after four months and 31% after five months.
CDC notes, however, that this latest figure is “obscure”, with a small number of people in the study receiving their third dose five months ago.
Overall, these results “strengthen the importance of additional observations for additional doses to maintain or improve protection against COVID-19,” the CDC wrote.
At a news conference Wednesday, White House Adviser on Health Crisis Dr Anthony Fowzie predicted that the elderly or those who are immunocompromised may need a fourth dose in the future.