The effectiveness of the booster dose of vaccines from Pfizer and Moderna declines over time, but remains high against hospitalizations four months after injection, according to a study released Friday by US health authorities.
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This effect is now well known after two doses of vaccines, but has been little studied after the third injection.
The study looked at 93,000 hospitalizations and more than 240,000 COVID-19-related visits to emergency departments in 10 US states.
It was conducted between the end of August 2021 and January 2022, covering both waves related to the Delta and Omicron variants.
During both periods, the effectiveness measured after a third dose was always higher than after two doses, noted the Centers for Disease Prevention and Control (CDC), which publishes this study.
Once Omicron became the dominant variant, efficacy against hospitalizations was 91% for those who received their third dose within two months, but fell to 78% for people who received it four months and more ago.
A percentage that “remains high”, according to the CDC.
Also after the arrival of Omicron, the effectiveness of the booster dose against emergency department visits was 87% within two months, 66% after four months, and only 31% over five months later.
The CDC notes, however, that this latest figure is “imprecise,” due to the small number of people in the study who received their third dose more than five months ago.
Overall, these results “reinforce the importance of additional considerations for additional doses, in order to maintain or improve protection against COVID-19”, write the CDC.
During a press conference on Wednesday, Dr. Anthony Fauci, White House adviser on the health crisis, had estimated that it was possible that vulnerable people, such as the elderly or immunocompromised, in the future will need a fourth dose.