Efficacy of booster dose against Covid drops in fourth month, study shows

The effectiveness of third doses of Pfizer and Moderna mRNA vaccines declines dramatically after the fourth month of administration, according to a new study from the US Centers for Disease Control and Prevention (CDC).

This effect was well known after administration of two doses of these vaccines, but little studied after the third.

The study examined 93,000 hospitalizations and more than 240,000 emergency room visits related to Covid-19 in 10 US states. It was carried out between August 2021 and January 2022 and covers the waves of the delta and omicron variants.

During both periods, the effectiveness measured after a third dose was always greater than after two doses, noted the CDC, which published this study.

Once the omicron became the dominant variant, the effectiveness against hospitalizations was 91% for those who received their third dose two months before contamination, but dropped to 78% for people who received it at least four months before.

A percentage that “remains high,” according to the CDC.

After the appearance of the omicron, the effectiveness of booster shots against emergency room visits was 87% over the next two months, 66% after four months, and only 31% five months later.

The CDC notes, however, that this last number is “inaccurate” due to the low number of study participants who received their third dose more than five months ago.

Overall, these results “reinforce the importance of further consideration of additional doses to maintain or enhance protection against Covid-19,” the CDC wrote.

During a press conference on Wednesday (9), Anthony Fauci, White House adviser on the health crisis, estimated that vulnerable people, such as the elderly or immunosuppressed, may need a fourth dose in the future.