Effectiveness of anti-COVID vaccine booster decreases after four months, says study

In a first analysis of the performance of the vaccines reinforcement against COVID-19 during the recent wave of the omicron variant in the United States, a decrease in its effectiveness has been detected, although they continue to offer strong protection against a serious illness.

This study, published by the Centers for Disease Control and Prevention (CDC), is considered a preliminary and limited look at the durability of brace protection during the omicron wave that broke out in December and January, but that has been attenuating in recent weeks.

“Over time, COVID-19 vaccine boosters remain safe and highly effective against serious illness,” said CDC spokeswoman Kristen Nordlund.

The researchers focused on patient visits to hospitals and urgent care centers in 10 states. They estimated the extent to which booster shots from Pfizer or Moderna prevented COVID-19-related visits to emergency rooms and urgent care centers, and to what extent vaccines prevented hospitalizations.

About 10% of the people who participated in the study received boosters.

Vaccine effectiveness was higher in people who had received boosters than in those who had only received the original series of vaccines.

But the researchers also found that during the time that the omicron variant has been predominant, the vaccine’s efficacy against outpatient visits was 87% in people who had received a booster two months earlier, but 66% at four months. Regarding the efficacy of the vaccine against hospitalization, it went from 91% at two months to 78% at the fourth month.

However, these results are based on a small number of patients—less than 200—who had been vaccinated four months earlier than the time of the omicron wave. It’s also unclear whether those people had received boosters earlier for medical reasons that might have made them more vulnerable to severe illness.

The effectiveness of boosters was higher last year, when the delta variant caused the majority of cases in the United States, the study found.

Health experts expect the protection from vaccines to decline. The booster campaign in the United States was based on evidence that emerged last year that vaccine protection was wearing off six months after people received their initial shots.

And from the beginning, vaccines have offered less protection against the omicron variant than earlier versions of the virus.