COVID-19 Booster effectiveness declines but remains strong

(New York) – An early look at the performance of COVID-19 booster shots during the recent Omicron wave in the US indicated a decline in effectiveness, although the shots still offered strong protection against serious illness.

The report, published Friday by the Centers for Disease Control and Prevention, is considered an early and limited look at the durability of booster protection during the Omicron wave that exploded in December and January but has been fading in recent weeks.

“COVID-19 vaccine boosters remain safe and remain highly effective against serious illness over time,” said Kristen Nordlund, a CDC spokesperson.
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The researchers looked at patient visits to hospitals and urgent care centers in 10 states. They estimated how well Pfizer or Moderna booster injections prevented COVID-related visits to emergency rooms and urgent care centers, and how well the vaccines prevented hospitalizations.

Read more: There’s good reason to be optimistic about Omicron

About 10% of the people in the study received a boost. The effectiveness of the vaccine was higher in people who had received boosters than in people who had only received the original series of injections.

But researchers also found that during the time when the Omicron variant was predominant, the vaccine’s efficacy against outpatient visits was 87% in people who had been boosted two months earlier, but 66% four months after that. The effectiveness of vaccines against hospitalization fell from 91% at two months to 78% by the fourth month.

However, those results were based on only a small number of patients — fewer than 200 — who had received a boost four months earlier at the time of the Omicron wave. And it’s unclear whether those people had received boosters early for medical reasons that might have made them more vulnerable to serious illness.

The effectiveness after a booster was higher last year, when the Delta variant caused the most U.S. cases, the study noted.

Health experts expect that protection against the vaccines will decrease. The US booster campaign was based on evidence emerging last year that vaccine protection declined six months after people received their first vaccinations.

And from the start, vaccines offer less protection against the Omicron mutant than previous versions of the virus. The study could not elaborate on how protection it will hold against the next variant coming.

Still, the finding of the new study was remarkable, said Dr. William Schaffner, a vaccine expert from Vanderbilt University.

“According to the data, I’m a little surprised that it’s already starting to decline,” he said, adding that he had expected higher estimates of vaccine effectiveness by the four months after the booster.

But Schaffner also said he’d like to see more research on booster protection’s durability, adding, “let’s take this with a grain of salt.”

dr. Michael Saag, an infectious disease physician at the University of Alabama at Birmingham, said 78% effectiveness against hospitalization is “still quite effective.”

“Anecdotally, I see very few people dying who got a boost,” he said, even among those with weakened immune systems. “The vaccines still work.”

AP medical writer Lauran Neergaard in Washington contributed to this report.

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