(New York) — An early look at the performance of COVID-19 booster shots during the recent Omicron wave in the U.S. hinted at a decline in effectiveness, though the shots still offered strong protection against severe illness.
Centers for Disease Control and Prevention published the report on Friday. The report is considered a limited and early look at booster protection’s durability during Omicron surge. However, it has started to fade in the recent weeks.
“COVID-19 vaccine boosters remain safe and continue to be highly effective against severe disease over time,” said Kristen Nordlund, a CDC spokesperson.
Researchers looked at 10 state patient visits to urgent care and hospitals. The researchers estimated the effectiveness of Moderna and Pfizer booster shots in preventing COVID-related trips to urgent care and emergency rooms, as well as how effective vaccines were at preventing hospitalizations.
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The study found that about 10% of the participants had received boosters. The vaccine effectiveness of boosters was greater than that in those who received the initial series of shots.
However, researchers found out that Omicron was the predominant variant of vaccine. Outpatient visits were 87% less in those who had received boosters two months before, and only 66% for those who received them four months later. The effectiveness of vaccines against hospitalization decreased from 91% in two months to 78% within the fourth month.
Those results, however, were based on only a small number of patients — fewer than 200 — who had been boosted four months earlier at the time of the Omicron wave. And it’s unclear if those people had gotten boosters early for medical reasons that may have made them more vulnerable to severe illness.
The study found that the effectiveness of a booster was greater last year than it was in the Delta variant, which was responsible for most U.S. cases.
According to experts, vaccine immunity will diminish. A U.S. booster campaign was founded on the evidence last year showing that vaccine protection is fading six to twelve months after initial vaccinations.
Since the very beginning vaccines offered lower protection than older versions against Omicron mutant viruses. The study couldn’t address how protection will hold up against the next variant to come along.
Still, the new study’s finding was notable, said Dr. William Schaffner, a Vanderbilt University vaccines expert.
“I’m a little surprised, according to the data, that it’s starting to wane already,” he said, adding that he would have anticipated higher estimates of vaccine effectiveness at the four-month post-booster mark.
But Schaffner also said he’d like to see more research about the durability of booster protection, adding “let’s take this with a grain of salt.”
Dr. Michael Saag, an infectious diseases physician at the University of Alabama at Birmingham, said 78% effectiveness against hospitalization “is still pretty effective.”
“Anecdotally, I’m seeing very few people die who got boosted,” he said, even among those with weakened immune systems. “The vaccines are still working.”
This report was contributed by Lauran Neergaard, an AP Medical Writer in Washington.