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Booster shots lose their bite four months in, CDC warns

Three shots provide strong protection against hospitalization, but a new study says immunity from Covid-19 begins to wane four months out from the booster.

WASHINGTON (CN) — The Centers for Disease Control and Prevention released a study Friday that shows Covid-19 booster shots begin to become less effective against the omicron variant after about four months but still provide substantial protection against hospitalization.

Researchers found that the Pfizer-BioNTech and Moderna booster shots are highly effective against serious illness from the virus for around two months, but protection wanes substantially four months out from someone's third shot.

Booster shots were 91% effective at preventing hospitalizations for two months, according to the study, but dropped to 78% efficacy after four months.

Protection against emergency room and urgent care visits dropped at a faster rate, from 87% for the first two months after being boosted to 66% four months out from a shot. This hints that additional versions of the booster shot may be needed down the line as the pandemic rages on, though the study does not clarify the severity of the symptoms that led to urgent care visits or how immunity differed by age group.

"There may be the need for yet again another boost — in this case, a fourth-dose boost for an individual receiving the mRNA — that could be based on age, as well as underlying conditions," Dr. Anthony Fauci, chief medical advisor to the president, said during a news briefing Wednesday.

Similar results showing waning protection months out from a third dose have also been reported in a study from Israel, according to he CDC's report.

Booster shots have become a critical tool in the U.S. fight against the omicron variant, which has shown higher resistance to a two-vaccine regimen than previous versions of the virus.

Two other CDC studies released late last month found that a third shot or "booster" brought protection from the virus back up to pre-omicron levels, with today's study for the first time using U.S. data to show how long that protection may last.

Health officials have urged Americans to get boosted in recent months since the emergence of the omicron variant. Almost 91 million Americans have gotten boosted, but the majority of Americans eligible for a booster have not yet gotten the shot.

"Covid-19 vaccines, booster shots can keep you out of the hospital and certainly can save your life," Fauci said Wednesday.

While Covid-19 cases are falling in most states, more than 900,000 Americans have died from the virus and health officials have warned the presence of Covid-19 in the U.S. and around the world is not yet over.

"I know there will come a time when we move from a phase of crisis to a point where Covid-19 is not disrupting our daily lives," Rochelle Walensky, director of the CDC, said Wednesday. "And as we all look forward to this next step, I want to instill in everyone that moving forward from this pandemic will be a process that’s led by our surveillance and our data."

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