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Booster shots of the Pfizer/BioNTech and Moderna COVID-19 vaccines lost some effectiveness after four months but still did a good job of keeping people out of the hospital during the Omicron surge, a study shows.
During the time when the Omicron variant dominated, the vaccines provided 87% effectiveness against emergency room visits and 91% effectiveness against hospitalizations two months after the booster, the study showed. Four months after the booster shot, effectiveness dropped to 66% against ER visits and 78% against hospitalizations.
The study published in the CDC’s Morbidity and Mortality Weekly Report looked at 241,204 emergency department visits and 93,408 hospitalizations in 10 states from August 2021 to Jan. 22, 2022. The CDC said about 10% of the people were boosted and more than half the people hospitalized were over 65.
The study was no surprise because previous research showed vaccine and booster effectiveness wanes over time, but it appears the booster effectiveness against the Delta variant was stronger than against Omicron, the CDC said. The highly transmissible Omicron variant now accounts for almost 100% of COVID cases in the United States.
The findings about the period when Omicron dominated were based on a small sample of fewer than 200 patients who’d gotten the booster at least four months earlier.
Overall, the study provided more proof that vaccines work and keep people out of the hospital, said Michael Saag, an infectious disease doctor at the University of Alabama at Birmingham.
“Anecdotally, I’m seeing very few people die who got boosted,” he told The Associated Press. “The vaccines are still working.”
In a separate report on Friday, the CDC changed its guidance on boosting for people with weakened immune systems. Those people should get boosted three months after completing the primary series of Pfizer or Moderna vaccines, not five months, the agency said.
About 7 million American adults are considered immunocompromised, Kaiser Health News has reported, including people who have certain medical conditions that impair their immune response or who take immune-suppressing drugs due to organ transplants, cancer, or autoimmune diseases.
The CDC recommended fourth shots for immunocompromised people in October.
The CDC also changed its guidance for immunocompromised people who received the one-dose Johnson & Johnson vaccine, saying they should get a second dose after 28 days, then get a booster of one of the mRNA vaccines.
Further, the CDC said people don’t need to delay COVID-19 vaccination after receiving monoclonal antibodies or convalescent plasma.
CDC: “Waning 2-Dose and 3-Dose Effectiveness of mRNA Vaccines Against COVID-19–Associated Emergency Department and Urgent Care Encounters and Hospitalizations Among Adults During Periods of Delta and Omicron Variant Predominance — VISION Network, 10 States, August 2021–January 2022”
The Associated Press: “Study: COVID booster effectiveness wanes but remains strong”
Kaiser Health News: “CDC Tells Pharmacies to Give 4th Covid Shots to Immunocompromised Patients”
CDC: “Interim Clinical Considerations for Use of COVID-19 Vaccines Currently Approved or Authorized in the United States