The effectiveness of the booster decreases after 4 months, research shows

February 14, 2022

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 peak, a study shows.

During the time when the Omicron variant dominated, the vaccines were 87% effective against emergency room visits and 91% effectiveness against hospitalizations two months after the booster, the study found. Four months after the booster injection, 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 room visits and 93,408 hospitalizations in 10 states from August 2021 to January 22, 2022. The CDC said about 10% of people received a boost and more than half of the people hospitalized were over 65 years of age.

The study came as no surprise because previous research showed that the effectiveness of vaccines and boosters decreases over time, but it appears that the booster’s effectiveness against the Delta variant was stronger than against Omicron, the CDC said. The highly transmissible Omicron variant is now responsible for nearly 100% of COVID cases in the United States.

The findings over the period when Omicron dominated were based on a small sample of fewer than 200 patients who had received the booster at least four months previously.

Overall, the study provided more evidence that vaccines work and keep people out of hospital, said Michael Saag, an infectious disease physician at the University of Alabama at Birmingham.

“Anecdotally, I see very few people die who have been boosted,” he told The Associated Press. “The vaccines still work.”

In a separate report on Friday, the CDC changed its guidelines for boosting people with weakened immune systems. Those people should receive a boost 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 reports, including people with certain medical conditions that impair their immune response or who are taking immune-suppressing drugs as a result of organ transplants, cancer or autoimmune diseases.

The CDC recommended a fourth injection for immunocompromised people in October.

The CDC also changed its guidelines for immunocompromised people receiving the Johnson & Johnson single-dose vaccine, saying they should receive a second dose after 28 days and then a booster of one of the mRNA vaccines.

Further, the CDC said people should not delay COVID-19 vaccination after receiving monoclonal antibodies or convalescent plasma.

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