TALLAHASSEE (CBSMiami/NSF) – A federal Centers for Disease Control and Prevention study released Wednesday shows the Pfizer and Moderna vaccines have become less effective in keeping nursing-home residents safe as the highly contagious delta variant of the coronavirus has spread and that booster shots might be needed.

The report came as the White House announced that booster shots will be made available to the public Sept. 20.

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The latest edition of the CDC publication “Morbidity and Mortality Weekly” shows that from March 1 to May 9, two doses of the Pfizer or Moderna vaccines were nearly 75 percent effective in keeping nursing home residents safe from COVID-19 infections. For the period from June 21 to Aug. 1, as the delta variant spread, the effectiveness of two doses of the vaccines dropped to 53.1 percent.

“Because nursing home residents might remain at some risk for (COVID-19) infection despite vaccination, multiple COVID-19 prevention strategies, including infection control, testing and vaccination of nursing home staff members, residents, and visitors are critical,” scientists wrote in the report. “An additional dose of COVID-19 vaccine might be considered for nursing-home and long-term care facility residents to optimize a protective immune response.”

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In reaching their conclusions, scientists reviewed tens of thousands of reports submitted by nursing homes. They found that the Pfizer and Moderna vaccines became less effective over time and less effective as the delta variant became the dominant strain of the virus.

For instance, the two vaccines proved to be 74.7 percent effective in the period from March 1 to May 9; 67.5 percent effective from May 10 to June 20; and 53.1 percent from June 21 to Aug. 1. But scientists said they could not differentiate the impact of the delta variant on the effectiveness of the vaccines from other potential factors, including the dwindling of vaccine effectiveness over time.

“Further research on the possible impact of both factors on (vaccine effectiveness) among nursing home residents is warranted,” the study’s authors wrote.

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CBSMiami.com Team