“The available data make very clear that protection against SARS-CoV-2 infection begins to decrease over time following the initial doses of vaccination, and in association with the dominance of the Delta variant, we are starting to see evidence of reduced protection against mild and moderate disease,” according to the statement signed by CDC Director Dr. Rochelle Walensky, acting FDA Commissioner Dr. Janet Woodcock, White House chief medical advisor Dr. Anthony Fauci and other U.S. health leaders. “Based on our latest assessment, the current protection against severe disease, hospitalization, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout.”

As a result, U.S. agencies are preparing to offer booster shots to all eligible Americans beginning the week of Sept. 20, starting 8 months after their second dose of Pfizer or Moderna’s vaccines, officials said. While they said recipients of Johnson & Johnson’s single-shot vaccine will likely need boosters, they are awaiting more data in the next few weeks before making a formal recommendation.