US officials on Wednesday formally announced plans to offer COVID-19 vaccine booster shots to Americans—and the plans are already under fire from experts at the World Health Organization.

US officials are recommending that all Americans vaccinated with two doses of an mRNA vaccine (either the Moderna or Pfizer/BioNTech vaccine) get a third booster dose of the same vaccine eight months after receiving their second dose. As such, boosters will be rolled out based on the order in which people were initially offered vaccines, i.e., with frontline health workers, nursing home residents, and other seniors at the front of the line.

US officials are prepared to begin offering booster shots the week of September 20. However, the timing of the boosters is pending authorization from the Food and Drug Administration and the recommendation of the Advisory Committee on Immunization Practices, an independent committee of experts that advises the Centers for Disease Control and Prevention.