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It remains unclear when public health agencies in the U.S. might recommend a SARS-CoV-2 booster dose for at least some vulnerable groups of fully vaccinated Americans (e.g.,immunocompromised groups or adults over 65 years old). There are currently three vaccines authorized for emergency use in the U.S.: the two-dose Pfizer-BioNTech vaccine, the two-dose Moderna vaccine, and the single-dose Janssen vaccine.

On 8 July 2021 Pfizer and BioNTech released a statement saying they “believe that a third dose may be beneficial to maintain the highest levels of protection” — they propose that this third dose would either be another dose of the original BNT162b2 vaccine or an updated version that targets the Delta variant’s spike protein. Later on 8 July 2021, the U.S. CDC and FDA issued a joint statement saying “Americans who have been fully vaccinated do not need a booster shot at this time.” In its 28 July 2021 second quarter results, Pfizer states that “newly disclosed data demonstrates that a third dose [of the original BNT162b2 vaccine] elicits neutralizing titers against the Delta (B.1.617.2) variant that are more than five times higher in younger people and more than 11 times higher in older people than after two doses.”

Moderna co-founder Derrick Rossi has also suggested that a booster will “almost certainly” be needed. There also remains ongoing discussion as to whether recipients of the single-dose Janssen vaccine will need a second dose to serve as a booster.