Current Covid-19 vaccines are not well-matched against the BA.2 sub-variant of Omicron, the US Food and Drug Administration (FDA) has said.
The comments were made as its panel of outside experts meet to discuss changes to future booster doses.
However, the regulator said booster shots protect against serious outcomes of the virus, compared with the two preliminary doses.
In late March, US health officials authorised a second booster dose of Moderna and Pfizer's vaccines for people aged 50 and older, citing data showing waning immunity and risks posed by Omicron variants of the virus.
"This discussion today is a much larger discussion - it's a discussion for what do we do about the entire population, and what do we do when we think the virus has evolved further," Peter Marks, director of the FDA's Center for Biologics Evaluation and Research, said.
A fourth dose of the Pfizer/BioNTech vaccine lowered rates of Covid among older people, but the protection against infection appeared short-lived, a large study from Israel found on Tuesday.
