By Reuters Staff

(Reuters) – The United States is preparing for the possibility that a booster shot will be needed between nine to 12 months after people are initially vaccinated against COVID-19, a White House official said on Thursday.

While the duration of immunity after vaccination is being studied, booster vaccines could be needed, David Kessler, chief science officer for President Joe Biden’s COVID-19 response task force told a congressional committee meeting.

“The current thinking is those who are more vulnerable will have to go first,” Kessler said.

Initial data has shown that vaccines from Moderna Inc and partners Pfizer Inc and BioNTech SE retain most of their effectiveness for at least six months.

Even if that protection lasts far longer, experts have said that rapidly spreading variants of the coronavirus and others that may emerge could lead to the need for regular booster shots – such as with annual flu shots.

The United States is also tracking infections in people who have been fully vaccinated, Rochelle Walensky, director of the U.S. Centers of Disease Control and Prevention told the House subcommittee hearing.

Of 77 million people vaccinated in the United States, there have been 5,800 such breakthrough infections, Walensky said, including 396 people who required hospitalization and 74 who died.

Walensky said some of these infections have occurred because the vaccinated person did not mount a strong immune response. But the concern is that in some cases, they are occurring in people infected by more contagious virus variants.