The White House said on Thursday it's preparing for the possibility that a third COVID-19 booster shot might be needed within a year after being vaccinated.
Initial data has shown that the vaccines from Moderna as well as from partners Pfizer and BioNTech stays mostly effective for at least six months.
But for how much longer that that has yet to be determined.
Earlier this month, Pfizer and BioNTech said their vaccine was about 91% effective, and cited trial data including more than 12,000 people who were fully inoculated for at least six months.
CNBC reported Thursday that Pfizer's Chief Executive Albert Bourla also said a third booster dose will 'likely' be needed within a year, and added the possibility of annual shots.
Experts have said that rapidly spreading variants of the coronavirus, as well as others that may emerge could lead to the need for regular booster shots, similar to annual flu shots.
The CDC is also tracking infections in people who have been fully vaccinated.
Its director Rochelle Walensky told Congress that of 77 million people already vaccinated in the United States, there had been 5,800 infections.
That includes almost 400 hospitalized, and 74 who died.
Walensky said some of those infections occurred in people with weak immune responses, while the concern is that some others were infected by more contagious virus variants.