(CNN) - Moderna and Pfizer booster shots updated to target Omicron coronavirus subvariants could be available in early fall, pending signoff by federal health agencies, the US Department of Health and Human Services said Friday.
The US Food and Drug Administration advised vaccine makers in June to update the boosters to add an Omicron BA.4/5 component and create a bivalent booster.
"Pfizer and Moderna have indicated that they anticipate the modified vaccines being available as early as September," an FDA spokesperson said.
HHS announced Friday an agreement to purchase 66 million doses of Moderna's bivalent booster shot for potential use in fall and winter.
That's in addition to 105 million bivalent boosters the US government has purchased from Pfizer.
The FDA spokesperson said that agency "has been working closely with vaccine manufacturers over the past several months to ensure that modified COVID-19 vaccines are available this fall to meet evolving public health needs, following FDA's review for safety and effectiveness. The agency will work expeditiously to review submissions to make COVID-19 vaccines available."
Updated shots would need to be authorized by the FDA and recommended by the US Centers for Disease Control and Prevention. Pfizer and Moderna have not responded to requests for comment.
Currently, people 5 and older are eligible for a first booster shot, but only those 50 and older and some immunocompromised people are eligible for second boosters.
Adults 50 and older who have not received a second booster dose with current Covid-19 vaccines "should consider doing so now," the FDA spokesperson said.
"You can still benefit from existing booster options and leave time to receive an updated booster in the fall."
At this time, the FDA is not considering authorizing a second booster for otherwise healthy adults under 50 with the original formulation of the Covid-19 vaccines.
"Once boosters containing a BA.4/5 component become available, individuals may consider getting one following an appropriate interval following their prior dose," the spokesperson said.
In late June, vaccine makers said boosters updated to target Omicron showed a stronger immune response than the current vaccines.
Combined, the US government's agreements with Moderna and Pfizer would make about 171 million bivalent vaccine booster doses available for the fall and winter, should they be authorized and recommended.
But the Biden administration added in its announcement Friday that that would not be enough for every US resident.
Although both agreements include options to purchase a total of 600 million doses -- 300 million from each company -- those options "can only be exercised with additional funding from Congress," the announcement said.
"We look forward to receiving these new variant-specific vaccines and working with state and local healthcare partners to make the vaccines available for free in communities around the country this fall," said HHS Assistant Secretary Dawn O'Connell, who leads the Administration for Strategic Preparedness and Response, in the HHS news release.