The new shots from Moderna and Pfizer-BioNTech target both the original strain of the coronavirus and the omicron BA.4/BA.5 subvariants that most people are catching now.
Pfizer and BioNTech are planning to ask the Food and Drug Administration to authorize a second COVID-19 booster shot for people age 65 and older.
"Given the public health emergency, FDA is advising that it is acceptable to use every full dose obtainable (the sixth, or possibly even a seventh) from each vial," the FDA said Wednesday.
As the first doses of the Pfizer-BioNTech vaccine made their way across the country on Monday, hospital employees celebrated the long-awaited medicine.
Pfizer and BioNTech's vaccine is the first to receive an emergency authorization from the Food and Drug Administration. Officials say it may be ready for widespread inoculations within days.
If approved, the vaccine could be distributed before the end of the year, BioNTech says.
The vaccine was found to be 95% effective in an updated study analysis. Safety data required by the Food and Drug Administration showed no serious concerns, the company said.