Updated COVID-19 booster shots are a step closer to being available in the United States after Pfizer and Moderna applied this week for emergency use authorizations.

Both of the shots are bivalent, targeting the original 2019 strain of the virus as well as the BA.4 and BA.5 omicron subvariants, which today account for 99.5% of COVID-19 infections in the country.

Pfizer, which partners with the German company BioNTech on its vaccine, announced Monday it submitted its application to the Food and Drug Administration, and Moderna said Tuesday it has applied. If authorized, Pfizer’s booster would be available to people 12 years of age and older, while Americans 18 and older would be eligible for Moderna’s.

The FDA and Centers for Disease Control and Prevention, both of which must review and sign off on the shots, did not respond to emails Wednesday with questions about the approval timeline. But last week, Dr. Ashish Jha, the White House’s COVID-19 response coordinator, said during a speech at a U.S. Chamber of Commerce Foundation event that the updated vaccines should be available by early to mid-September.

NBC News, citing two people familiar with the discussions, reported Tuesday that the FDA plans to authorize the new shots around Labor Day. The agency does not plan to convene its advisory panel on vaccines before making a decision, one person told the network. 

The CDC’s Advisory Committee on Immunization Practices, which votes on whether the agency should issue recommendations, has meetings scheduled for Sept. 1 and 2. CDC Director Dr. Rochelle Walensky will make the final call for the CDC on the shots, which could come within hours of a vote by the advisers.

The Wall Street Journal reported Tuesday that the Biden administration has completed its plans for a fall booster campaign and that the new boosters would be shipped immediately. The administration has told states, pharmacies and other entities they can preorder the shots through  the end of August.

The federal government has agreed to purchase 105 million doses of Pfizer’s updated vaccine and 66 million of Moderna’s, with options to buy more at a later time.

According to CDC data, about two-thirds of Americans have received their primary series of COVID-19 vaccine, but less than half of those have been given at least one booster, indicating the Biden administration may face challenges convincing many Americans to get the new shots.

Vaccine effectiveness wanes over time. The CDC recommends two boosters for most Americans age 50 and older, and one booster for most people 5 to 49.

If authorized, the updated shots would be only available as boosters. People receiving their primary series will continue to be given the original version of the vaccines, according to the CDC’s fall vaccination planning guide.

Earlier this year, Pfizer and Moderna approached the FDA with bivalent boosters that targeted the original version of the virus as well as the first strain of the omicron variant, BA.1. However, the FDA in June directed the vaccine manufacturers to change their designs to start targeting BA.4 and BA.5. 

The companies submitted clinical data to the FDA for the BA.1 version of the vaccine, but the agency is not requiring them to do so again for the BA.4/BA.5 version, Dr. Peter Marks, head of the FDA’s Center for Biologics Evaluation and Research, told Reuters last month. Instead, Pfizer and Moderna have submitted the clinical data for the BA.1 bivalent vaccines and preclinical data from animal studies on the BA.4/BA.5  shots in their latest applications.

Pfizer and Moderna both say clinical trials for their BA.1-targeting vaccines showed superior antibody responses compared to current booster shots. Moderna said their clinical trial also found that the BA.1 booster generated potent neutralizing antibodies against BA.4 and BA.5, which should theoretically be further enhanced by the updated shots. Pfizer said its preclinical data on their BA.4/BA-5-targeting vaccine showed a strong neutralizing antibody response.

“Given the ongoing evolution of SARS-CoV-2 (the virus that causes COVID-19) and its variants, it's of great importance that vaccines can be rapidly adapted to the major circulating Omicron lineages,” Moderna CEO Dr. Ugur Sahin said in a statement Monday. “In less than three months after the FDA provided its guidance for adapted vaccines in the U.S., we are ready to ship the first doses of our Omicron BA.4/BA.5-adapted bivalent vaccine, pending regulatory authorization, to provide people in the U.S. with the possibility to get a booster adapted to the currently most dominant strain of the virus.”

Said Moderna CEO Stéphane Bancel: “We have worked closely with the FDA to ensure that Americans will have access to Moderna's updated, bivalent booster, which, if authorized, may offer higher, broader, and more durable protection against COVID-19 compared to the currently authorized booster.”

The drugmakers did not say in their announcements this week if they expect the updated boosters to reduce the number of breakthrough infections in vaccinated people, although health experts believe they will continue to offer strong protection against hospitalization and death.

The U.S. is reporting nearly 90,000 COVID-19 infections a day. While that is believed to be a vast undercount — due to unreported at-home tests and fewer health agencies and states providing updates — it is down 31% since July 16. The seven-day average for COVID-19 deaths is 387.