It’s official: Both Moderna and Pfizer-BioNTech’s Omicron-specific boosters are (almost) a go.

On Wednesday, the U.S. Food and Drug Administration authorized the retooled shots, which target the BA.4 and BA.5 Omicron subvariants, in anticipation of a fall and winter Covid-19 surge. The boosters still need to be reviewed by the CDC’s vaccine advisory panel, which is scheduled to meet Thursday and Friday to discuss final recommendations. Then CDC director Rochelle Walensky will give the final okay. If all goes well, we could expect to see the new boosters rolled out after Labor Day.

“As we head into fall and begin to spend more time indoors, we strongly encourage anyone who is eligible to consider receiving a booster dose with a bivalent COVID-19 vaccine to provide better protection against currently circulating variants,” FDA Commissioner Robert M. Califf said in a tweet on Wednesday. “The COVID-19 vaccines, including boosters, continue to save countless lives and prevent the most serious outcomes (hospitalization and death) of COVID-19.”

The federal government has already secured 105 million doses of the Pfizer bivalent booster and 66 million doses of Moderna’s equivalent shot, reported Reuters. Additionally, Pfizer said it has doses to ship off immediately and can deliver up to 15 million doses by September 9. Moderna said its Omicron-targeting jab will be available at vaccination sites nationwide in the coming days, reported STAT News.

Since Omicron’s identification in late 2021, vaccine makers have been tinkering with a bespoke shot to provide better and stronger immunological protection against new circulating versions of the virus behind Covid-19, SARS-CoV-2. In June, both Pfizer and Moderna presented clinical trial data to the FDA on BA.1-targeting vaccines. But the advisory panel instead recommended Pfizer and Moderna consider creating a bivalent vaccine against the BA.4 and BA.5 subvariants, futureproofing for any inevitable surge brought on by these strains. BA.5 now dominates globally and accounts for 90 percent of cases in the US, according to the CDC.

It’s important to note for the retooled boosters, authorization is being made based on animal data, which has not been publicly released yet. At the June FDA meeting, Pfizer presented preliminary data that showed when mice were given BA.4/BA.5 as their third dose, the animals showed an increased response to all Omicron variants — BA.1, BA.2, BA.2.12.1, BA.4, and BA.5 — compared to mice that received the original vaccine as a booster, reported Science.

“The mouse data are helpful,” Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, told the Washington Post. “But the reason to have data for humans is to say the immune response is as good or better” as the one triggered by the original vaccine or an alternative.

Other experts like Peter Hotez, co-director of the Center for Vaccine Development at Texas Children’s Hospital, said the move to include new subvariants makes sense.

“In an ideal world, there is an advantage of combining BA.5 with the original lineage because BA.5 is still with us,” Hotez told NBC News. “And if there’s a new variant of concern that emerges later this fall or in the winter, it’s a possibility it’ll look more like BA.5 than the original lineage.”

But Hotez added that while revamped vaccines could provide more effective protection against Omicron sublineages, it’s uncertain without hard and fast data in humans (although mice are a good proxy). Pfizer and Moderna are gearing up for clinical trials in humans next month.

Considering that SARS-CoV-2 is here to stay with some sort of seasonal regularity, authorization without new human data might be a move we will see more of with future boosters, similar to the FDA’s approach with annual flu shots.

“Bivalent and multivalent vaccines are very common and modifying a vaccine to include different virus strains often does not require a change in other ingredients,” Califf tweeted last week. “FDA has extensive experience with reviewing strain changes in vaccines, as is done with the annual flu vaccine.”