• The FDA authorized booster shots that target the omicron BA.5 variant.
• It is the first time the FDA has authorized an updated vaccine formula since the shots started in December 2020.
• The new boosters are expected to roll out to the public after Labor Day weekend.
• The Centers for Disease Control and Prevention has to sign off before pharmacies can start administering them.

The Food and Drug Administration authorized Covid booster shots Wednesday that target the omicron BA.5 subvariant as the U.S. prepares for another surge of infections this fall and winter.

It is the first time the FDA has authorized an updated vaccine formula since the original shots rolled out in December 2020. Pharmacies are expected to start administering the new boosters after Labor Day weekend.

The U.S. has secured 171 million doses of Pfizer's and Moderna's updated shots so far, according to the Health and Human Services Department.

Pfizer's new booster dose is authorized for people ages 12 and older, while Moderna's new shots are authorized for adults ages 18 and older. The eligible age groups can receive the boosters two months after completing their primary series or their most recent booster with the old shots.

The U.S. will no longer use the original vaccines as booster doses for individuals ages 12 and older now that the the updated shots have been authorized, according to the FDA.

The Centers for Disease Control and Prevention has to sign off on the boosters before pharmacies can give them to patients. The CDC's independent advisory committee is scheduled to meet Thursday and Friday to review the data and issue its recommendations for health-care providers.

Bivalent vaccines

Public health officials believe the redesigned boosters will provide longer-lasting protection against the virus and reduce hospitalizations this fall and winter. The new boosters target both the original strain that emerged in China more than two years ago, which scientists refer to as the "wild type," and omicron BA.4 and BA.5, which are now the dominant variants in the U.S.

Shots that target two different strains are called bivalent vaccines.

The vaccine makers developed the original shots against the strain of Covid that first emerged in Wuhan, China, in 2019. But the virus has mutated dramatically since then. Omicron and its subvariants have drifted so much from the original Covid strain that the virus is able to slip past the protective antibodies induced by the vaccines.

As a consequence, the shots' effectiveness at preventing infection and mild illness has declined substantially as the virus has evolved. Though the vaccines are still generally preventing severe disease, the protection they provide against hospitalization has slipped over time as well.

"There is declining effectiveness against hospitalization and severe illness. The problem has been persuading the American people to get boosted on a regular basis," said Dr. Peter Hotez, an infectious disease expert at the Baylor College of Medicine in Texas. Hotez led a team that developed a Covid vaccine based on protein technology that is authorized in India.

Original vaccines losing effectiveness

About 76% of people ages 12 and older have received their first two vaccine doses in the U.S., according to CDC data. About 50% of those individuals have received their first booster dose.

For adults ages 18 and older, three doses of Pfizer's or Moderna's original vaccines were 55% effective at preventing hospitalization from the omicron BA.2 subvariant four months after the third shot, according to CDC data.

Three shots were 19% effective at preventing infection from omicron five months after the third shot, according to CDC data from August 2021 through May 2022. The rapidly spreading BA.4 and BA.5 subvariants have since driven omicron BA.2 out of circulation.

Dr. Peter Marks, head of the FDA office responsible for reviewing vaccines, said the hope is that the updated boosters will restore the high level of protection against disease that vaccines demonstrated when they were first authorized in December 2020.

"We don't know for a fact yet whether we will get to that same level, but that is the goal here. And that is what we believe the evidence that we've seen helps point to," Marks told reporters during a news conference after the authorization Wednesday.

The Biden administration moved rapidly over the summer to get updated shots ready for the fall. Public health officials are worried the U.S. is on the verge of another wave of infection as more transmissible omicron variants spread, immunity from the original vaccines wears off, and people head indoors to escape colder weather.

Pfizer and Moderna were originally developing boosters to target omicron BA.1, the variant that caused the massive wave of infection last winter. But the FDA told the vaccine makers in late June to switch gears and target BA.4 and BA.5 instead as those variants quickly gained ground. The sudden change in plans left little time for clinical trials in humans before a fall rollout.

As a consequence, the authorization is based on human clinical trials from the BA.1 shots, which produced a better immune response than the original doses, according to the FDA. But it's unclear how the BA.5 boosters will perform in humans since the data is based on BA.1.

Marks said it will likely be at least another two months before human clinical data on the BA.5 shots is made available to the public.

The most common side effects from the human trials of the BA.1 shots was pain, redness, swelling at the injection site, fatigue, headaches, muscle pain, joint pain, chills, nausea, vomiting and fever, according to the FDA. The Covid vaccines also have a well-established safety profile after administration to millions of people over the course of the pandemic, the FDA said.

Mouse data

In addition to human data from the BA.1 shots, the authorization was also based on animal studies from the BA.5 boosters, Marks said. In June, Pfizer also presented data to the FDA's independent vaccine advisory committee that showed the bivalent omicron BA.5 shots increased antibodies in mice that protect against infection by about 2.6-fold compared with the original vaccine.

Marks said the FDA used the same process for the authorization that it relied on in the past for switching the strains in flu vaccines.

"We're pretty confident that or what we have is very similar to the situation that we've done in the past with influenza changes where we don't do clinical studies for them in the United States," Marks said. "We know from the way the vaccine works, and from the data that we have, that we can predict how well the vaccine will be working."

But some infectious disease and vaccine experts say the FDA should have waited for human data from the BA.5 shots before authorizing them. Dr. Paul Offit, a member of the FDA's advisory committee, said data based on mice studies is not sufficient to justify authorizing the new boosters.

"You have to show some evidence in people that the immune response that you're getting with the bivalent vaccine is clearly better, and those data haven't been presented," said Offit, an infectious disease and vaccine expert at Children's Hospital of Philadelphia.

Human trials

"You can't ask millions of people to get this booster dose without showing some human data that you have a dramatic increase in neutralizing antibodies to the BA.4/BA.5 strains as compared to boosting with the ancestral type," Offit said, referring to the currently authorized shots based on the version of Covid that emerged in China, more than two years ago.

Michael Osterholm, a leading epidemiologist and director of the Center for Infectious Disease Research and Policy at the University of Minnesota, also said more data needs to be presented on how the BA.5 shots perform in humans.

"It's not that I don't think it could work," Osterholm said. "But I think we need the data first to show that the immune response to this vaccine is equivalent to or better than what we have already."

But CDC Director Dr. Rochelle Walensky, in a radio interview, said waiting longer for human data from the BA.5 shots could mean the boosters become outdated if a new variant emerges. Walensky said the change in the vaccine formula is small and should not affect safety.

"There's always a question here of being too slow versus too fast," Walensky told "Conversations on Health Care" in a radio interview. "One of the challenges is if we wait for those data to emerge in human data [...] we will be using what I would consider to be a potentially outdated vaccine."