(AP) — Pfizer said Wednesday that a booster of its COVID-19 vaccine may offer important protection against the new omicron variant even though the initial two doses appear significantly less effective.

Pfizer and its partner BioNTech said that while two doses may not be strong enough to prevent infection, lab tests showed a booster increased by 25-fold people’s levels of antibodies capable of fighting off omicron. For people who haven’t yet had a booster, the companies said two doses still should prevent severe disease or death.

Health authorities in the U.S. and other countries have urged eligible people to get a third dose even before these results.

“Go and get your third boost as soon as possible,” Dr. Mikael Dolsten, Pfizer’s chief scientific officer, told The Associated Press. “This is comforting and a very positive message that we now have a plan that will induce immunity that is likely to protect from infection, symptomatic illness and severe disease from now across the entire winter season.”

President Joe Biden said the Pfizer booster finding is “very encouraging” although he cautioned, “that’s the lab report. There’s more studies going on.”

Pfizer and BioNTech tested blood samples taken a month after a booster and found people harbored levels of omicron-neutralizing antibodies that were similar to amounts proven protective against earlier variants after two doses. For the lab tests, researchers grew samples of so-called “pseudoviruses” that hold the worrisome new mutations.

Scientists don’t yet know how big a threat the omicron variant really is. Currently the extra-contagious delta variant is responsible for most of the COVID-19 cases in the U.S. and other countries.

But the omicron variant, discovered late last month, carries an unusually large number of mutations and scientists are racing to learn how easily it spreads, whether it causes illness that is more serious or milder than other coronavirus types — and how much it might evade the protection of prior vaccinations.

Pfizer’s findings, announced in a press release, are preliminary and haven’t yet undergone scientific review. But they’re the first from a vaccine maker examining whether the booster doses that health authorities are urging people to get may indeed make a key difference.

Moderna and Johnson & Johnson also are testing how their vaccines may hold up, but health authorities will be closely watching for real-world evidence of how omicron spreads in highly vaccinated populations.

If it becomes dominant and causes serious illness, then regulators will have to decide whether vaccines should be tweaked to better match omicron — changes to the recipe that manufacturers already are beginning, just in case.

Scientists have speculated that the high jump in antibodies that comes with a third dose of current COVID-19 vaccines might be enough to counter any decrease in effectiveness.

Despite the large number of mutations that omicron bears, “it is still not a complete escape variant, it is a partial escape variant,” BioNTech CEO Ugur Sahin said in a press conference.

Antibody levels predict how well a vaccine may prevent infection with the coronavirus but they are just one layer of the immune system’s defenses. Pfizer said two doses of the vaccine should still protect against severe disease, because omicron’s mutations don’t appear to hamper another defense — T cells that fight the virus after infection sets in.

A small lab study in South Africa also concluded people may be more susceptible to breakthrough omicron infections after just two Pfizer doses. Scientists from the Africa Health Research Institute in Durban found a sharp drop in antibody strength against omicron compared to other variants — although they didn’t test boosters. Pfizer boosters aren’t yet available in South Africa, but health care workers are being offered extra doses of the single-shot J&J vaccine.

The preliminary South African results suggested people vaccinated after an earlier bout of COVID-19 retained more protection — reflecting that initial shots are known to trigger a huge antibody jump after prior infection.

Even if there are more breakthrough infections after two doses, most experts believe vaccines will still work against the omicron variant because of the other immune defenses they trigger, said Willem Hanekom, a co-author of the South African study.

“The more antibodies you can have on board, the better you will do, at least in these lab experiments,” Hanekom said. “So booster vaccines might be very important.”

A U.S. expert agreed the preliminary booster findings are encouraging although more information is needed.

But if omicron winds up causing severe disease and becomes common globally, “it may be much better addressed through vaccines that are adjusted to specifically protect against that variant,” said Dr. Jesse Goodman of Georgetown University, a former Food and Drug Administration vaccine chief.

Vaccine makers already are tweaking their vaccine recipes to create an omicron-specific dose in case it’s needed. Pfizer predicted its candidate could be ready for regulators to consider in March.

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Associated Press journalists Andrew Meldrum and Frank Jordans contributed to this report.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education. The AP is solely responsible for all content.