(AP) — COVID vaccine makers are studying updated boosters that might be offered in the fall to better protect people against future coronavirus surges. Moderna has an experimental shot—combining the original with protection against the omicron variant—that appears to work, the company announced Wednesday.

Moderna’s preliminary study results show people given the combination shot experienced an eight-fold increase in virus-fighting antibodies capable of targeting the omicron mutant, the company announced. The drugmaker said that its new study found that, one month after the combo shot, recipients had higher omicron-fighting antibody levels—and cross-protection against other prior variants—than the original vaccine triggers.

However, antibodies naturally wane, so it’s not clear how long the protection against infection will last. The study was performed on 437 people, and safety was similar to today’s boosters, Moderna said. The results were announced in a press release and haven’t undergone scientific review.

Today’s COVID vaccines all are based on the original version of the coronavirus. They’re still providing strong protection against severe disease, hospitalization, and death even after the appearance of the super-contagious omicron variant—especially if people have had a booster dose. But the virus continues to mutate rapidly in ways that let it evade some of the vaccines’ protections and cause milder infections.

That’s why U.S. regulators and the WHO are considering whether to order a change in the vaccine recipe for a new round of booster shots in the fall, when cold weather and kids returning to school are expected to drive yet another surge. But how do they make that change without losing the continued strong protection against COVID’s worst outcomes?

And what’s the right variant to target? After the huge winter omicron surge, that mutant’s genetically distinct siblings now are the main threats, including one that’s fueling the current U.S. wave of infections.

The FDA has set a meeting in late June for its scientific advisers to debate those questions and evaluate data from vaccine makers’ tests of potential new formulas. Pfizer also is studying a combination shot, what scientists call a bivalent vaccine.