UNITED STATES (AP)— The COVID-19 vaccines are on track for a big recipe change this fall.

Today’s vaccines still contain the original coronavirus strain, the one that started the pandemic — even though that was long ago supplanted by mutated versions as the virus rapidly evolves.

Thursday, the Food and Drug Administration’s scientific advisers reviewed whether the next round of shots in the US should only include protection against the newest variants that are now dominant worldwide — a branch of the omicron family tree named XBB.

While infections have declined, the virus could be a real concern next winter, FDA’s vaccine chief Dr Peter Marks said as the daylong meeting began.

“We’re concerned that we may have another wave of COVID-19 during a time when the virus has further evolved, immunity of the population has waned further, and we move indoors for wintertime,” he said.

Here are some things to know:

– Why another round of shots?

The FDA told Americans to expect an updated fall vaccine against COVID-19, just like they get a new flu shot every fall. Even though most of the population has either been infected or had at least one round of vaccinations, the coronavirus keeps churning out new varieties.

What’s in use in the US now are combination shots from Pfizer and Moderna that mix the original strain with protection against last year’s most common omicron variants, called BA.4 and BA.5. But just 17 per cent of Americans rolled up their sleeves for a combo booster.

And while the FDA did allow seniors and others at high risk to get an extra booster dose this spring, most people will be many months beyond their last shot by fall.

Those currently available shots do still help prevent severe disease and death even as XBB variants have taken over. But protection gradually wanes over time and was short-lived against milder infection even before the virus, inevitably, evolved again.

– Sharpening protection

Last year when regulators were struggling to decide how to update the vaccine, a combo shot seemed like the safest bet. Omicron was pretty new, and there was no way to know how long it would stick around or if the next big coronavirus change would more resemble the original strain.

Now, “there’s no reason really to keep the original strain in there,” said Dr Rachel Presti, an infectious disease specialist at Washington University in St Louis. She is not on the FDA panel.

Doing so actually has a downside called “imprinting.” After repeated exposure to the original strain, people’s immune systems tended to recognise and respond more strongly to it than to the half of the new booster dose that was brand new.

So the FDA is leaning toward dropping the original strain and picking a fall shot recipe that targets just XBB. The hope is that the XBB variants are different enough to spark the immune system to make more diverse, cross-protective virus-fighting antibodies.

The World Health Organization’s vaccine advisers and European regulators recently made similar recommendations.

– Predicting what will hit this winter

Another challenge: Deciding which XBB variant to target — since what’s spreading now likely will have mutated again by winter.

Regulators will be making their best guess, just like they do every year in setting the recipe for the fall flu vaccine.

The FDA has identified three XBB subtypes as the top strain choices. Vaccine makers already have been developing XBB-targeted formulas, and FDA’s outside advisers on Thursday will review new data on how well those choices rev up the immune system.

– How many mire shots?

The FDA’s advisers will debate that, too. Older adults and others at high risk because of seriously weak immune systems continue to have the highest rates of hospitalisation from COVID-19 even as cases have declined. One question is whether they’ll be urged to get a single fall shot or be eligible for more than one.

Another is how many doses the youngest children who’ve never been vaccinated would need.

Ultimately, the Centers for Disease Control and Prevention will make the final recommendations.