U.S. health authorities are facing a critical decision: whether to offer new COVID-19 booster shots this fall that are modified to better match recent changes in the shape-shifting coronavirus. 

Moderna and Pfizer have tested updated shots against the Omicron variant, and advisers to the Food and Drug Administration (FDA) will debate Tuesday if it's time to make a switch – setting the stage for similar moves by other countries.

The Moderna and Pfizer shots still offer strong protection against the worst outcomes – severe illness and death – especially after a booster dose. 

But those vaccines target the original coronavirus strain and between waning immunity and a relentless barrage of variants, protection against infections has dropped markedly. The challenge is deciding if tweaked boosters offer a good chance of blunting another surge when there's no way to predict which mutant will be the main threat.

Moderna and Pfizer found their combo shots substantially boosted levels of Omicron-fighting antibodies in adults who'd already had three vaccinations, more than simply giving another regular dose. 

Recipients also developed antibodies that could fight Omicron's newest relatives named BA.4 and BA.5, although not nearly as many. It's not clear how much protection that will translate into, and for how long. 

Antibodies are a key first layer of defense that form after vaccination or a prior infection. They can prevent infection by recognizing the outer coating of the coronavirus – the spike protein – and blocking it from entering your cells. 

But antibodies naturally wane and each new variant comes with a different-looking spike protein, giving it a better chance of evading detection by remaining antibodies.

Separate studies published this month in Nature and the New England Journal of Medicine show the newest Omicron relatives are even better at dodging antibodies – both in the vaccinated and in people who recovered from the original Omicron.

That first booster people were supposed to get strengthened immune memory, helping explain why protection against hospitalization and death is proving more durable. If the virus sneaks past antibodies, different defenders called T cells spring into action, attacking infected cells to curb illness.

As people get older, all parts of their immune system gradually weaken. There's little data on how long T cell protection against COVID-19 lasts or how it varies with different mutations or vaccines. 

Scientists recently petitioned the FDA to quit focusing solely on antibodies and start measuring T cells as it decides on vaccination strategy.

(With input from AP)