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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 of the shape-shifting coronavirus.

 and  have tested updated shots against the super-contagious omicron variant, and advisers to the Food and Drug Administration will debate Tuesday if it’s time to make a switch — setting the stage for similar moves by other countries.

“This is science at its toughest,” FDA vaccine chief Dr. Peter Marks told The Associated Press, adding that a final decision is expected within days of the advisory panel’s recommendation.

Current COVID-19 vaccines  around the world in just their first year of use. And 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.