More than two years into the COVID-19 pandemic, U.S. health officials are beginning to grapple with how to keep the vaccines updated to best protect Americans from the ever-changing coronavirus.

On Wednesday, a panel of vaccine advisers to the Food and Drug Administration spent hours debating key questions for revamping the shots and conducting future booster campaigns. They didn’t reach any firm conclusions.

The questions facing the experts included: How often to update the vaccines against new strains, how effective they should be to warrant approval, and whether updates should be coordinated with global health authorities.

Last week, the FDA authorized a fourth dose of the Pfizer or Moderna vaccines for anyone 50 or older and for some younger people with severely weakened immune systems. It’s an effort to get ahead of another possible surge.

But the FDA’s vaccine chief Dr. Peter Marks acknowledged at the meeting “we simply can’t be boosting people as frequently as we are.” He called the latest booster update a “stopgap” measure to protect vulnerable Americans while regulators decide whether and how to tweak the current vaccines.

Marks cautioned that waning vaccine protection, new variants, and colder weather in the fall could raise the risk of more surges.

“Our goal here is to stay ahead of future variants and outbreaks and ensure we do our best to reduce the toll of disease and death due to COVID-19,” said Marks, adding that he expects more meetings of the vaccine panel in coming months.