The U.S. on Friday opened COVID-19 booster shots to all adults and took the extra step of urging people 50 and older to seek one, aiming to ward off a winter surge as coronavirus cases rise even before millions of Americans travel for the holidays.

Until now, Americans faced a confusing list of who was eligible for a booster that varied by age, their health and which kind of vaccine they got first. The Food and Drug Administration authorized changes to Pfizer and Moderna boosters to make it easier.

Under the new rules, anyone 18 or older can choose either a Pfizer or Moderna booster six months after their last dose. For anyone who got the single-dose Johnson & Johnson vaccine, the wait already was just two months. And people can mix-and-match boosters from any company.