By LAURAN NEERGAARD and MATTHEW PERRONE
U.S. regulators are telling COVID-19 vaccine makers to tweak any booster doses intended for fall so they protect against the newest omicron relatives. There’s no way to know what coronavirus variant will be the main threat as cold weather returns. But the Food and Drug Administration decided Thursday that combination shots adding a type of omicron protection to the original vaccine is the best booster strategy for fall. Pfizer and Moderna already had been testing combo shots targeting the omicron strain responsible for last winter’s surge, and now will need another tweak for even newer versions.