WASHINGTON—Johnson & Johnson said Thursday that U.S. regulators extended the expiration date on millions of doses of its COVID-19 vaccine by six weeks. The company said a Food and Drug Administration review concluded the shots remain safe and effective for at least 4 1/2 months. In February, the FDA originally authorized J&J’s vaccine for up to three months when stored at normal refrigeration levels. Thursday’s announcement comes after state officials warned that many doses in storage would expire before the end of the month. The change gives health providers more time to try and use remaining J&J shots sitting at pharmacies, hospitals and clinics. Many states have adopted a “first-in-first-out” approach to try and use their oldest vaccines first. Vaccine expiration dates are based on information from drugmakers on how long the shots stay at the right strength. J&J said the FDA added six weeks based on data from ongoing …