The company gave an extra dose to 306 people at that point and recorded levels of virus-fighting antibodies threefold higher than after the earlier shots. Earlier this week, two top FDA vaccine regulators joined a group of international scientists in rejecting boosters now for otherwise healthy individuals, citing the strong continuing protection against severe disease. The U.S. already offers an extra dose of the Pfizer or Moderna vaccines to people with severely weakened immune systems. Moderna also has asked FDA to allow a booster dose of its vaccine, and on Wednesday posted its own study of waning immunity. It’s not clear how soon FDA will consider a Moderna booster, or one for people who received the single-shot Johnson & Johnson vaccine.