The Food and Drug Administration authorized a second booster shot of the Moderna or Pfizer coronavirus vaccine — but who should get the fourth shot and when is the best time to do so? Boston doctors gave their best tips on Greater Boston.

The shots have been cleared for people over the age of 50 and those who are immunocompromised, according to the FDA. The fourth dose should be given at least four months after the first booster.

Dr. Paul Sax, clinical director of Brigham & Women's Hospital Infectious Disease Division, said he would "strongly recommend" a fourth shot to those who are medically vulnerable or elderly, and the time is now. If you do not have such medical conditions, he said the decision on timing can be made with your doctor.

Dr. Sabrina Assoumou, infectious disease physician at Boston Medical Center, said the FDA made the right call because the best data on second boosters shows the most benefits for older adults.

Both Assoumou and Sax said although the second booster is important for those with qualifying conditions, many people still haven't gotten a first booster yet.

"If you haven't been boosted yet, please go ahead and get boosted. It's not an extra dose, it's actually a dose that you actually require to get the level of protection that's needed in your community to protect you from the currently circulating variant," Assoumou said.

Watch: Boston doctors discuss who needs a second booster