Millions of Americans are now eligible to receive a Pfizer booster shot to help increase their protection against the worst effects of the coronavirus. Boosters are free and available at health departments, clinics and drugstores. People 65 and older who got the Pfizer shots qualify for a booster, along with others with certain health conditions or risky jobs. Still waiting are those who got Moderna or Johnson & Johnson shots initially. They aren’t eligible for boosters yet. There’s not enough data on mix-and-match vaccines, and government regulators are waiting to review evidence submitted by the companies for their own booster.