NEW YORK (AP) — Are COVID-19 boosters the same as the original vaccines?

Yes, COVID-19 boosters use the same recipe as the original shots, despite the emergence of the more contagious delta variant. The vaccines weren’t tweaked to better match delta because they’re still working well.

The vaccines work by training your body to recognize and fight the spike protein that coats the coronavirus and helps it invade the body’s cells. Delta’s mutations, fortunately, weren’t different enough to escape detection.

The increased protection you might get from a booster adjusted to better match the delta or other variants would be marginal, says Dr. Paul Goepfert, director of the Alabama Vaccine Research Clinic at the University of Alabama at Birmingham.

Manufacturing doses with a new formula would have also delayed the rollout of boosters.

Moderna and Pfizer are studying boosters tweaked for the delta and other variants to be ready if one’s ever needed. Health authorities would have to decide if and when a vaccine formula swap would be worthwhile.

“What we don’t know,” Goepfert noted, “is if you have a delta vaccine compared to the regular vaccine, does it actually work better in preventing transmission or asymptomatic infection?”

The U.S. has authorized booster doses of the Pfizer, Moderna and Johnson & Johnson vaccines for certain people, and a few other countries also are using boosters of those shots or other COVID-19 vaccines.