President Joe Biden on Friday urged those now eligible for boosters of Pfizer’s COVID-19 vaccine to get the added protection a day after the Centers for Disease Control and Prevention endorsed the extra doses for millions of older or otherwise vulnerable Americans.

Now public health officials must clear up confusion over exactly who should get a booster, and why — as they juggle vaccinating the unvaccinated who still make up the vast majority of the nation’s coronavirus cases.

People 65 and older, nursing home residents and those ages 50 and up who have chronic health problems such as diabetes should be offered a booster once they’re six months past their last Pfizer dose, CDC Director Dr. Rochelle Walensky ruled late Thursday.

And a broad swath of other adults can decide for themselves if they want a booster once they reach that six-month mark: Younger people with underlying health problems — plus people at increased risk of infection because of their jobs, such as health workers, or their living conditions, such as jails or homeless shelters.