A county health worker fills a syringe with the Moderna COVID-19 vaccine in West Chester, Pa., in late December. Rutgers University will require students enrolling for the fall semester to show they have received a COVID-19 vaccine.

FILE – In this Dec. 29, 2020, file photo, Penny Cracas, with the Chester County, Pa., Health Department, fills a syringe with the Moderna COVID-19 vaccine before administering it to emergency medical workers and healthcare personnel at the Chester County Government Services Center in West Chester, Pa. President Joe Biden’s pledge that all U.S. adults will be eligible for coronavirus vaccines by May 1, 2021, is putting a new challenge in front of the nation. For that to happen, the federal government must deliver doses to hundreds of new vaccination sites and recruit more health care workers to administer the shots. (AP Photo/Matt Slocum, File)