The U.S. vaccination effort has quickened in recent weeks, but supply issues remain an issue. About 64 million vaccine doses have been administered as of 9 a.m. Monday, according to the CDC.
The CDC recommends people who have had COVID-19 still get vaccinated because a previous infection does not guarantee future protection. People's immune systems react differently and milder infections might not produce enough antibodies to offer long-term protection, infectious disease experts say.
Nearly 28 million Americans have tested positive for the coronavirus. If this population only needs one dose for optimum protection, changing the guidelines would free up doses for other people, researchers say.