All Americans ages 6 months and older should receive one of the new Covid-19 vaccines when they become available this fall, the Centers for Disease Control and Prevention said on Thursday. The recommendation comes as the nation faces a summer wave of Covid, with the number of infections rising in at least 39 states and territories.

Most Americans have acquired immunity against the coronavirus from repeat infections or vaccine doses, or both. The vaccines now offer an incremental boost, remaining effective for only a few months as immunity wanes and the virus continues to evolve. Still, across every age group, a vast majority of Americans who were hospitalized for Covid did not receive one of the shots offered last fall, according to data presented at a meeting on Thursday of the C.D.C.’s Advisory Committee on Immunization Practices.

Dr. Mandy Cohen, the agency’s director, accepted the unanimous advice of the panel on Thursday to recommend another round of immunizations. “Professionals and the public in general do not understand how much this virus has mutated,” said Carol Hayes, the committee’s liaison to the American College of Nurse-Midwives. “You need this year’s vaccine to be protected against this year’s strain of the virus.”