The C.D.C. now says adults 50 and older should get a second booster, amid rising U.S. cases and hospitalizations.

She said in Massachusetts, a state with a high rate of vaccination, officials estimate that as many as 70 percent of hospital patients who test positive for the virus were admitted primarily for unrelated illnesses. However, coronavirus infections can also exacerbate underlying medical conditions, of which many Americans have at least one.

The death rate from Covid, although a lagging indicator, may be a more reliable gauge of the degree of Covid’s impact, because physicians must note the cause of death on the death certificate, according to Dr. Gandhi and other experts.

Recently, deaths have remained low. About 275 deaths have been recorded each day on a seven-day average, Dr. Walensky said Thursday. The number of new deaths has actually dipped slightly in recent weeks. According to the Times database, the overall toll of U.S. deaths surpassed one million on Thursday, the highest confirmed total of any nation.

Those aged 50 or older have been eligible for a second booster shot since late March, but federal health officials have said too few people are taken advantage of that. Only one-fourth of those 65 and older who have gotten one booster dose, for instance, have gotten a second, the C.D.C.’s data show.

Dr. Walensky also said this week that the C.D.C. and the Food and Drug Administration are now discussing whether to broaden eligibility to those younger than age 50.

This week, the agencies cleared a booster shot of the Pfizer-BioNTech vaccine for 5- to 11-year-olds earlier, expanding eligibility for a first booster dose to a younger age group. Among other factors cited in its decision memo, the F.D.A. cited “the continued relaxation” of preventive measures, including mask mandates, social distancing and isolation of infected individuals.

The agency also noted the risk of long Covid, which it said “can cause significant morbidity after initially mild infection.”