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 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 Thursday of the CDC'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."

A vaccine by Novavax will target JN.1, the variant that prevailed for months in the winter and spring. The shots to be made by Pfizer and Moderna are aimed at KP.2, which until recently seemed poised to be the dominant variant.

But KP.2 appears to be giving way to two related variants, KP.3 and LB.1, which now account for more than half of new cases. All three variants, descendants of JN.1, are together nicknamed FLiRT, after two mutations in the virus's genes that contain those letters.

The mutations are thought to help the variants evade some immune defenses and spread faster as a result, but there is no evidence that the variants cause more severe illness.

Emergency department visits related to COVID in the week ending June 15 increased by nearly 15%, and deaths by nearly 17%, over the previous week's totals. Hospitalizations also appear to be increasing, but the trends are based on data from a subset of hospitals that still report figures to the CDC even though the requirement to do so ended in May.

"COVID is still out there, and I don't think it's ever going away," Dr. Steven P. Furr, the president of the American Academy of Family Physicians, said in an interview.

The biggest risk factor for severe illness is age. Adults ages 65 and older account for two-thirds of COVID hospitalizations and 82% of in-hospital deaths. Yet, only about 40% of Americans in that age group were immunized with a COVID vaccine offered last fall.

"This is an area where there's a lot of room for improvement and could prevent a lot of hospitalizations," said Dr. Fiona Havers, a CDC researcher who presented the hospitalization data.

Although younger adults are much less likely to become severely ill, there are no groups completely without risk, CDC researchers said. Children — particularly those younger than 5 — are also vulnerable, but only about 14% were immunized against COVID last fall.

Many parents mistakenly believe that the virus is harmless in children, said Dr. Matthew Daley, a panelist and senior investigator at Kaiser Permanente Colorado.

"Because the burden was so high in the oldest age groups, we lost sight of the absolute burden in pediatric age groups," Daley said.

Even if children do not become ill themselves, they can fuel circulation of the virus, especially once they return to school, Furr said.

"They're the ones that, if they're exposed, are more likely to bring it home to their parents and to their grandparents," he said. "By immunizing all groups, you're more likely to prevent the spread."

Among children, infants younger than 6 months have been hardest hit by COVID, according to data presented at the meeting. But they are not eligible for the new shots.

It is "critical that pregnant persons get vaccinated, not only to protect themselves but also to protect their infants until they are old enough to be vaccinated," Dr. Denise Jamieson, one of the panelists and the dean of the Carver College of Medicine at the University of Iowa, said in an interview.

Among both children and adults, vaccine coverage was lowest among the groups most at risk of COVID: Native Americans, Black Americans and Hispanic Americans.

In surveys, most Americans who said they probably or definitely would not receive the shots last fall cited unknown side effects, not enough studies, or distrust of the government and pharmaceutical companies.

The CDC has said that the vaccines are linked to only four serious side effects, but thousands of Americans have filed claims for other medical injuries they say were caused by the shots.

At the meeting, CDC researchers said they had, for the first time, detected that Pfizer's COVID vaccine may have led to four additional cases of Guillain-Barré syndrome, a rare neurological condition, per 1 million doses administered to older adults. (The numbers available for Moderna and Novavax vaccines were too small for analysis.)

The risk may not turn out to be real, but even if it is, the incidence of GBS is comparable to the rate observed with other vaccines, the researchers said.

The CDC has also investigated a potential risk of stroke after vaccination, but the findings thus far are inconclusive, agency scientists said. In any case, the benefit from the vaccines outweighs the potential harms, they said.

The panelists bemoaned the sharp drop in health care providers who counsel patients about the importance of COVID vaccination. Nearly half of providers said they did not recommend the shots because they believed their patients would refuse.

There has also been increasing physical and verbal abuse in hospitals and health care settings, said Dr. Helen Keipp Talbot, a professor of medicine at Vanderbilt University and the committee's chair.

"Some of our physicians may not be recommending it due to concerns about safety of them and their staff," she said.

Although the panelists unanimously recommended COVID vaccination for people of all ages this time, they debated the feasibility of universal recommendations in the future. The vaccines are much pricier than other shots, and they are most cost-effective when given to older adults.

At an individual level, the Affordable Care Act requires insurers, including Medicare and Medicaid, to cover vaccines recommended by the advisory committee at no cost. But up to 30 million Americans do not have health insurance.

The Bridge Access Program, a federal initiative that makes the vaccines available to underinsured and uninsured Americans, will end in August.

Unless the price of the vaccines drops, the cost of immunizing all Americans may not be sustainable, the panelists said.

"As more and more of society is exposed either to vaccine or disease, it will become much less cost-effective," Talbot said. "We will need to have a less expensive vaccine to make this work."