The virus sends up to 160,000 people over 65 to hospitals every year. But just 15 percent have gotten the newly available shots.

Toby Gould was an early adopter. In September, Mr. Gould, 78, went to a pharmacy in Hyannis, Mass., to get one of the new vaccines for respiratory syncytial virus, known as R.S.V. He has asthma, which would heighten his risk of serious illness if he were to be infected.

Carol Kerton, 64, knew R.S.V. could be dangerous: Her 3-year-old granddaughter had such a severe case that she was taken to an emergency room. Ms. Kerton was vaccinated in September at a local supermarket in Daytona Beach, Fla.

Sam Delson, 63, received the R.S.V. vaccine last month in Sacramento. His doctor recommended it, he said, “because I’m over 60 and have a somewhat weakened immune system” after a long-ago bout with cancer.

They are the exceptions. So far, only about 15 percent of Americans over 60 have received one of the two new R.S.V. shots, which the Food and Drug Administration approved in May and are the first-ever vaccines against the disease. Just 16 percent more said they definitely planned to, according to the Centers for Disease Control and Prevention.

By contrast, more than 62 percent of adults over 65 have received the recommended flu shot this fall, and a third have gotten the updated Covid-19 vaccine.

“It’s a new vaccine, and people are trying to figure out whether they need it or not,” said Dr. Preeti Malani, a geriatrician and infectious disease specialist at University of Michigan Health.

That is if they know about the R.S.V. vaccines at all. A national survey this summer of people ages 60 through 80 found that nearly half hadn’t heard about them.

The C.D.C. recommends the R.S.V. vaccines for people over 60, after having individual discussions with their health care providers, something called “shared clinical decision-making.” Medicare Part D, Medicaid and most private insurers will cover the entire cost.

The fact that older people are vulnerable to R.S.V. is an unfamiliar concept to many people. For decades, the virus was mostly considered a threat to infants and young children. Most physicians, “when they went to medical school, were taught that R.S.V. was a pediatric illness,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. “It’s still the leading cause of hospitalization in infants in the U.S.”

But the F.D.A. estimates that the virus sends 60,000 to 160,000 people over 65 to hospitals each year and causes 6,000 to 10,000 deaths. Other published estimates are even higher.

“It is a very contagious virus,” Dr. Malani said. Though children can become quite ill, more often, “a 4-year-old with a runny nose could have R.S.V. and not become very sick; it looks like a regular cold,” she said. However, she added: “The grandparents could get pneumonia.”

A close-up of an assembly line apparatus moving small vials of vaccine in production.
Pfizer’s R.S.V. vaccine in production. Only about 15 percent of Americans over 60 have received one of the two new R.S.V. shots.Credit…Pfizer, via Reuters

The risk of becoming seriously ill from R.S.V. increases markedly with age. Hospitalization rates rise sharply for those in their 70s and 80s, especially for those with chronic heart and lung diseases like asthma, heart failure and chronic obstructive pulmonary disease. Older adults who have diabetes or liver and kidney disease, or weakened immune systems, also face higher risk. Adults can be infected repeatedly, and there’s no drug that ameliorates the disease, as there is for the flu and Covid-19.

A study published in The New England Journal of Medicine in 2005 followed patients over four winters and reported that, among high-risk patients (their average age was 70) with heart failure or pulmonary disease who contracted R.S.V., 16 percent required hospitalization. In another cohort of older patients hospitalized with respiratory symptoms (with an average age of 75) and diagnosed with R.S.V., 15 percent wound up in intensive care.

The new R.S.V. vaccines are highly effective. Clinical trials results showed that Arexvy, the shot made by GSK, was 94 percent effective against severe illness in older adults. Pfizer’s shot, called Abrysvo, was 86 percent effective against severe illness.

So why haven’t the vaccines caught on more with their intended recipients?

One reason: A shared decision-making recommendation from the C.D.C. can depress vaccination rates, Dr. Schaffner said, because “you can’t promote it with quite the intensity and assurance as with a blanket recommendation” — like the one recommending flu shots for everyone over 6 months old.

Also, older people now receive multiple public health messages about seasonal vaccinations. “A few years ago, we were all recommending one vaccine each winter — flu,” Dr. Schaffner said. “We haven’t yet organized ourselves to be persuasive in getting people to accept three seasonal vaccines,” for influenza, Covid-19 and now R.S.V. (Getting two or three at the same time is fine, the C.D.C says.)

When R. Jessica Jones, 76, who lives in Haiku, Hawaii, texted her doctor about seasonal vaccinations, he replied that she should get the Covid-19 booster and a flu shot, but that getting an R.S.V. vaccine was “optional.”

Ms. Jones, surprised, asked why. He told her he thought the data on their safety and efficacy was “limited” (the F.D.A. disagreed), so she skipped getting one.

“When providers are confused, patients are also confused,” Dr. Malani, of University of Michigan Health, said. “If we really want uptake in the population that could benefit, we need to provide clear information to doctors and others.”

While some health care providers hope to improve the vaccination rate among older Americans, the vaccines’ manufacturers are apparently delighted with the number of people seeking out the shots so soon after shipping them to pharmacies, hospitals and doctors’ offices last summer. The manufacturers are collecting data on the vaccines’ effectiveness and side effects and — a central unanswered question — how often people will need to be revaccinated to maintain protection.

“For a new class of vaccines, this is really fantastic,” said Dr. Len Friedland, who directs public health for GSK Vaccines.

“There will always be hiccups,” he said. “But in general it’s gone very well, and we’re not hearing that there are access problems for patients.”

Dr. Nathaniel Hupert, co-director of the Cornell Institute for Disease and Disaster Preparedness, was more cautious. Fifteen percent is “a lot better than zero,” he said, pointing out that until last summer, there was no prevention available against R.S.V. But, he said, “if you’d like to stamp out R.S.V., that’s not going to happen with this level of coverage.”

Other manufacturers have R.S.V. vaccines in development, and older Americans may eventually have greater protection as more pregnant women and babies are immunized, as the C.D.C. recommends. “Kids have the distribution franchise for these respiratory viruses every winter,” Dr. Schaffner, of Vanderbilt University Medical Center, said.

Over time, “we’ll probably see less R.S.V. transmission from kids to their grandparents,” Dr. Hupert said. “But we’re not there yet.”

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