Most Americans have had one or more shots of the flu and Covid vaccines. New this year are the first shots to protect older adults from respiratory syncytial virus, a lesser-known threat whose toll in hospitalizations and deaths may rival that of flu.

Federal health officials are hoping that widespread use of these three vaccines will head off another “tripledemic” of respiratory illnesses, like the one seen last winter. For people with insurance, all of the vaccines should be available for free.

“This is an embarrassment of riches,” said Dr. Ofer Levy, director of the precision vaccines program at Boston Children’s Hospital and an adviser to the Food and Drug Administration.

Here’s what he and other experts say about who should receive which vaccines, and when.

What respiratory illnesses are coming our way?

The coronavirus, flu and R.S.V. are all likely to resurge this fall, but exactly when and how much damage they will do are unknown. That’s in part because the restrictions in place during the pandemic altered the seasonal patterns of the viruses.

This past winter, the flu peaked in December instead of in February, as it typically does. The virus may have caused as many as 58,000 deaths, a higher number than usual. Covid kept up a steady number of infections and deaths most of the season, with a peak in January.

Compared with its pattern before the pandemic, R.S.V. peaked several weeks earlier last year, and it circulated for longer than usual.

R.S.V. is increasingly recognized as a major respiratory threat, particularly to older adults, immunocompromised people and young children. “R.S.V. has a burden of disease similar to flu in older adults — it can make you very, very sick,” said Dr. Helen Chu, a physician and immunologist at the University of Washington.

Scientists expect respiratory viruses to return to their prepandemic patterns eventually, but “it’s going to be unpredictable for the next two years,” Dr. Chu said.

Which vaccines should I seek out?

Everyone should have at least the flu and Covid shots this fall, experts said.

The annual flu vaccine is recommended for everyone 6 months and older, but it is most important for adults aged 65 and older, children under 5 and people with weak immune systems.

Updated Covid shots are coming this fall from Pfizer, Moderna and Novavax, and all are designed to target XBB.1.5, the Omicron variant that currently accounts for roughly 27 percent of cases. The full recommendations will not be available until the F.D.A. authorizes the shots and the C.D.C. reviews new data.

Federal health officials aren’t talking about a primary series of shots followed by boosters. (Officials aren’t even calling the shots “boosters” anymore.) Instead, they are trying to steer Americans toward the idea of a single annual immunization with the latest version of the vaccine.

“Like a seatbelt in a car, it’s a good idea to keep using it,” Dr. Camille Kotton, a physician at Massachusetts General Hospital and an adviser to the Centers for Disease Control and Prevention, said of the Covid vaccine.

R.S.V. is a frequent cause of respiratory illness among older adults, particularly those 75 or older who have other conditions, such as cardiovascular disease, chronic lung disease or diabetes.

The new R.S.V. vaccine is not approved for Americans younger than 60. The C.D.C. recommends that people aged 60 and older sign up for the shot after consulting with their doctors.

While it’s true that risks posed by any of the three viruses increases with age, remember that “65 is not a magical cutoff point,” Dr. Chu said.

“Even those with no pre-existing conditions can become quite sick with all three of these viruses,” she said.

When should I get the vaccines?

No one knows when these viruses will re-emerge, so you should get the shots early enough in the fall to build immunity against the pathogens. Most people will not want or be able to make multiple trips to a clinic or pharmacy to space the shots apart.

That probably means September or October. Most Americans may want to consider receiving the flu and Covid shots at the same time, so they are prepared to face either virus. Older adults who are in poor health — who have heart or lung disease, for example, or are on home oxygen — should get all three shots, some experts said.

They should “get them as quickly as possible and definitely before the season, and do it all at once,” Dr. Chu said.

Adults 50 and older should also get the vaccine for shingles, if they haven’t already, and those 65 and older should sign up for the pneumococcal vaccine. But those vaccines don’t need to be given in the fall and should be scheduled for a different time, Dr. Chu said.

Is it safe to get these vaccines at once?

The flu and Covid shots were often given together last fall and seemed to work well. Because the R.S.V. vaccine is new, however, there is little information on how it might interact with the other two vaccines.

“The available data pertaining to the administration of influenza and Covid-19 vaccines at the same time do not indicate safety concerns,” the Department of Health and Human Services said in a statement to The New York Times.

“F.D.A. and C.D.C. systems monitor vaccine safety year round and will remain in place,” the department said. “If any new potential safety signals are identified, the F.D.A. and C.D.C. will conduct further assessment and inform the public.”

Some research suggests that the R.S.V. and flu vaccines produce lower levels of antibodies when given together than when delivered one at a time. But those levels are probably still high enough to protect people from the viruses, experts said.

There is also limited data on the safety of the two R.S.V. vaccines. Clinical trials recorded six cases of neurological problems, including Guillain-Barré syndrome, compared with none in the placebo groups.

But the numbers were too small to determine whether the cases were a result of the inoculations. More clarity will come from surveillance while the vaccines are administered on a large scale, Dr. Chu said.

The C.D.C. is expected to make recommendations on administration of the vaccines together in the coming weeks.