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 and infants from respiratory syncytial virus, a lesser-known threat whose toll in hospitalizations and deaths may rival that of the flu.

Federal health officials are hoping that widespread adoption of these immunizations 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 at no cost.

“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 immunizations, and when.

What respiratory illnesses are coming our way?

The coronavirus, the flu and R.S.V. are all likely to send thousands of Americans to the hospital this year, but exactly when, and how severe the toll will be, is unknown. That’s in part because the restrictions in place during the pandemic altered the seasonal patterns of the viruses.

Last winter, the flu peaked in December instead of in February, as it typically does. 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. But this year, it is now beginning to pick up in the South, suggesting that the virus may return to its prepandemic patterns.

R.S.V. is the least familiar of the three viruses, but increasingly it is 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.

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 aged 6 months and older, but is most important for adults ages 65 and older, children under 5, and people with weak immune systems.

Updated Covid shots from Pfizer and Moderna are now endorsed by the F.D.A. and the C.D.C. (A third, from Novavax, is expected to arrive in the next few weeks.) The recommendations:

• Americans aged 5 and older may receive one dose, at least 2 months after their last dose of any Covid vaccine.

• Children aged 6 months through 4 years who have already been vaccinated may receive one, or two, doses of the new vaccines. The timing and number of doses depends on the previous vaccine received.

• Unvaccinated children aged 6 months through 4 years may receive three doses of the new Pfizer-BioNTech vaccine or two doses of the new Moderna vaccine.

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 C.D.C., said of the Covid vaccine.

What about R.S.V.?

Two vaccines are now available for adults aged 60 and older: Abrysvo, by Pfizer, and Arexvy, by GSK. They are not universally recommended; patients may choose to get them in consultation with their doctors. The vaccines, which may have rare but serious side effects, are mostly likely to benefit older people with such underlying conditions as heart disease and asthma.

Abrysvo and Arexvy are not yet approved for most Americans younger than 60.

The C.D.C. now recommends another new shot against R.S.V. — Beyfortus, a monoclonal antibody — to protect infants less than 8 months old, as well as infants 8 months to 19 months old if they are at risk for severe illness.

In August, the F.D.A. approved Abrysvo for pregnant women as a way to protect infants from the virus. The vaccine, to be given in the last weeks of pregnancy, may prevent severe respiratory illness in infants up to 6 months.

While risks posed by any of these respiratory viruses increase 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.”

When should I get the vaccines?

You should get the shots early enough to build immunity against the pathogens, but the timing may depend on your particular circumstances.

If you do not want or are unable to make multiple trips to a clinic or pharmacy to space the shots apart, experts recommend getting the shots together. But if at all possible, it may be wise to time the shots to provide maximum protection.

Covid is already on the rise, so getting that shot as soon as possible makes sense. Flu may not peak until December, so getting the flu vaccine in October may be wisest — your antibodies will not have waned so much by the time the virus comes most prevalent.

Adults aged 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 can be scheduled for different times, Dr. Chu said.

Is it safe to get these vaccines all at once?

Getting the Covid and flu shots all at once does not significantly affect the protection or produce worse side effects compared with getting either one alone, according to a recent study by Israeli scientists.

“F.D.A. and C.D.C. systems monitor vaccine safety year round and will remain in place,” the Department of Health and Human Services said in a statement to The Times. “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.

Because the R.S.V. vaccine is new, however, there is little information on how it might interact with the other two vaccines.

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.

Christina Jewett contributed reporting.