The vast majority of people at risk for illness and death following infections with these viruses are those 75 and older. In that group, the benefit from each of the vaccines clearly outweighs any safety concerns, Dr. Kotton and other experts said.
Up to 85 percent of flu-related deaths in recent years were among those age 65 and older, according to the C.D.C. The agency recommends that older adults get a high-dose flu vaccine or one with an adjuvant, an ingredient that can produce a stronger immune response.
Hospitalizations and deaths from Covid also occur primarily in the oldest Americans, and Covid boosters are now thought to be beneficial primarily for older adults and people with weakened immune systems.
In June, the F.D.A. advised Pfizer-BioNTech, Moderna and Novavax to manufacture Covid shots designed to target XBB.1.5, the Omicron variant that accounts for roughly 27 percent of cases. That variant seems to be receding, however, and a newer variant, XBB.1.16, is on the upswing.
R.S.V. is the leading cause of infant hospitalizations in the United States, and among the top killers of young children in low- and middle-income countries. The virus was underappreciated as a respiratory threat to adults until recently.
The virus may lead to as many as 160,000 hospitalizations and 10,000 deaths among older adults each year, according to the C.D.C. — and those numbers are likely to be underestimates. For every one million adults age 65 and older who get the vaccine, 25,000 outpatient visits, 2,500 hospitalizations and 130 deaths would be prevented, according to one analysis presented to the agency’s advisers.
For decades, vaccines against R.S.V. proved challenging to design. A breakthrough in 2013 galvanized efforts by several companies. In a recent trial, the GSK vaccine, to be sold as Arexvy, retained much of its potency into the second year, and its efficacy is being studied for an even longer period.