When the Food and Drug Administration approved several new vaccines against respiratory syncytial virus last year, the shots were celebrated as a “powerful new tool” to protect infants and older people. Pharmaceutical giants projected billions of dollars in sales, assuming a large swath of the eligible population would roll up their sleeves.

But for all the anticipation, relatively few people sought the vaccines out: Only about 24 percent of eligible older adults received it last season. It was a disappointing response, one experts blamed largely on vaccine fatigue, the cost of the shots and confusion over who should get them.