What Is the Contagion Window for RSV?

My 18-month-old tested positive for RSV on Sunday. We are supposed to travel this weekend to see family for the holiday. How long is my son contagious? Visiting grandparents and a new baby.
—Amanda

The length of contagiousness for viruses is always a little hard to measure — for COVID, for example, we still do not really know the distribution. For RSV, our best data comes from a detailed study of a set of Kenyan households where members were tested repeatedly for RSV, regardless of whether they had had symptoms. In that study, they can measure the duration of viral shedding.

The average duration of viral shedding is 11 days. This number is longer in young children — 18 days in infants on average, nine days in adults. Viral shedding is not exactly the same as being contagious, but it is a close proxy.

If you were visiting only healthy adults, I think you could make a case for this being close to the line — it’s about 11 days between the positive test and Thanksgiving, and the infectious period likely started considerably before the positive test. But with a new baby and older adults, I am afraid I would say skip it unless you get another negative RSV test before you go. I’m sorry!

(Update 8/24/2023: A post on the newly FDA-approved RSV drug for infants and toddlers is here.)