The largest opportunity for an RSV antiviral is in peds. I don't know the sales potential that has been thrown out by companies and analysts along the way, but it is large enough that large pharmas were/are pursuing it - Gilead, JNJ, PFE, etc.
RSV vaccines are only in adults right now, and the CDC schedule for Beyfortus is designed such that they think there will be about 50% coverage. There are something like 850K outpatient pediatric RSV visits, and Beyfortus is expected to cut that down by a little over 100K visits. so how many of the remaining 700K-750K get therapy? I have no clue (note this includes upper respiratory infections not just LRTI). Given that the risk of progression to severe disease and hospitalization is much higher than flu (which is higher than the common cold), I do think an antiviral will see uptake at a decent clip in this population. The CDC estimates that Beyfortus will lower ER visits by 38K, so post introduction still 200K ED visits annually in a typical year. You get the idea it is still a large underserved market
I think the street is discounting the RSV prospects because of the history of failures w prior agents, the recent failure of the PFE drug (although that was a fusion inhibitor like most of the other drugs), and the fact the ENTA compound failed to show a benefit in low risk patients already. If they hit on the peds trial it is a pretty big deal IMO (probably has read through to high risk adult population) and all told at least a 500M annual opportunity as a ballpark guess
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