Note that this is a monoclonal antibody injection and not a vaccine. This means that it will be way more expensive and protection will only last a few months at most. This is still a good thing for that are young and don’t mount great immune responses, but an actual vaccine approved for this age group is still needed.
No this drug is actually approved for prevention and not treatment.
It probably would reduce the chance they would get sick and get natural immunity. However a RSV vaccine for infants is hopefully not far off so they can gain immunity that way.