The weakened immune systems is the theory, no?
That would be the nursing mother's theory they mentioned, which could be. But I'm sure is far from proven.
The other part was why older siblings are spreading it more but that doesn't answer why it would be more severe.
Assuming we are talking about this part:
“Nursing mothers, who would [typically] protect their children through passive immunity, aren’t able to give that robust immunity to their children this year because the mothers were not exposed to these viral illnesses last year as they usually are,” the doctor said.
“Also, a lot of the toddlers who get mild illness skipped the typical RSV season last year, have no immunity to RSV, and thus pass it to their siblings or neighbors.”
