2) I'd also like to note that one thing animal immune researchers have found is social animals have different immune systems than solitary ones, because we need to prevent pathogens from ever causing issues with certain key organs, such as, for instance, causing permanent organ damage to completely eradicate any pathogen that evolves to infect the heart; while solitary animals have immune systems which won't do that. If so, then one way a collapse in herd immunity could be expected to play out is a collapse in this, and a rise in pathogens able to infect hearts, brains, and other key organs.

This was not a problem until now because even though some people have compromised immune systems, they are not common enough for pathogens to be able to spread from person to person far before they run into immune systems which will go into overdrive and cause serious collateral damage in order to destroy them.

Sadly I haven't been able to track down the study again, but it makes evolutionary sense.