It's not about the number of cases out there but rather the severity. If everyone is getting covid but no one is getting sick enough to die or need intensive care, there won't be a need anymore for a vaccine. The natural life cycle of any virus is for it to become more infectious and less dangerous to the hosts since that's the best way for the virus to survive. The multitude of viruses that cause the common cold have already undergone that evolution. Influenza is a very unique case because of the structure of the virus that allows recombination among and between strains from different species means novel flu strains can happen any time without much warning and our immunity can be very variable. There are other outliers like HIV but most viruses that have ever infected humans have become relatively inert. A great example of the most extreme form of becoming inert is JC virus which is entirely asymptomatic and inconsequential unless someone is prescribed certain immunosuppressive medications. If you do get put on one of those medications you have to be tested for it otherwise it can reactivate and cause progressive multifocal leukoencephalopathy which is almost universally fatal.

Or Russian intelligence.

If we were talking about a single river they might be but we're talking about multiple rivers that are tributaries of one another.

Yes. There seems to be a lot of confusion here about what this means so I'll try to clarify. This is the furthest boat trip you can take along natural waterways from the ocean to a point inland without crossing land or using a canal. Taking a boat all the way from the Gulf of Mexico, up the Mississippi, up the Missouri, and into Montana is a very far boat ride. You cannot do this further anywhere else on Earth without using a canal or taking your boat over land. I honestly didn't think this was such a confusing concept but apparently people think it is.