The best evolutionary strategy in human viruses is probably to cause mild upper respiratory tract or gastrointestinal disease such that it increases coughing and sneezing or causes diarrhoea.
Basically, the most successful viruses—colds and Norwalk like viruses—modify our behaviour by hijacking one of our body's defence mechanisms so that they can spread more easily.
I would guess that the mildest form of disease sufficient for such behaviour changes would probably be the best strategy. This would keep the infected host active enough to circulate and spread the virus.
My layman's opinion on covid is that it will become weaker the more it spreads through the population, to the point that when most of us inevitably get it it will cause symptoms like a cold or mild flu.
There is a paper that showed that the flu virus makes people more socialable.
It was done with students who took flu vaccine. They found people socialised more after they had the vaccine (and thus, were a little bit infected) compared to before.
A literature review looked at the study and came up with the negative explanation - that people felt more secure or relaxed after getting the vaccine as they felt safer. Or were more nervous about needles before.
Ancedote:
I think I have covid-19 and the days before symptoms I felt unusually anxious to get outside and amongst fellow humans. I'm usually an introvert and stay inside mostly so it was notable.
Suggestion:
Online interview people with symptoms now to collect data on peoples behaviour in the days before symptoms showed, when they were infected but not showing, compared to 2 weeks before.
A vaccine does not make people "a little bit infected". Vaccines are deactivated virii, so your body develops the antibodies, without any actual infection.
That's not totally true the live Polio Vaccine used a live weak infectious virus. Which had the advantage of conferring immunity to unvaccinated people. At the expense of very rarely reverting and causing full blown disease. Far as I know it's no longer being used because of that.
