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The following is highly simplified and should only be used as an example. Natural immunity does play a significant part, lower in early part of spread and more later on. I have not attempted to account for the natural immunity. It only shows the impact of vaccines and social distancing on the various viruses and their varying R0 values.
Flu R2. 50% effective vaccine in 50% of population. No social distancing. Effective R0 at R1.5
1/1.5/2.3/3.5/5.3/8/12/18 (continues to spread)
Flu R2 50% effective vaccine in 50% of population so same R1.5 Social distancing, Virtual school.. Hygiene. Far less air traffic. Limited attendance concerts/sporting events. Masking. Restaurants seating/takeout. Etc. Effective R0 drops slightly more than 1/3 from R1.5 to R0.9.
1/0.9/0.81/0.73/0.66/0.59/0.53/0.48 (never gets a foothold, dies out)
Alpha variant R3. No vaccine. Same social distancing etc. Effective R0 drops the same 1/3 to R2.
1/2/4/8/16/32/64/128 (exponential growth)
Alpha variant at R3. Social distancing etc to an effective R2 (same 1/3 drop) Highly effective vaccine against Alpha that eventually reaches 50% of population. Effective R0 at R1 and lower.
1/1/1/1/1/1/1/1 ( no growth)
Delta variant at R5. Social distancing etc (same 1/3 drop) to an effective R3.4. Far less effective vaccine against variant with 50% of population vaccinated cuts it by a quarter (50-%effective in 50% of population). Effective R2.6
1/2.6/6.7/17.4/45/117/304/790 (exponential growth)
Flu spreads with vaccine and no social programs.
Flu dies out with vaccinations and social programs. Never had initial foothold in Fall of 2020.
Alpha continues to spread without vaccines even though social programs in place, though at a lower pace.
Alpha slowly dies out with effective vaccine and social programs.
Delta continues to spread easily with less effective vaccine and social programs.
Who knows what the flu season will be like this year. There should be far less natural immunity since we had no flu season last year. The vaccine is always a crap shoot and will be even more so this year since we have so little data from last year to work with. Also, much of the social distancing is no longer in place. My guess is an above avg year. And yes, I received my super-duper old folks vaccine this year.
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