Then why do the 2nd dose when it’s the 2nd more likely to cause adverse effects? Overkill? 🤷🏻♀️
95% is significantly better than 90% and the adverse effects are things like fever, body aches, soreness at the injection site, etc. Pretty mild stuff comparatively. Also, that study out of Israel showing "up to 90%" is still pretty new and not yet replicated (that I've seen). Actual efficacy - particularly against the South Africa or Brazil variants - could be a lot less.
The polio shot is >90% effective at two doses. It's 99% effective at 3. You aren't considered "fully vaccinated" unless you've had four. It's a risk:reward calculation. When the risk of real problems is very low and the reward is a significant reduction in the risk for real problems, you do it; sore arms and body aches be damned.
(If first Pfizer shot can give up to 90% protection) “why do the second shot?”
Gas_Dr posted, that the literature indicates that the second shot drives TCell memory (longer term immunity).
Cathi posted a link to a Scottish study (not yet peer reviewed or published) that indicated the protection (against hospitalization for COVID), peaked four weeks after a first shot (Pfizer or AZ), and declined in weeks 5 and 6.
Perhaps a second shot really s important.