There was one field scale test of the polio vaccine, in 1954. It included about ~700,000 children. There was not three years of staged testing, let alone millions of children within those three years.
There were 623,000 children in the field trial (placebo/non-placebo). There was an additional million who acted as controls (not directly a part of the study, but used a baseline measure of polio incidents in the population surrounding the subject).
While the entire study took over a year, different populations were "monitored" for, at best, six months, many less.
The trial size was massive, but its generally agreed to have been overkill.
The results didn't differ much from the first 150 kids they tried it on.
Covid vaccines were tested on tens of thousands in 8-10 months.
As I said, MMR components were 5-8 years tested and then mass-deployed by the time of combination.
Mandates didn't come until the late 60s/70s for either MMR or Polio vaccines, therefore decades.
All that dealing with much higher (by YLL/DALY) impact than Covid.