> Simple math: if Pfizer says their vaccine is 90% effective at preventing severe disease then 10% of people are at risk of getting severe disease.

That’s not quite how vaccine efficacy works. Suppose the risk of getting sick is 0.5% per thousand person hours when unvaccinated. A 90% vaccine efficacy means that the risk of getting sick is reduced to 0.05% per thousand person hours, not that 10% of people who get vaccinated are still vulnerable.