Unlikely worst case: there are rare side-effects that get only discovered very late. For example: 70% of those who get the vaccine develops liver complications

Unlikely worst case: there are rare side-effects that get only discovered very late. For example: 70% of those who get the vaccine develops liver complications within 3 years and 60% die a horrible death with no way to cure. Or every women who gets the vaccine becomes infertile within a decade. This is super unlikely, and moreover could only partially detected in a Phase 3. But the longer you give people to look for such complications, the more likely that they're discovered before mass-application. Realistic worst case: there are some minor complications, and some people will get hurt. This has happened before: https://www.cdc.gov/vaccinesafety/concerns/concerns-history.html https://www.cdc.gov/vaccinesafety/concerns/concerns-history.... , which is why we have such strict guidelines for vaccine development today. Realistic bad case: the vaccine is not at all effective and does absolutely not protect from (re-)infection, but gives everyone post-covid19-fatigue. It will also be giving anti-vaxxers actual arguments and getting Trump re-elected.