Historically, most vaccine problems show up early. First, there's the risk of an immediate allergic reaction, which, if it's going to happen, happens within minutes to hours.

Then there's the risk of an inactivated or weakened virus vaccine causing the disease it's supposed to prevent. That happened in a big way in 1955, with a defective batch of polio vaccine. The mRNA vaccines can't do that - they don't have an entire virus.

There's the Guillain-Barré syndrome with some vaccines, including the Janssen COVID-19 vaccine. If it shows, it does so in the first two months.