That's my first guess too, but these long terms effects seem fairly rare (esp. cytokine storm-related) and I'm not convinced the trial population would be big enough to take this into account.

Moderna's third stage trial consisted of 30,000 recipients (presumably that means 15,000 since half of them would have received a placebo.) Added with the previous two trials, you'd expect even the rarest reactions to start showing up already.

You can expect that if there are indeed super-rare novel responses, they'll come to light over the approximately year-long roll out.

Additionally, even though vaccines will be rolling out, Moderna is holding 88 trials over the next 2 years. So they're not just ending the trials and injecting everyone they can.

That's just the one company. Pfizer and the others should be similar.