The main non-mRNA alternatives were the adenovirus vector vaccines, which caused blood clots in young people - quite a few countries ended up withdrawing them f

The main non-mRNA alternatives were the adenovirus vector vaccines, which caused blood clots in young people - quite a few countries ended up withdrawing them from use in those age groups because of this. Novavax, which is a more traditional protein subunit vaccine, is probably safer (and certainly seems to be better tolerated in terms of the more common side effects) but wasn't available until late on in the pandemic, still hasn't been approved in the US, and doesn't have a huge amount of real-world evidence because it arrived too late to be widely used. Safe vaccine design is just hard, as is determining their safety. Anyone who'd paid attention to the history of emergency pandemic vaccinations would've known this, there was just a push to ignore it largely for US domestic political reasons.