Long-term efficacy and absence of side effects of anti-covid vaccines
You are correct that we have been creating and approving vaccines for a very long time, so the procedures for looking at efficacy and side effects of vaccines in general long-term are quite well developed. They occur via so-called &quot;Phase IV&quot; trials, also known as post-marketing surveillance.
However, one complicating factor with at least the Pfizer/BioNTech and Moderna vaccines is that they're the first mRNA vaccines ever approved. While I hope the companies have been following some of the volunteers from previous clinical studies of other mRNA vaccines, we're in somewhat uncharted territory right now. The vaccines appear to be safe and effective, at least on the scale of tens of thousands of Phase III test subjects, but we just don't know exactly how they'll behave long-term. They were approved in the US as &quot;Emergency Use&quot; products, meaning the upside of having them available outweighs any known or potential downsides at this point.
So, the technical answer to your question &quot;how does one prove the long-term efficacy and absence of side effects in such a short time?&quot; is that they can't prove it. Based on the available data to date, there's no reason to suspect that some previously-unobserved deleterious effect will become common, but we can't be 100% certain until the products have been in widespread use for years and possibly decades. However, given the seriousness of COVID-19 and its currently unchecked spread across much of the world, the benefits of having &quot;highly, strongly, very most likely safe&quot; vaccines (that are also very effective) are huge.