There is one argument in the vaccine sceptics’ list that sounds more reasonable and scientific than others. It’s the concern about the unknown long-term effects of the mRNA vaccines.
The technology we are using for these vaccines is new – so the argument goes – and the trials have monitored side effects over a little more than one year. Shouldn’t we be concerned about the possibility of longer-term side effects?
I find the argument unconvincing because of three wrong assumptions that it is based on.
The first one is the idea that this technology was invented from scratch and in a hurry during the pandemic. The technology behind mRNA vaccination is at least 20 years old1. Multiple human trials have been run in that period, for a number of different diseases. Before 2020 these attempts hadn’t ended up yet in approved vaccines because of a mix of challenges, both logistic and safety related. Crucially, the safety challenges were all about conventional, well understood short-term side effects 1 (allergic reaction, immune response etc.), which are not present in the COVID-19 vaccines. No long-term effect has been observed.
The second wrong assumption is that the alternative to vaccination is just living our lives as if nothing happened. Unfortunately this is not true. The alternative to vaccination is the virtual certainty of contracting the virus, sooner or later. And unlike the mRNA vaccines, we already know for sure that COVID-19 does have long-term effects – and bad ones, even for young people2.
The third implicit assumption is a wrong idea about the way science works: people concerned about long-term effects seem to think that the only way we know about things in science is through empirical observations. That the only reason we know the vaccines are safe in the short-term is because we collected short-term observations, but we can say nothing about the long-term effects until we get long-term data.
But that’s not how the scientific method works in practice. When a scientist has high confidence in a particular prediction (e.g. the prediction “this vaccine won’t cause long-term side effects”), that high confidence is only in part due to immediate empirical evidence. A big part is actually linked to how consistent the prediction is with pre-existing theories that do have good empirical evidence3.
To go back to the vaccine, there are no known biological mechanisms by which the mRNA vaccines should have long-term effects. The “induced DNA mutation” some people are worried about is simply impossible, because of the known difference between mRNA and DNA (as well as the fact that the mRNA injected with the vaccine gets quickly degraded, disappearing in a few hours). Which means the prediction “the vaccine won’t cause long-term side effects” is much more consistent with our existing relevant knowledge (genetics, epidemiology, virology etc) than the prediction “the vaccine will cause long-term side effects”. And this fact should greatly affect our confidence about the safety of the vaccine – even before collecting any long-term observation – since our existing relevant knowledge has a great track record at predicting things.
Does it mean that mRNA vaccines are definitely safe long-term? Of course not. Theories are always imperfect and we never know where they’ll break.
Does it mean it is reasonable to be anxious about that? Not more reasonable than being anxious about Wi-Fi. Most of us have been using Wi-Fi networks for less than 20 years now. So far they don’t seem to be a health hazard, but how can we be sure they won’t cause brain cancer after 25 years of exposure? Technically we can’t, because we haven’t collected data for that long. And we are talking about electromagnetic fields fluctuating around our heads 24/7. Are the vaccine sceptics losing sleep over that? I bet they aren’t, because (consciously or not) they trust the relevant theories about electromagnetism and brain physiology. They may as well trust our theories about genetics and help us end this pandemic.
 In the words of physicist and philosopher David Deutsch, our scientific theories “have reach” to unobservable facts about the world.