A while ago I ran into a report of an alleged side effect of the COVID-19 vaccine. It was a “this happened to my cousin” story, so it’s low on credibility. Some news outlets, though, claimed it couldn’t be true because there was no previous report of this effect.

That’s not a valid refutation. The vaccines are fairly new, and it’s plausible that a few people could have side effects that weren’t previously recorded. The media argument amounted to “We never heard of it before, so it couldn’t have happened.”

That’s sometimes called the black swan fallacy. Someone might say, “I’ve never seen or heard of a black swan, so there can’t be any.” Plumage is variable within families of birds, so that’s not a good argument. In fact, there is a black swan, Cygnus atratus, which is native to Australia. I hadn’t known that till I researched this post.

When analyzing claims in your writing, you have to keep two considerations in mind First, the burden of proof is on someone who makes a positive claim. If someone says “There are black swans,” you should expect some supporting evidence. In this case, there’s plenty on the Internet. In many cases of unusual reactions to vaccines, there’s nothing but rumor.

However, the second consideration is that things that haven’t been encountered before can happen. Coelacanths were thought to have been extinct for millions of years, and the first reports of live ones met a lot of skepticism, but it turned out there are still living coelacanths. There can be odd individual reactions to medical treatments.

(An additional point: If a side effect is real but there’s only one confirmed instance, that suggests it’s extremely rare, and people shouldn’t worry about it.)

The unproven and the impossible are two different things. You should insist on evidence for unlikely claims, but you need more than a lack of previous confirmation to declare them debunked.