You may have heard that some European countries have postponed the use of what is called the AstraZeneca vaccine against Covid-19 amid fears that it might lead to blood clots. (For what it's worth, that vaccine hasn't yet been approved for use in the U.S.)
This Washington Post article [paywall, probably] argues that such fears are unfounded. Public health officials state that there is no proven link between taking that vaccine and the onset of blood clots. They also state that what evidence there is seems to back them up.
The only evidence for the supposed blood clot/AstraZeneca link is "anecdotes." Some people who have taken the vaccine have had blood clots. And the public health officials claim that the incidence of clotting is actually somewhat lower among AstraZeneca takers than what we might expect among the general population.
Fine. I buy the argument and accept it. (And for what it's worth, I'm terrified of blood clots. They can kill you. And the main way to treat and manage them is to take blood thinners, which have a narrow therapeutic range and come with their own complications, such as nosebleeds that are difficult to stop. As a epistaxiphobe, that terrifies me, too. I have also heard, though I'm not sure how well-established this point is, that Covid-19 itself can lead to blood clots.)
Unfortunately, the same article relies includes these two paragraphs, one after the other and without a hint of self-aware irony:
Evans [Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine], who is 77, said he has received two doses of the vaccine and did not experience any serious adverse effects.Anecdotes are not what epidemiologists deal with....unless they're relating their own personal experience with the vaccine.
The current concerns surrounding the vaccine are an example of “anecdotes being turned into data, which is not what epidemiologists deal with,” he said.
I don't really mean any of this as a criticism of Mr. Evans. The reporter probably asked him, "have you taken the vaccine and experienced any adverse effects." He probably honestly said, "no," and then went on to explain that anecdotes aren't data.
I also don't mean this article's use of anecdotes to combat other anecdotes disprove the article's argument.
I do suggest two things, though. The first is that this is sloppy reporting, or sloppy editing. The Washington Post article seems clueless about the irony here. It uses an anecdote to refute other anecdotes, all the while insisting that we shouldn't rely on anecdotes.
Second, it's quite possible that anecdotes are indeed "data." Almost by definition, anecdotes are not systematic data. They're not collected and related in a controlled process, with a control group. But they are one piece of real, lived experience. We shouldn't build public health policy on anecdotes, at least not as a general rule. But they might clue us in to something. Early studies and comparisons with general populations might simply be wrong. And that one nugget of information we dismiss as "anecdote" today might prove tomorrow to have signaled something.