Thread by @GidMK

Ugh both the headline and the study it's based on are...pretty dubious

Let's look at some science on twitter 1/n

2/n Paper is here, it's really your bog-standard epidemiological study. The authors took a large database of people who had tested positive to COVID-19 and compared them to people who hadn't in Israel
sciencedirect.com/science/articl…

3/n The two groups were different in loads of ways. However, after putting all the variables into a statistical model, they found that asthma appeared to still be significantly connected to risk of COVID-19!

4/n They then went on to suggest that this statistically significant result may mean that asthma is protective against COVID-19, although the mechanism is unknown

5/n Now, most epidemiologists are probably already groaning, because this is a very beautiful example of the Table 2 fallacy, which is a pretty common problem

6/n The basic issue here is that you have to be VERY CAREFUL with your statistical models. If you just lump everything in together it can make the results a bit meaningless

7/n There's also a problem in this study that boils down to selection bias

You see, the study wasn't looking at every person infected with COVID-19

It was looking at everyone who TESTED POSITIVE TO COVID-19

8/n This creates an issue known as selection bias. The group of people that is selected to be studied (due to who got a test for COVID-19 in Israel earlier this year) may not be representative of the population in crucial ways that could have biased the results

9/n For example, we know that asthma diagnoses are very variable by many characteristics, which may also make people more or less likely to go get tested for COVID-19 (young men, for example - lots of asthma, might avoid testing)

10/n It also makes correcting for confounding challenging, because we do not know which biases are influencing the SELECTION of participants

11/n Ultimately, it's a bit hard to say more than that people with asthma were less likely to have a positive COVID-19 test in the group of people presenting for testing in Israel earlier this year

12/n While that's a bit less sensational than the headline, it's also a bit more true

13/n I should also note - the study itself is fine, generally, but the limitations section doesn't really discuss some of the biggest drawbacks of their methodology which I don't love

14/n Anyway, the most likely explanation to me for fewer people with asthma getting COVID-19 is that we told them to be careful not to catch the respiratory infection because they could easily die and so they were more careful

Inhalers? Ehhhhh