Does COVID Make People Stupid?

Edit: the answer appears to be reverse causation. COVID does not make people stupid.

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4884504

ORIGINAL POST

Anatoly Karlin recently posted a study about whether COVID infection could have caused decreased IQs:

Of the 141,583 participants who started the online cognitive assessment, 112,964 completed it. In a multiple regression analysis, participants who had recovered from Covid-19 in whom symptoms had resolved in less than 4 weeks or at least 12 weeks had similar small deficits in global cognition as compared with those in the no–Covid-19 group, who had not been infected with SARS-CoV-2 or had unconfirmed infection (−0.23 SD [95% confidence interval {CI}, −0.33 to −0.13] and −0.24 SD [95% CI, −0.36 to −0.12], respectively); larger deficits as compared with the no–Covid-19 group were seen in participants with unresolved persistent symptoms (−0.42 SD; 95% CI, −0.53 to −0.31). Larger deficits were seen in participants who had SARS-CoV-2 infection during periods in which the original virus or the B.1.1.7 variant was predominant than in those infected with later variants (e.g., −0.17 SD for the B.1.1.7 variant vs. the B.1.1.529 variant; 95% CI, −0.20 to −0.13) and in participants who had been hospitalized than in those who had not been hospitalized (e.g., intensive care unit admission, −0.35 SD; 95% CI, −0.49 to −0.20). Results of the analyses were similar to those of propensity-score–matching analyses. In a comparison of the group that had unresolved persistent symptoms with the no–Covid-19 group, memory, reasoning, and executive function tasks were associated with the largest deficits (−0.33 to −0.20 SD); these tasks correlated weakly with recent symptoms, including poor memory and brain fog. No adverse events were reported.

In the following figure, the researchers estimate the effect of COVID infection on cognitive ability using linear regression in the solid bars and propensity score matching in the lined bars, where they compare people who are matched in demographic variables like sex, race, and age. People who suffer from “unresolved COVID” suffer from the greatest losses in cognitive in cognitive ability. Even as a long-COVID skeptic, that does look somewhat concerning.

I looked through their supplement, and found that within their regression model, taking the vaccine once or twice was associated with slightly lower levels of cognitive ability (b = 0.057, t = 4.1 for two doses). Given that intelligence assessed prior to dose is associated with slightly greater probabilities of taking the vaccine (at least in Sweden), it is likely that the vaccine causes some cognitive decline (0.8 IQ points, based on the observed effect). In the case of infection, the decrease is about 3.5 points; because of self-selection effects, I don’t think it would be appropriate to compare these effect sizes.

As for whether getting COVID causes cognitive decline, there’s the problem of self-selection - cognitive ability could be causing people to act in certain ways, and those actions could be causing infection. The time trend of the effect is a complication here - you would have to hypothesize that the selection effect differs across time.

It is always difficult to make causal claims about correlative data, and medical science has a tendency to replicate poorly. That said, the study is of reasonable quality - they had more than enough people to detect any effect size (n = 100k), they did control for things that should be controlled for, and all of their statistical analysis methods suggested the same results.

There is also the question of how the effect works - a German man took 217 COVID shots and was still human enough to interact with police, so it can’t be the case that the vaccine lowers IQ every time it is taken. Taking this into consideration, I propose the following potential realities:

• The vaccine/COVID lowers IQ at first exposure by a small amount, but not subsequent ones.

• The vaccine/COVID lowers IQ, but only within some people. There may or may not be subsequent exposure effects.

• One of the effects is real, but the other isn’t.

• Some combination of the three realities above (e.g. only COVID infections lower IQ at first exposure, and only in some people, but the vaccine has no effect)

• The paper’s results are not correct.

I would give all five realities roughly the same probability of being true. It is possible that one of the effects exists but not the other, but because the infection and the vaccine are physiologically similar, if one of the mechanisms has an effect, then that increases the probability that the other should have an effect as well. Because the infection is more severe than the vaccine, the theoretical case for it reducing IQ is stronger, but the evidence for the vaccine lowering IQ is stronger, as the self-selection effect is already known to go in the opposite direction.