Dozens of media outlets have run stories today on a study that seems to show a link between baldness and the chance of severe coronavirus infection.

But on closer inspection, the relationship isn’t so clear.

The study

Researchers in the US and Spain teamed up for this study of 175 coronavirus patients who were treated at three hospitals in Madrid during March and April.

The “pre-print” version of the paper, which is due to be published in the Journal of the American Academy of Dermatology, is titled: “Androgenetic Alopecia Present in the Majority of Hospitalized Covid-19 Patients”.

The study does not adjust for age, which is known to be the biggest single risk factor when it comes to getting severe, or even fatal, coronavirus illness.

However, the paper does make some initial comparisons. The researchers found 79 per cent of men in their sample who were hospitalised with coronavirus were bald, versus a 31-53 per cent baldness rate in an “age-matched”, “similar Caucasian population”.

Put side by side, that seems to show a very striking relationship between hair loss and the chances of severe covid-19 illness – and one that does not appear to be explained by age.

The limits

But there’s a catch. The source for the claim that 31-53 per cent of people in a “similar Caucasian population” are bald is an Australian study from 2003 which looked at men aged between 40 and 69.

That’s a problem when we consider that the age group most at risk from coronavirus complications is the over-70s.

As the authors of the Madrid study say: “The precise AGA [androgenic alopecia, the type of baldness that most often affects men] rate in age-matched, not admitted covid-19 population is still unknown to draw further conclusions.”

In other words, until we have more precise data on the rate of baldness in the general population broken down by age, we cannot say with any certainty if there is a correlation between hair loss and severe coronavirus.

What do we know about baldness among older people?

That said, there is other research that could shed some light on hair loss rates among the elderly.

A 2005 study of 739 Danish twins found that just 28 per cent of men over 70 in their sample had “almost all hair intact”.

That means the remaining 72 per cent had some degree of baldness (as measured on the same scale used by today’s Madrid researchers).

These findings are consistent with a 1975 study of baldness in Caucasian men, which found 70 per cent were bald by the age of 79.

We’re not in a position to make “age-matched” comparisons with the patients in the Madrid study.

But if it is as common for men over 70 as this research suggests, it seems the rate of hair loss among severely ill coronavirus patients in the Madrid sample is about what we’d expect, given baldness in the wider population of those most likely to have severe covid-19 (the elderly).

Further caveats

In any case, we should remember that today’s report is a study of relatively few patients (175) in a small geographic area (one city), and that, as the researchers describe: “The patients were randomly examined by dermatologists that were assisting with the overwhelming amount of admitted patients”.

All of these factors leave open the possibility that we are not seeing a truly representative snapshot of covid-19 patients from which to draw wider conclusions.

And of course, the comparisons with a Caucasian population are not very helpful when we don’t know the ethnic make-up of the patients in the Madrid sample.

FactCheck verdict

A medium-sized study of patients in three Madrid hospitals has attracted attention today after it appeared to show a strong correlation between baldness and the chance of severe coronavirus illness.

The study does not adjust for age, which we know is the biggest single risk factor for serious covid-19 disease. However, it does compare the rate of baldness among severely ill patients (79 per cent) with the level of baldness in an “age-matched”, “similar Caucasian population”, which the researchers say is estimated at 31 to 53 per cent.

This suggests a much higher rate of baldness among severely ill coronavirus patients than would be expected given levels of hair loss in the general population – and that the relationship cannot be explained fully by age.

But there’s a catch: the “31-51 per cent” baseline comes from a study of men aged 40 to 69. So it doesn’t tell us about men over 70, the group at the highest risk of severe coronavirus illness.

Separate studies have estimated that around three quarters of Caucasian men aged over 70 had some degree of hair loss. So it’s possible that the rate of baldness among patients in the Madrid sample is about in line with what we’d expect in men of that age. Though, as the researchers concede, we can’t say for certain without more detailed data on hair loss in the wider population.