Covid risk and blood type O

23andMe reported findings that people with type O blood had a lower rate of COVID-19 than others in a recent blog post. The study looked at 23andMe customers and volunteers who had been diagnosed with COVID-19, and compared them to customers without a COVID-19 diagnosis. The 23andMe genetic tests discovered that the genotype for type O blood is associated with having 88% of the chance of a COVID-19 diagnosis than those without. This finding echoes several previous studies.

Do masks help part II: A study claiming a natural experiment in Germany

The claim is made in "Face Masks Considerably ReduceCOVID-19 Cases in Germany:A Synthetic Control Method Approach", Mitze et al., that by using synthetic controls, the effect of mandatory face mask use can be measured. Mitze et al. estimate that masks reduce the rate of reported infections by around 40%.

I don't find this paper convincing.

Do masks help part I: Methodological Limitations

One of the big themes in the popular news on COVID research these days is studies that report on how much masks help reduce the spread of COVID. These studies generally have some serious methodological limitations; I have yet to see one that can distinguish between the hypothesis that masks limit the spread of COVID by blocking stuff from the hypothesis that people wearing masks will be more mindful of other restrictions, and thus reduce the spread of COVID due to better adherence to social distancing and hand-washing guidelines.

This is the start of a series of posts discussing some of these studies. This post will give an overview of the different types of evidence typically used to argue this point; specific studies will be addressed in future posts.

Comparing reported rates of COVID across communities is hard

On June 10th, there were 2,000,000 COVID cases in the US, with 113,000 deaths. In California, there were about 130,000 cases, with 4,600 deaths. What can we conclude from this? California has slightly over 10% of the US population; does this mean it has a lower infection rate than the country overall (even after removing New York and New Jersey’s infections)? Is its 3.5% fatality rate evidence of better medical care than average for the US with a 5.6% fatality rate? Or the world’s 5.7% fatality rate?

We can conclude nothing.

Be careful who you measure

One of the challenges with the observational studies conducted on COVID-19 is that the studied population may be different from the larger population, limiting the validity of drawing conclusions for making policies for general populations. Beyond that, many of the studies either take advantage of differences that may have unusual or undocumented causes, or attempt to match to other, similar people. However, differences may exist that were not measured or used in the matching that can affect the conclusions.

Be careful what you measure

When evaluating studies, a lot of subtle mistakes can under-cut the conclusions authors claim. Most health studies can be separated into one of two categories. Randomized clinical trials occur when people meeting some criteria are randomized into treatment and control groups. This is widely considered the gold-standard in how to do clinical science. However, it is not always practical or ethical to perform randomized clinical trials, so instead a lot of science is done through observational studies. Because groups being compared are not otherwise identical, measurements are taken, and scientists use models to correct for these differences. Unfortunately, what is measured can be a proxy for what actually is causing the effect of interest, so we need to be careful that when we observe a difference between two groups based on an observable difference. What we base our group separation on may not be causing the differences observed!

Put another way, correlation does not imply causation, however much we wish it did. One example many people find amusing is that Ice Cream sales are correlated with murder rates - both go up in the summer! So even though we can estimate murder rates based on Ice Cream sales, reducing Ice Cream sales will, I believe, have no discernable impact on murder rates.

Addendum to Cox Proportional Hazard Model results may not mean what you think they mean

A recent paper in the New England Journal of Medicine reports on a double-blinded clinical trial that was ended early due to beneficial effects. “Remdesivir for the Treatment of Covid-19 — Preliminary Report” It reports a median time to recovery of 11 days for those treated, as opposed to 15 days for those in the control group. There is a VERY low chance of this being a difference due to chance. The 14-day mortality rates of 7% for the treatment group and 12% for the control group were not statistically significant. If we conclude that Remdesivir has no effect on survival rates based on this non-significant difference (not a conclusion I’d support), then this appears to behave like the hypothetical drug of the previous post on the issues with the Cox Proportional Hazards model. Thankfully, this was not how they analyzed the relevant data.


Disclosure: I know, and have written papers with, people involved with the study.

Caution: Cox Proportional Hazards Model results may not mean what you think they mean

The currently statistical methods for identifying specific risk factors and possible beneficial treatments for Covid-19 are asking the wrong question. Many, if not most, of these studies use analysis of patient hospital records as their primary data source. The standard statistical analysis asks what rate people die in the hospital under different conditions. Unfortunately, under a simple scenario, this can conclude a beneficial medicine that also reduces the length of hospital stays of patients who will recover is harmful.