Weekly seasonality in COVID-19 deaths reported in Maryland

On its dashboard, the Maryland Department of Health is reporting confirmed deaths due to COVID-19 in two ways: by date of report and by date of death (updated as amendments to the death record are received). The definition of confirmed death is:

A death is classified as confirmed if the person had a laboratory-confirmed positive COVID-19 test result.

What I was intrigued is that reporting seems to follow a pattern influenced by the day of the week (see figure below). The top chart (cumulative) is just an addition. A plateau would be welcome: it would indicate death rate is slowing down. Today, the COVID-19 death rate is 41 / 100,000 population. The bottom chart shows the number of deaths due to COVID-19 reported each day: the black line represents the number of deaths each day they were reported; the grey line represents the number of deaths each day they occurred.

Evolution of coronavirus confirmed deaths in Maryland, as of June 3, 2020

One could see that in both lines, there are two kind of patterns. The first in an overall trend upwards until beginning of May, followed by a decrease since then. The second trend has a big peak being followed by a decrease with 2 smaller peaks and a big dip – then an up, decrease with 2 peaks and a big dip – etc.. As data was reported, we saw intuitively that the big dip came on Sundays, the big peak on Tuesdays and the rest of the week was a decrease towards Sunday.

And this is confirmed by the analysis of seasonality for confirmed death by reported date:

Here, the top chart is just the data we observed before. Below, the trend shows that, indeed, there was an increase up to end of April and we then see a slow decline. The third graph (“seasonal”) shows the pattern I mentioned earlier. This confirms the lowest reporting on Sundays and the highest reporting on Tuesdays. The bottom chart (“irregular”) shows that, even if there is a pattern, there are a lot of irregularities added to the seasonality.

The same patterns can be observed for the deaths by date of death (when they occurred; see chart below). This shows we are currently also in a decreasing number of deaths, each day (fortunately!). The pattern here is that the number of deaths increase from the lowest on Saturday to the peak on Friday (with an intermediary peak on Wednesday). Again, note the important number of irregularities (at the bottom).

In my opinion, this regular patterns come from the reporting system. I don’t see why COVID-19 patients would die more towards the end of the week and less during the weekend. But please tell me if you have more information about this (in the comments below or by email)!

To be continued …

As usual, you’ll find other graphs on my page about COVID-19 in Maryland and the data, code and figures are on Github.

P.S. I’m not counting probable deaths. The MD Department of Health reports this variable but, as it is dependent of a confirmation, it is highly fluctuating and not necessarily representative of deaths due to COVID-19. If confirmed, these probable deaths are accounted in the confirmed deaths (counted here).

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s