Was there a spike in COVID-19 cases due to the differently-timed lockdowns along the Irish border?

Short answer: no

Andrew Parnell https://www.maynoothuniversity.ie/hamilton (Hamilton Institute, Maynooth University)https://www.maynoothuniversity.ie/hamilton , Bruna Wundervald https://www.maynoothuniversity.ie/hamilton
07-27-2020

tl;dr

Background

Back in April, there was a bit of a media storm over whether a spike in cases along Irish border (notably Cavan) was caused by the differential travel restrictions between Northern Ireland (NI) and Ireland This was most vocally stated by Dr Gabriel Scally, the President of the Epidemiology and Public Health section of the Royal Society of Medicine, in the Irish News, but was also picked up in the Irish Times, and the Guardian amongst many others.

The key point raised in the article was:

“Co. Cavan has the highest incidence rate of coronavirus in the south per 100,000 of the population, followed by Dublin and Monaghan.”

At this particular point in the pandemic, there were two slightly different lockdown strategies. Ireland had broadly a two-stage strategy, with schools, many workplaces, etc, closed on the 12th March, with everyone advised to stay at home from 27th March. Meanwhile the UK government closed bars on the 20th March, but didn’t enter into a ‘full’ lockdown until 28th March.

In theory, this means that those who travel to school or work in NI might have been doing so during those weeks between 12th March and 20th March, and so transferring the virus across the border.

The issue of the border county spike was downplayed by some of the senior NPHET members, most notably Tony Holohan said that it was ‘unlikely to be a spillover’:

“This is a disease and you get regional concentrations”

However, no-one seemed to really quote any evidence for or against these patterns. We thought we would have a look at traffic data to see if we could make a more informed opinion.

Was there a spike in cases by county?

If you download the county data for Ireland and plot the number of cases per 100k residents you will see the following:

Hover your mouse over the graph to identify individual counties. It’s not that clear to see the differences between the counties so let’s highlight the border counties of Cavan, Donegal, Leitrim, Monaghan and Sligo:

It certainly seems that Cavan and Monaghan are very high, with Cavan above even Dublin when corrected for population size. But perhaps some of the others (Leitrim, Sligo, etc) are pretty in line with the others. Was this due to the transport across the border? Or was it just regional variation in diseases due to something else, such as older populations, more care homes, etc?

Traffic data

Transport Infrastructure Ireland have a wonderful website where you can get hourly traffic counts for all of the stations in the country:

Each one of these green dots records the number of cars crossing that station in each direction every hour. We downloaded all the hourly traffic counts for all the stations to see when people were travelling and where they were doing.

To help with the understanding of this analysis, you can imagine a few different hypothetical scenarios:

  1. Aoife lives in Dundalk and is constantly out and about. After the lockdown started she can still see her friends in Northern Ireland carrying on as normal so she does pretty much the same. So we might expect the traffic stations around Dundalk (e.g. on the M1 or the N52) to record higher traffic activities than normal compared to non-border counties.
  2. Brendan lives in Letterkenny but likes to do his shopping in Strabane. After the lockdown started he carried on doing his shopping there because everything was still open, rather than shopping more locally. The station on the border on the N14 thus might still see a high count throughout the day as people travel back and forth to do their shopping compared to other stations in the same county.
  3. Caiomhe lives in Cavan and travels to work every day in Enniskillen. When the lockdown started on 12th March in Ireland she still had to go to work. The traffic station on the N3 at the border would still be recording a count of her car going to work in the morning and coming home in the evening, but the count of cars going south in the morning would be low, because people in the North are not travelling south to work.

Let’s see if we can check if whether Aoife, Brendan, or Caiomhe were guilty of that later spike in the border counties. Reminder: these are all hypothetical people! Other reminder: all the traffic station data we have (and all the cases by county) are from Ireland. Unfortunately, right now, we don’t have any comparable data from Northern Ireland.

Results

First up, here’s a plot of all the stations in the country by date. We’ve marked the two Irish lockdown dates of 12th and 27th March as black vertical lines. You can see that there is a big effect of the first lockdown and a levelling off after the 27th. Some of the stations have very high numbers of cars (over 150k per day); these are likely to be on the main motorways