COVID waves: Europe and US compared

Waves have been roughly synchronized, but mortality impacts have been anything but similar

The US and Europe have gone through several waves of COVID. How closely did they synchronize? And what about their impacts? This post compares COVID waves across and within Europe and the US. It dissects the patterns across the four Census Bureau regions of the US and the four continental regions of Europe. A look underneath the aggregate picture reveals that while cases are relatively synchronized, mortality impacts have been anything but similar across regions and over time. 

Roughly synchronized waves

Focusing first on the infectious waves themselves, the chart below plots the evolution in daily cases per capita across the regions of Europe and the US. We show this as mirror images, with the US at the top and Europe at the bottom. The outer contour lines are group averages and the stacked values (the vertical distance spanned by each color code) are the contributions of individual regions to the group average.  The chart thus compares trends in case rates between Europe and the US as a whole and across the four different regions (each) that they consist of.   

COVID waves across Europe and the US have been roughly synchronized. That is hardly surprising given the various common factors at play:

  • Seasons. As US and Europe are both in the Northern Hemisphere, the end-of-year winter period fuels case growth as people spend more time in indoors. 
  • Holiday and school calendars. Holiday schedules and school calendars show similarities and thus contribute to spread when adults return to work and kids to school when schools reopen.  
  • Variants. The spread of new variants, which over the course of the pandemic have become more transmissible, is also a common factor.
  • Fatigue. Also similarly shared across the pond is pandemic fatigue, which has undeniably assisted the more transmissible variants in finding suitable hosts. 
  • Testing. Differences in testing also help explain things. A key reason for low case rates in both the US and Europe during the first wave was limited testing. A key reason for low case rates in recent months is…. limited testing.

But the association is far from perfect and there is much variation among the underlying regional drivers of the aggregate case rate. In the US, the South and West regions seem to be contributing the most. In Europe, Western Europe appears to be the main driver. Of course, contributions may be large not only when regional case rates are high, but also when the population share of the region in the country/continent is high (a point explored in more detail later on). 

Progressively smaller mortality impact

The chart below is an exact replica of the earlier one, except that it now shows the trends in daily mortality rates. Again we show Europe and the US as mirror images, the outer contour lines reflect the mortality rates for Europe and the US as a whole, and the regional values are stacked to represent the contributions to the overall mortality rate.

The general impression is that the mortality impact was more severe for the US, but the patterns differ considerably depending on the time period:

  • Initial wave of 2020. The initial wave produced a higher peak in Europe but the tail was longer in the US. 
  • Summer of 2020. The US continued to sustain high mortality rates in the summer of 2020, which were largely avoided in Europe. 
  • Winter 2020/21. Europe and the US were both affected the worst during this period, with the highest peaks observed in the US.
  • Autumn 2021 through spring 2022. The spread of Delta and subsequently Omicron raised mortality for a long time, again with higher peaks in the US. 
  • Post-spring 2022. Mortality rates decreased considerably, even though again the US sustained higher rates than Europe. 

What stands out however is the regional diversity underneath the averages:

  • In Europe, the initial wave was carried very broadly by all regions except Eastern Europe. During most of the rest of the pandemic, however, Eastern Europe became the main contributor to European mortality. This is especially visible during the Delta/Omicron period. 
  • In the US, the Northeast carried the brunt of the impact of the initial wave, but later on made way for other regions that contributed the most to the US mortality rate.  The South and West regions of the US played a disproportionate role during most of the pandemic period. 

Underneath the aggregate picture

Let us now look at the actual case and mortality rates across regions – as opposed to the contributions to such rates at the aggregate. We will compare them with simple line charts and also, less conventionally, as contour charts. The latter will allow us to identify how long certain regions have been outliers relative to their regional peers. 

US regions

Starting off with US regions, the chart below shows daily confirmed cases and deaths divided by the population of the region. Note how the scales in the chart below have a larger range. That is because the earlier contributions looked at cases and deaths divided by the population of the aggregate (Europe or US as a whole).  In other words, the values shown here are scaled up by the inverse of the population share of each region. 

Two impressions obtain. One, we notice that the regions like to huddle together: the trends are highly correlated. This is particularly so for cases and less so for mortality. Two, major outliers pop up left and right.  Let us look at these in some more detail now. 

The chart below shows a multi-colored line, which is the upper contour of the case rate. For each date, this line shows the maximum case rate across regions and colors it by color code of the region that the maximum belongs to. The dashed line in black is the case rate for the US as a whole, which by definition (maxima always weakly exceed averages) wiggles below the contour line. Also featured is a box that displays the percentage of time (number of days in total pandemic days) that a particular region tops the ranking among regions. 

It appears that US regions are quite “democratically distributed” in terms of their outlier status with respect to cases.  The South region exceeded other regions during 32% of the full pandemic period, the West region 19% and the other regions somewhere between these extremes.

The chart below plots the contour lines for daily mortality rates. It reveals a totally different picture than what we saw for case rates. 

Mortality rates across US regions were initially dominated by the Northeast. But in later stages the region hardly popped up and over the full course of the pandemic it claimed outlier status only 16% of the period. Contrast that with the South, which topped the regional ranking for mortality rates 48% of the time. Notice also that the lowest value is for the West at 12%, with the Midwest claiming the remainer of 24%.

European regions

We can now repeat the same exercise for Europe. The chart below plots case and mortality rates across Europe’s four continental regions. The patterns, both for cases and deaths, are far less homogeneous than they were in the US. Of course, we are dealing with different aggregates, with Europe representing an entire continent and its regions groups of countries, so we would expect more heterogeneity. But the differences among them are nevertheless instructive.

The above suggests that Western Europe seems to have had the highest case rates on several occasions. But on the mortality side, we see greater divergence, where the trends in Eastern Europe, which rises to ever greater prominence as the pandemic matures, are particularly notable. 

What do the contour charts tell us about the outliers?

The above chart on case rates confirms the dominance of Western Europe in the rankings across European continental regions: it is at the top 38% of the time. Next in line are Southern Europe (at 25%) and Northern Europe (at 28%). Eastern Europe only featured 10% of the period as an outlier – a result we can partly attribute to differences in testing particularly in light of the region’s high mortality rates as discussed next.

The above chart shows the contours lines for mortality rates across European continental regions. Notice how Eastern Europe stands out: it claimed the highest mortality rate 47% of the time. Conversely, Western Europe was an outlier over this period close to 0% of the time. Blame this on differences in vaccine coverage, where Eastern Europe has lagged global and European levels on both primary and booster vaccination. Southern Europe and Northern Europe registered high mortality rates too, particularly in the earlier spring wave in 2020 and the winter wave of 2020/21. They topped the rankings 33% and 20% of the pandemic period, respectively.

Parting thought

While the evolution of case rates appears roughly synchronized across Europe and the US, this aggregate picture changes considerably when we look at the diversity underneath, especially, the mortality statistics. The amplitude of the mortality impact varies widely across regions. The regions of Eastern Europe and the US South have registered mortality rates that were higher than any regional peer during 50% of the entire pandemic period. 

The usual caveats about measurement apply. Self-testing has become more common and this leads to fewer positives being centrally reported. The diminished severity of Omicron infection among better-immunized populations has also played its part, as fewer people end up in hospital where they normally would be tested and counted. 

But the fact remains that actual fatalities are less easily ignored or obfuscated than suspected cases. In light of this, we should attribute greater credibility to the mortality picture, the sheerly concentrated nature of which has illustrated how unevenly the pandemic has impacted upon both Europe and the US. 

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