What role did the summer holiday season play in increasing COVID-19 infections in Germany? Exploiting the staggered nature of school holidays in Germany, Thomas Plümper (Vienna University of Economics) and Eric Neumayer (LSE) estimate the effect of school holidays on growth in infections across Germany. They find that the estimated effect equates to 48.7% of the average growth rate across German districts during their respective final week of holidays. The effect differs across the 16 federal states and is conditioned by how rich a district is and by the share of foreigners amongst its resident population.

In April 2020, Germany, like many European countries, had managed to repress the spread of COVID-19, allowing its citizens a summer of seeming normality. Yet the party was short-lived. The second wave of COVID-19 started much earlier and more ferociously than many epidemiologists had expected. Seasonal effects may have played a role, of course, but the rise in the number of new infections began too early to be entirely caused by lower temperatures and an increase in indoor activities. In Germany, the public debate on the root causes of the second wave focuses on two factors: large private social events and international holiday travel to high-risk areas. Our recent research shows that, on average, the effect of school summer holidays accounts for about half of the increase in growth rates in infections in German local districts towards the end of the holiday period.

The substantial effect of summer holidays on infections was widely predicted. Travelling via bus, train, or plane adds to the risk of becoming infected, though probably only to a small extent. More importantly, holidaymakers and migrant workers, regardless of whether they travel internationally or internally, change their social behaviour and engage in more intense social interactions, often with people that they do not know. Thus, holidaymaking also reduces the ability of health authorities to successfully trace the close contacts of people that are infected with COVID-19.

A paucity of evidence on the effects of holidaymaking on the pandemic

Since the accelerating effect of holidaymaking on the pandemic was to be expected, it is surprising that we know of no academic study besides ours that actually assesses the impact that the school summer holiday season on the pandemic. To be sure, we are not the first to report relevant numbers for Germany. In reports of positive cases that local health authorities send to Germany’s Robert Koch Institute (RKI), the most likely place of infection is typically (though not always) noted, with the notification of multiple places possible. In its daily update from 13 October, the Institute reports:

“In the initial phase (…) the share of all cases with an exposure location abroad was 46% in reporting week 11. Due to travel restrictions, this share has steadily fallen, to 0.4% in reporting week 19. Since the 25th reporting week, there have been the first border openings, initially in Europe. From then onwards, the proportion of cases with a likely exposure location abroad increased again and reached its peak in reporting week 34 at 49%.”

After the peak in week 34 (mid-August), the share of infected with a likely infection location abroad declined to about 4% in mid-October. Based on numbers reported in RKI, we calculate that, on average, in about 27% of weekly cases over the entire summer school holiday period in Germany, a foreign country was mentioned as the most likely place of infection.

This does not necessarily mean the effect of holidaymaking is on average 27% and at most 49% of all infections. On the one hand, infections may not have occurred abroad or may not have been linked to holidaymaking. On the other hand, the RKI excludes the impact of holidaying within Germany by design. The RKI numbers also understate the effect of holidaymaking via second-order infections. Consider the case of a person who became infected while being abroad and then infects one or more contacts after returning to Germany. Implicitly, the second-order infections not only do not count as foreign travel-related; they even bring down the share of infections occurring abroad.

Exploiting an idiosyncratic feature of school holidays in Germany

Our research design estimates the effect of the holiday season on the growth rate in all new infections. We exploit an idiosyncratic feature of school holidays in Germany, namely, the ex-ante coordination of holidays across the 16 German states. States stagger their school holidays in order to reduce the probability and scale of traffic jams on Germany’s motorways. As a consequence, school holidays are spread over a period of approximately 12 weeks, though the maximum length of holidays in each state is six weeks or a few days more. This feature allows us to estimate the effect of summer school holidays on the growth rate in districts that are on holiday with the growth rate in districts that are not on holiday as the presumed counterfactual. To mitigate the impact of potential confounders, we control via district fixed effects for time-invariant heterogeneity and we account for the common trend by including a lagged dependent variable. Our sample covers 401 German districts (combining districts in Berlin into a single unit) over ten weeks (the six holiday weeks plus two weeks before and after the holidays). We include the two weeks after holidays officially end to capture infections from those returning toward the end of the holiday period, the incubation period, and delays in getting tested.

The effects by week, by state, and by district

Disaggregating the effect week by week of the holiday season, we find that the estimated effect equates to 48.7% (95% confidence interval: 36.6–60.6) of the average growth rate across German districts during their respective final week of holidays and to 49.5% (95% confidence interval: 40.0–61.0) and 46.1% (95% confidence interval: 34.8–57.4) of the average growth rate during their respective first two weeks after holidays end. However, this effect is not identical for all German states. In contrast, if we allow for causal heterogeneity across states, we find virtually no holiday effect for Thüringen, Brandenburg, and Saxony-Anhalt. On the other end of the effect size continuum, we find the strongest holiday effects in Lower Saxony, Bavaria, Baden-Württemberg, Schleswig-Holstein, Bremen, and Hesse.

A plausible explanation for this heterogeneity comes from another of our findings, namely, that the holiday effects are stronger in districts that are richer and in which foreigners make up a larger share of the resident population. Richer people are more likely to go on holiday for longer and foreign citizens are likely to use the holiday season for returning to their home country for family visits (possibly in addition to taking other holidays). The states that see the highest effects also tend to be ones with richer districts and districts with an above average share of foreign residents.

We illustrate the two conditioning effects in Figures 1a and 1b, in which we use a heatmap with dark and red colours indicating low levels of growth and yellow and white indicating high levels of growth in district infection numbers.  The x-axis displays the period from seven days before the summer school holidays begin in a district to eight weeks after the beginning of holidays at a daily level. On the vertical axis of Figure 1a, the 401 German districts are sorted in descending order by taxable income so that districts with higher taxable income appear at the top and districts with a lower taxable income appear at the bottom. Similarly, on the vertical axis of Figure 1b districts are sorted by the share of foreigners amongst residents so that the districts with the highest share appear at the top and districts with the lowest share appear at the bottom of Figure 1b.

Figure 1: The conditioning effects of taxable income and share of foreigners on the holiday effect

a) By income