Influence of Seasonality and Public-Health Interventions on the COVID-19 Pandemic in Northern Europe

Abstract

Background: Most government efforts to control the COVID-19 pandemic revolved around non-pharmaceutical interventions (NPIs) and vaccination. However, many respiratory diseases show distinctive seasonal trends. In this manuscript, we examined the contribution of these three factors to the progression of the COVID-19 pandemic.

Methods: Pearson correlation coefficients and time-lagged analysis were used to examine the relationship between NPIs, vaccinations and seasonality (using the average incidence of endemic human beta-coronaviruses in Sweden over a 10-year period as a proxy) and the progression of the COVID-19 pandemic as tracked by deaths; cases; hospitalisations; intensive care unit occupancy and testing positivity rates in six Northern European countries (population 99.12 million) using a population-based, observational, ecological study method.

Findings: The waves of the pandemic correlated well with the seasonality of human beta-coronaviruses (HCoV-OC43 and HCoV-HKU1). In contrast, we could not find clear or consistent evidence that the stringency of NPIs or vaccination reduced the progression of the pandemic. However, these results are correlations and not causations.

Implications: We hypothesise that the apparent influence of NPIs and vaccines might instead be an effect of coronavirus seasonality. We suggest that policymakers consider these results when assessing policy options for future pandemics.

Limitations: The study is limited to six temperate Northern European countries with spatial and temporal variations in metrics used to track the progression of the COVID-19 pandemic. Caution should be exercised when extrapolating these findings.

Keywords: COVID-19 pandemic; Northern Europe; epidemiology; public health; seasonal variation; vaccination.

Figure 1

Seasonal influence of human beta-coronavirus in the six Northern European Countries. (a) Northern European countries examined in this study as defined by the World Geographical Scheme for Recording Plant Distributions modified from base map created by Rkitko, 2 June 2014. CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:WGSRPD_Northern_Europe.svg (accessed on 25 July 2023). (b) Seasonal incidence of human beta-coronaviruses OC43 and HKU1 from 2010–2020 recorded by the University Hospital, Stockholm, Sweden (red dot, (a)). (c) Average observations over 10 years (2010–2020) beginning at epidemiological week 26 through to the same week of the next year. Data from Neher et al. (2020) [45].

Figure 2

Progression of the COVID-19 pandemic in Denmark. COVID-19 progression measured by: (a) cases (per 100,000); (b) deaths (per 100,000); (c) positivity rate; (d) time-lagged death (3-week lag, expressed per 100,000); (e) hospitalisations (per 100,000); (f) ICU occupancy (per 100,000); (g) time lag between cases (blue line) and deaths (black line). Note that the secondary y-axis for (g) has been split into three scales to allow easier comparison of the peaks of the waves. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data” (https://ourworldindata.org/coronavirus; accessed on 25 July 2023 [69]).

Figure 3

The relative stringency of non-pharmaceutical interventions compared to the progression of the COVID-19 pandemic for each Northern European country. COVID-19 pandemic measured by time-lagged deaths (assuming a 3-week lag between infection and death, expressed per 100,000). Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data” (https://ourworldindata.org/coronavirus; accessed on 25 July 2023 [69]).

Figure 4

Time-lagged Pearson correlation tests between the stringency of NPIs and the progression of the COVID-19 pandemic. Correlation of the (potential) time-lagged effects of non-pharmaceutical interventions (NPIs) with the progression of the COVID-19 pandemic as measured by (a) COVID-19 cases/100,000 or (b) COVID-19 deaths/100,000) for weekly time lags. Non-significant values are indicated by NS. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data” (https://ourworldindata.org/coronavirus; accessed on 25 July 2023 [69]).

Figure 5

The percentage of the population fully vaccinated compared to the progression of the COVID-19 pandemic for each Northern European country. COVID-19 pandemic progression measured by deaths (assuming a 3-week lag between infection and death, expressed per 100,000). Data covering the period 1 March 2020 to 6 May 2023 taken from “Our World in Data” (https://ourworldindata.org/coronavirus; accessed on 25 July 2023 [69]).

Figure 6

Pearson correlation test of Northern European vaccination time series compared to COVID-19 pandemic progression. Pandemic progression measured by (a) COVID-19 cases, (weekly time-lagged cases per 100,000), (b) deaths time series, (weekly time-lagged deaths per 100,000). Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data” https://ourworldindata.org/coronavirus; accessed on 5 July 2023 [69].

Figure 7

The average seasonal incidence of human beta-coronavirus cases in Stockholm (2010–2020) compared to the progression of the COVID-19 pandemic. COVID-19 pandemic progression measured by time-lagged death (assuming a 3-week lag between infections and deaths, expressed per 100,000). Chronological data on COVID-19 variants, V0 = Wuhan strain, V1 = Alpha, V2 = Delta, V3 = Omicron BA.1, V4 = Omicron BA.2, V5 = Omicron BA.5 and V6 = Omicron BQ.1, sourced from GISAID, via CoVariants.org accessed on 1 November 2023. COVID-19 data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data” (https://ourworldindata.org/coronavirus; accessed 5 July 2023 [69]). The weekly beta-coronavirus (HCoV-OC43 and -HKU1) data (1 January 2010 to 2 April 2020) were recorded by the University Hospital in Stockholm, Sweden [45].

Figure 8

Pearson correlation test of weekly average human beta-coronavirus cases in Stockholm compared to progression of COVID-19 pandemic. Progression of pandemic measured by (a) COVID-19 cases (cases per 100,000) and (b) deaths (3-week time lag, expressed per 100,000). Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data” https://ourworldindata.org/coronavirus; accessed on 5 May 2022 [69] and weekly beta-coronavirus (HCoV OC43 and HCoV HKU1) cases (1 January 2010 to 2 April 2020) from the University Hospital in Stockholm, Sweden [45].

Figure 9

Statistical fitting of NPIs, vaccinations and seasonality to time-lagged deaths in six Northern European countries. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data”, https://ourworldindata.org/coronavirus; accessed on 5 July 2023 [69] and weekly beta-coronavirus (HCoV-OC43 and HCoV-HKU1 cases (1 January 2010 to 2 April 2020) from the University Hospital in Stockholm, Sweden [45].

Figure 10

Statistical fitting of NPIs, vaccinations and seasonality to cases of COVID-19 in six Northern European countries. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data”, https://ourworldindata.org/coronavirus; accessed on 5 July 2023 [69] and weekly beta-coronavirus (HCoV-OC43 and HCoV-HKU1) cases (1 January 2010 to 2 April 2020) from the University Hospital in Stockholm, Sweden [45].

Figure 11

Statistical fitting of NPIs, vaccinations and seasonality to positivity rate in six Northern European countries. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data”, https://ourworldindata.org/coronavirus; accessed on 5 July 2023 [69] and weekly beta-coronavirus (HCoV-OC43 and HCoV-HKU1) cases (1 January 2010 to 2 April 2020) from the University Hospital in Stockholm, Sweden [45].

Figure 12

Statistical fitting of NPIs, vaccinations and seasonality to hospitalisations in six Northern European countries. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data”, https://ourworldindata.org/coronavirus; accessed on 5 July 2023 [69] and weekly beta-coronavirus (HCoV-OC43 and HCoV-HKU1) cases (1 January 2010 to 2 April 2020) from the University Hospital in Stockholm, Sweden [45].

Figure 13

Statistical fitting of NPIs, vaccinations and seasonality to ICU occupancy in six Northern European countries. Data covering the period 1 March 2020 to 6 May 2023 were taken from “Our World in Data”, https://ourworldindata.org/coronavirus; accessed on 5 July 2023 [69] and weekly beta-coronavirus (HCoV-OC43 and HCoV-HKU1) cases (1 January 2010 to 2 April 2020) from the University Hospital in Stockholm, Sweden [45].