Introduction During the first wave of COVID-19 it was hypothesized that COVID-19 is subject to multi-wave seasonality, such as other respiratory viral infections since time immemorial, including earlier respiratory pandemics. It has already been observed that the COVID-19 community outbreaks appear to have a similar pattern as other influenza like illnesses (ILI). One year into the pandemic, we aimed to test the seasonality hypothesis for COVID-19. Methods We gather and calculated the average ILI annual time series based on incidence data from 2016 till 2019 in the Netherlands. And, compared this with two independent COVID-19 time series during 2020/2021 for the Netherlands, plotted on a logarithmic infection scale. We tested our hypothesis by calculating correlation coefficients and, as a sensitivity analysis, by performing univariate regression analysis. Results The COVID-19 time series strongly and highly significantly correlates with the ILI time series r(45) = 0.75 (p < 0.00001) and (r(45) = 0.798, p < 0.00001). Also the univariate regression analyses that we performed as a sensitivity analysis are all highly significant: respectively F(1, 43) = 61.45, p < 0.0001, and F(1, 43) = 81.18, p < 0.0001 and the correlations (r2) are moderate to strong. Conclusions Given the strong, and highly significant, correlations between the ILI and COVID-19 time series, we conclude that COVID-19 behaves as seasonal as ILI in a country in the temperate climate zone, such as the Netherlands. Moreover, the COVID-19 peaks are all during flu season, and lows are all in the opposing period as expected. Furthermore, the COVID-19 time series satisfies the two characteristics of earlier pandemics, namely a short first wave at the tail-end of a flu season, and a longer and more intense second wave during the subsequent flu season.

Eur J Clin Invest 2010; 40 (9): 770–789