We analyze data from Twitter to uncover early-warning signals of COVID-19 outbreaks in Europe in the winter season 2019–2020, before the first public announcements of local sources of infection were made. We show evidence that unexpected levels of concerns about cases of pneumonia were raised across a number of European countries.
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Figure 1a shows the cumulative distribution functions of the normalized number of tweets mentioning the word “pneumonia” in the selected European countries: France, Germany, Italy, The Netherlands, Poland, Spain, UK. To better understand the change in slope exhibited by the curves in the first few weeks of 2020, for each country we conducted a two-sample Kolmogorov–Smirnov (K–S) test of the null hypothesis that the cumulative distributions over two corresponding winter seasons (2018–2019 and 2019–2020) are the same against the alternative hypothesis that they differ. Figure 1b suggests that, with the exception of Germany, the distributions in the two winter seasons are statistically different for all countries: at the 0.10 level of significance for Poland, and at the 0.05 level of significance for the remaining countries (see also Supplementary Table S1 for the details on the specific time periods in which the distributions differ). To check for robustness, we also computed the Anderson–Darling (A–D) test and obtained similar results (Supplementary Fig. S1; Supplementary Table S2). Finally, we further performed similar robustness checks (i.e., KS and AD tests) by comparing the 2019–2020 winter season with each of the corresponding winter seasons since 2014 (i.e., 2014–2015, 2015–2016, 2016–2017, 2017–2018, and 2018–2019), and obtained similar findings (Supplementary Figs. S2, S3; Supplementary Tables S3, S4).
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With the exception of Germany, the series of cumulative mentions of pneumonia unmask unexpected statistically significant variations in public interest in pneumonia-related cases already in January 2020 (Fig. 1a). Interestingly, findings suggest a significant increase in tweets mentioning pneumonia in most of the selected European countries well before the outbreak of COVID-19 was officially reported in the news. In Italy, for example, where the first lockdown measures to contain an emerging threat of endemic COVID-19 infections were introduced on 22 February 2020, the rate of increase in mentions of pneumonia during the first few weeks of 2020 (shaded bar B, inset of Fig. 1a) substantially differs from the rate observed in the same weeks in 2019