The article explores the hypothesis of pre-epidemic phenomena through influenza-compatible cases that were negative for influenza, i.e. through negative surveillance of influenza syndromes. In historical terms, it is common for any epidemic to have prodromes; after all, the intrinsic factors that will facilitate its spread and impact are already present before the epidemic as such. They need not necessarily be generated by the same agent, and in many cases, as in this work, it is not possible to determine what the pathogen might have been, but indirect estimates allow us to calculate when the first anomalies in disease distribution patterns were detected. 

Could the dates of detection of outliers in Europe coincide with the current state of the art? Perhaps in Spain, but in the UK it is a bit more confusing due to the year-end peak in 2019 and the second peak in March 2020. In the United States, on the other hand, the pre-covid-19 peak phenomena were not so clear, but the peak of influenza-negative cases would have been very close to the covid-19 peak. This difference would have been much greater in Germany, which would support the thesis that the timing of non-drug interventions reached European countries at very different stages of the pandemic, with a greater impact in those countries where the period between influenza-negative anomalies and the peak of covid cases was longer. 

Therefore, the paper builds on a hypothesis in the literature, uses known (non-direct) estimation methodologies and is compatible with the existing chronology of events and epidemiological data. However, it is subject to important limitations in extrapolating the data. In any case, as the authors propose, the percentage of influenza-negative cases among cases consistent with acute respiratory infection could be an early detection indicator to be given more weight in preparedness and response, although this would require re-running the analysis in other waves and in more countries.