I recently read in the news from Reuters that Covid-19 was already present in the Europe in March 2019 as reported by the study by Chavarria-Miró et al (2020). According to Chavarria-Miró et al they found traces of SARS-CoV-2 in March 12, 2019 sewage samples from Barcelona.
I am interested in this news because I do research on effects of covid-19 pandemic and government responses to covid-19 on macreconomic indicators. In macroeconomcis we are unable to run randomized control trials but sometimes nature can create natural experiments or situations that allow for use of quasi-experimental statistical techniques, and I think I can use the situation above in such way.
Consequently, I have the following questions:
How conclusive is that Chavarria-Miró et al study? Are there any red flags that would indicate the conclusion drawn from the study should not be taken at face value? Essentially, I am interested in knowing if the study is reliable or if there are possibly other corroborating studies.
Is the SARS-CoV-2 found in march 12 the same SARS-CoV-2 that was circulating in 2020? Or we can't tell? Or to what degree it is similar if it is possible to know (in terms of infectiveness and mortality/morbidity)? If it is not possible to answer this empirically, is it at least possible to answer it theoretically based on how similar viruses mutate? (Here I would also appreciate some references to literature/data if possible).
The reason why I ask this second question is that given how dangerous and infectious is Covid-19 considered, I am surprised that there weren't spikes in deaths in Spain. In fact, according to statistics provided by Statista the number of deaths in 2019 was highest in January and even declined after March. When the number of death is totaled across whole year 2019, and compared to years prior years in data from Statista here, again the number of deaths was comparable to 2018. I realize that just looking at a crude naïve statistics like this is not rigorous but I would expect at least some visible effect if Covid-19 was already spreading in March 2019.