Effects of 105 biological, socioeconomic, behavioural, and environmental factors on the risk of SARS-CoV-2 infection and a severe course of Covid-19: A prospective longitudinal study

The confirmed number of SARS-CoV-2 infections up to 30 August 2021 is 217 mil. worldwide but information about factors affecting the probability of infection or of a severe course of Covid-19 remains insufficient and often speculative. Only a small number of factors have been rigorously examined, mostly by retrospective or cross-sectional studies. We ran a preregistered study on 5,164 internet users who shared with us information about their exposure to 105 risk factors and reported being Covid negative before the beginning of the fourth wave of Covid-19 in the Czech Republic. After the fourth wave, in which 709 (13.7%) of participants were infected, we used a partial Kendall test controlled for sex, age, and urbanisation to compare the risk of infection and of a severe course of the disease in subjects who originally did and did not report exposure to particular risk factors. After the correction for multiple tests, we identified 13 factors - including male sex, lower age, blood group B, and the larger household size - that increased the risk of infection and 16 factors - including mask wearing, borreliosis in the past, use of vitamin D supplements, or rooibos drinking - that decreased it. We also identified 23 factors that increased the risk of a severe course of Covid-19 and 12 factors that decreased the risk.