Prospective analysis: does ice bathing harden against COVID-19 and is B RhD-the least viral resistant blood group in Polish ice bathers?

Background. Amateur ice bathing has recently become very popular, with studies revealing health benefits, including en-hanced viral resistance. Such interest in this issue has been heightened even more due to the COVID-19 pandemic. The study predates widespread COVID-19 vaccination in Poland. Objectives. The aim of the study was to investigate the effect of short-term cold-water immersion on COVID-19 resistance in Polish ice bathers. Additionally, a possible relation between the ABO blood group and RhD antigen of winter swimmers and COVID-19 incidence was also studied. Material and methods. A survey was conducted on 2,534 Polish ice bathers who were questioned about their demographic data, ice bathing habits, ABO and RhD antigens, symptoms of infection and SARS-CoV-2 RT-PCR test results. For prospective statistical analysis, a Bayesian prediction was carried out to accommodate for missing COVID-19 RT-PCR test data following the selected explanatory clinical covariates. Furthermore, a taxonomic method was used to cluster the ice bathers. Results. In experienced ice bathers, a 7% higher resistance to COVID-19 has been estimated compared to beginners. Additionally, the probability of COVID-19 infection in ice bathers with blood group B RhD-is predicted to be the highest, while group O RhD+ and A RhD+ tend to be more resistant against the virus. Conclusions. There is an increasing immunological resistance of ice bathing, especially in O RhD+ and A RhD+ carriers, against COVID-19, although this does not appear to be strong. Our findings suggest that ice bathing should be considered as an additional means to enhancing the resistance to infection.