Republican voters are more likely to be elderly, more likely to be men, more likely to live in rural areas with lower hospital capacity, and importantly, more likely to reject social distancing, masks and vaccines. All of these factors make them more susceptible to the virus. (COVID’s higher fatality rate among the elderly is well-known; it’s also more deadly to men, for reasons that are less clear.)
Counterbalancing this, COVID hit earliest and hardest in densely populated cities, which lean more Democratic. Also, Democrats are more likely to be Black or from other marginalized groups which, for historical reasons, are more likely to have comorbidities and more likely to lack access to health care.