  Multiple studies have suggested the spread of COVID-19 is affected by factors
such as climate, BCG vaccinations, pollution and blood type. We perform a joint
study of these factors using the death growth rates of 40 regions worldwide
with both machine learning and Bayesian methods. We find weak, non-significant
(< 3$\sigma$) evidence for temperature and relative humidity as factors in the
spread of COVID-19 but little or no evidence for BCG vaccination prevalence or
$\text{PM}_{2.5}$ pollution. The only variable detected at a statistically
significant level (>3$\sigma$) is the rate of positive COVID-19 tests, with
higher positive rates correlating with higher daily growth of deaths.
