Air temperature influences early Covid-19 outbreak as indicated by worldwide mortality

The Covid-19 outbreak has triggered a global crisis that is challenging
governments, health systems and the scientific community worldwide. A
central question in the Covid-19 pandemic is whether climatic factors have
influenced its progression. To address this question, we used mortality
rates during the first three weeks of recorded mortality in 144 countries,
during the first wave of the pandemic. We examined the effect of climatic
variables, along with the proportion of the population older than 64 years
old, the number of beds in hospitals, and the timing and strength of the
governmental travel measures to control the spread of the disease. Our
first model focuses on air temperature as the central climatic factor and
explains 67% of the variation in mortality rate, with 37% explained by the
fixed variables considered and 31% explained by country-specific
variations. We show that mortality rate is negatively influenced by warmer
air temperature. Each additional Celsius degree decreases mortality rate
by ~5%. Our second model is centred on the UV Index and follows the same
trend as air temperature, explaining 69% of the variation in mortality
rate. These results are robust to the exclusion of countries with low
incomes, as well as to the exclusion of low- and medium-income countries.
We also show that the proportion of vulnerable age classes and access to
healthcare are critical factors impacting the mortality rate of this
disease. The effects of air temperature at an early stage of the Covid-19
outbreak is a key factor to understand the primary spread of this
pandemic, and should be considered in projecting subsequent waves.