COVID-19 Infection Rates Linked to Global Seasonal Changes

COVID-19 Infection Rates Linked to Global Seasonal Changes
Digital generated image of globe inside a corona virus cell on dark grey background.

Now that COVID-19 has been with us for more than a year, researchers find evidence that seasonal influences may be in play, at least partially explaining the incidence of COVID-19 infections and deaths around the globe.

In a new study  from the University of Illinois, researchers collected COVID-19 related data from 221 countries from  April 15, 2020, including information on disease incidence, mortality, recovery cases, active cases, testing rate, and hospitalization. They matched that data with seasonal influences, such as each country’s latitude, longitude, and average temperature.

The date of April 15, 2020 was selected since it represents the middle of the Spring season, a time when temperature correlates highly with seasonality. That date also brackets maximal temperature variation among regions of the world.

The expectation was that warmer countries closer to the equator would be the least affected by the disease.

“Indeed, our worldwide epidemiological analysis showed a statistically significant correlation between temperature and incidence, mortality, recovery cases, and active cases. The same tendency was found with latitude, but not with longitude, as we expected,” says Gustavo Caetano-Anollés, professor in the Department of Crop Sciences, affiliate of the Carl R. Woese Institute for Genomic Biology at Illinois, and senior author on the paper.

But, when they analyzed mutational changes and mutational change over time in 55, 453 SARS-CoV-2 genome sequences, they found no correlation with temperature and geographic coordinates. This data suggests that others drivers of virus mutation are at play, not necessarily factors related to seasonal effects like temperature or latitude.

In sharp contrast to the global pattern, however, the researchers did not find a strong link in the U.S. between latitude and COVID-related variables. The reason could be that in the U.S., temperature does not vary as much as in other countries of the world. However, they did find an unexpected connection between longitude and COVID-19 incidence, mortality, number of active cases and hospitalization rate in the US. The authors conclude this association is related to the spread of the disease in both highly populated coasts of the US, and the differential response to the disease across US states.

The research team also assigned each country a risk index after studying other variables related to epidemiological variables – such as public health preparedness and incidence of co-morbidities in the population. The idea was that if the disease was surging in countries with inadequate resources or higher-than-average rates of diabetes, obesity, or old age, the risk index would appear more important in the analysis than temperature. But they found that these variables did not correlate with COVID-19 metrics at all.

While temperature and latitude were correlated with COVID-19 cases globally, the researchers emphasized other factors also impacted seasonal COVID-19 incidence worldwide. These influences included the impact of public health measures and policy regarding mask wearing and social distancing, and differing cultural expectations.