Illinois researchers found that the number of COVID-19 cases significantly correlated with temperature and latitude across 221 countries, and was independent of comorbidities among the population.
The seasonal nature of viral diseases is widespread. With the rapid increase of global COVID-19 active cases and deaths in the fall and winter, there is comprehensive consensus: coronavirus infection, like flu, could also be seasonal.
In a recent research paper published in the journal Evolutionary Bioinformatics, Illinois researchers have addressed the issue and showed that the number of COVID-19 cases significantly correlated with temperature and latitude across 221 countries.
First, the researchers collected relevant epidemiological data such as disease incidence, mortality, recovery cases, active cases, testing rate, and hospitalization from 221 countries, along with the latitude, longitude, and average temperature of the respective countries.
The research team then used statistical methods to test if epidemiological variables were correlated with temperature, latitude, and longitude.
Since high temperatures and humidity affect the half-life of the SARS-CoV-2 virus in fomite transmission environments, the researchers thought 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, a professor in the Department of Crop Sciences at Illinois and senior author on the paper.
Prevalence of comorbidities such as diabetes, obesity, or old age has been shown in many studies as an increased risk factor for COVID-19 case severity.
The researchers compared epidemiological data with the incidence of the risk factors to assess if the higher-than-average incidence of the population’s comorbidities plays a more critical role than the temperature. But they did not find any such relationship—the comorbidity index did not correlate with the infection index at all.
To illustrate the mechanism, how the pattern of seasonality influences the number of COVID-19 active cases, the researchers mention our immune system could be partially responsible for this disparity.
Our immune system is heavily influenced by temperature and nutritional status, including vitamin D. During the winter, lower sun exposure doesn’t let us make enough of that vitamin, which was found very low in many COVID-19 severe cases.
The authors of the research paper note that more research is needed to determine if correlations are more likely the result of effects associated with the environment or the host’s innate immunity.
They also suggest the impact of policy, such as mask mandates, and cultural factors, such as the expectation to look out for others, are key players. However, they upheld the importance of understanding seasonality in battling the virus.
“We know the flu is seasonal, and that we get a break during the summer. That gives us a chance to build the flu vaccine for the following fall,” Caetano-Anollés says.
“When we are still in the midst of a raging pandemic, that break is nonexistent. Perhaps learning how to boost our immune system could help combat the disease as we struggle to catch up with the ever-changing coronavirus.”