Research shows there are more COVID-19 cases as temperature and humidity fall. A recent study points to more severe cases in cold and dry weather. Do these findings suggest COVID-19 is seasonal? Experts disagree.
Why are these findings so controversial, and why has the United States seen most cases during its hot and humid summer? In this special feature, we investigate which weather conditions are most associated with COVID-19 cases.
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We look at what factors might confound these studies and make them hard to interpret. And we describe how one international study tries to get around these problems.
There are good reasons to expect a respiratory virus to show seasonal variation. Infections from influenza and respiratory syncytial virus are more common during winter in temperate areas of the world.
“But the fact is that respiratory viruses are generally seasonal, probably as viruses that transmit on water droplets do so less well if the droplet dries up faster, and temperature, humidity, and UV may be part of the lull in transmission we are now seeing. The flip side, alas, is that the opposite will be true in the autumn and beyond.”
– Prof Ian Jones, Professor of Virology, University of Reading, United Kingdom
Studies of the first SARS-CoV in 2003 suggest weather might be important for coronavirus spread. While this virus did not circulate long enough to establish any potential seasonal pattern, daily weather was associated with the number of cases. In Hong Kong, new cases were 18 times higher in lower temperatures — under 24.6°C, 76°F — than more elevated temperatures.
The epidemic died out during a warm, dry July in 2003, but tight public health control measures were also in place. A recent review of the seasonality of respiratory infections describes how cold, dry winter weather makes us more susceptible to viruses in general.
In these conditions, the mucous lining in our noses dries up, which in turn impairs the function of cilia, the tiny hairs that line the nasal passage. These beat less often, meaning they may fail to clear viruses from the nose. The review concludes that a relative humidity of 40–60% might be ideal for respiratory health.
Americans spend 87% of their time indoors, so how does the outside weather affect them so much? When cold, dry air meets warm air from indoors, it reduces the air’s humidity inside by up to 20%. During winter, indoor humidity levels are 10–40%, compared to 40–60% in fall and spring. The lower humidity aids the spread of virus aerosols and could make the virus more stable.