Study shows COVID-19 cases rise 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 unique feature, we investigate which weather conditions are most associated with COVID-19 cases.
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 necessary 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, 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.
Humidity and Rainfall
Laboratory and observational studies of cases of COVID-19 patients show an impact of humidity on the SARS-CoV-2 virus.
A laboratory-generated aerosol of SARS-CoV-2 was stable at a relative humidity of 53% at room temperature, 23°C, 73°F. The virus had not degenerated much even after 16 hours and was more robust than MERS and SARS-CoV. This helps explain its higher levels of airborne infectivity.
Laboratory studies do not necessarily predict how a virus will behave in the real world. However, a survey of 17 cities in China with more than 50 cases of COVID-19 found a link between rises in humidity and reductions in COVID-19 cases.
The team measured humidity as absolute humidity, or the total amount of water in the air. For every gram per cubic meter (1 g/m3) increase in absolute humidity, there was a 67% reduction in COVID-19 cases after a lag of 14 days between the humidity increase and the number of cases.
The way temperature and humidity interact provide different weather patterns, which are determined by latitude.
A comparison of climate data looked at eight cities with high levels of COVID-19 spread:
- Wuhan, China
- Daegu, South Korea
- Tokyo, Japan
- Madrid, Spain
- Milan, Italy
- Paris, France
- Seattle, U.S.
- Qom, Iran
These cities were matched against 42 other cities worldwide with a low COVID-19 spread. All of the first eight cities lay in a narrow band between 30°N and 50°N latitudes.
Between January and March 2020, the affected cities had low mean temperatures of 5–11°C, 41–52°F, and low absolute humidity of 4–7 g/m3.
The authors conclude these findings are:
Studies of influenza show tropical areas where rainfall drives humidity and has a higher transmission in humid-rainy conditions.
American researchers established a threshold of 18–21°C (64–70°F) and specific humidity below 11–12 g/kg, approximately equivalent to 13–14 g/m3, for increased winter transmission. Tropical countries with temperature and humidity above these had higher influenza transmission when rainfall was high, defined as greater than 150 mm per month.
Brazilian researchers looked at rainfall worldwide, and confirm COVID-19 cases also increase with more significant precipitation. For each average inch per day of rain, there was an increase of 56 COVID-19 cases per day. No association was found between rainfall and COVID-19 deaths.
However, higher temperatures are associated with a lower number of cases in Turkey, Mexico, Brazil, and the U.S., but it appears there is a threshold. Higher temperatures do not cause a further decline in COVID-19 transmission, which could account for some of the disparities.
This is consistent with laboratory studies that show the SARS-CoV-2 virus is highly stable outside the body at 39.2°F (4°C) but increasingly unstable at temperatures above 98.6°F (37°C).
Hours of Sunshine and UV Light
A study in Spain found after five days of lockdown, the longer the hours of sunshine, the more cases there were of COVID-19. This positive association held with a lag — between sunshine hours and cases — of 8 and 11 days. There was no link between the hours of sunshine before lockdown and during the first five days.
This contradicts findings from influenza research, which suggests lower transmission with longer hours of sunshine. The authors say:
“The positive sign of sunshine may well be another instance of behavioral adaptations, whereby compliance with lockdown orders weakens on sunny days.”
In contrast, there appears to be no effect of solar UV light, as the wavelength required to kill viruses and bacteria is under 280 nanometers (nm).
This type of UV light (UVC) does not reach Earth as it is absorbed in the ozone layer. If it did reach Earth, humans would suffer severe burns to their skins and eyes within minutes.
Some minor effects of UVB light, defined as 280–320 nm, have been proposed to explain the contradictory findings of lower transmission of COVID-19 in cold and dry conditions at a higher altitude. However, other factors, such as higher vitamin D levels within people in these regions, might be more crucial.