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.
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.
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 study 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.
Experts report similar associations between the number of cases and humidity in Australia, Spain, and for both cases and deaths in the Middle East.
The way temperature and humidity interact provides 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
- Tokyo, Japan
- Daegu, South Korea
- Qom, Iran
- Milan, Italy
- Paris, France
- Seattle, U.S.
- Madrid, Spain
These cities were compared with 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:
“consistent with the behavior of a seasonal respiratory virus.”
Studies of influenza show tropical areas where rainfall drives humidity have 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 levels 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 greater 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.
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