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How weather changes affect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been the subject of several studies. Researchers’ concern about this aspect of the 2019 coronavirus (COVID-19) pandemic stems from observations that the flu virus and other coronaviruses follow seasonal patterns. Understanding how SARS-CoV-2 transmissibility is affected by seasonal changes can therefore help us develop more informed preventive measures against its spread.
Laboratory studies have shown that SARS-CoV-2 – the causative pathogen of COVID-19 – is not very stable in warmer temperatures and high humidity. Other studies have found both positive and negative associations between time and viral transmission, based on daily confirmed case statistics.
However, using daily positive cases may not be very useful for studying the climate effect of SARS-CoV-2 transmission. For example, these data do not include the many undocumented cases, the delay between infection and symptom onset, and the delay between testing and reporting. Furthermore, many studies have not controlled for other confounding factors such as the implementation of health interventions, socioeconomic factors and other environmental factors.
To account for these issues, the authors used the mean reproduction number to allow for delayed reporting, unreported infections, and population movements. The authors also controlled for other factors such as smoking, air pollution, and obesity and strengthening the model.
Researchers from Yale University and Columbia University in the United States have published a paper on the medRxiv * prepress server that further explores the relationship between SAR-CoV-2 transmissibility and time. The team examined the association between air temperature and humidity on SARS-CoV-2 transmission in the United States based on the virus reproduction number, a metric used to calculate the infection rate of a pathogen in a population. . They define the reproduction number as the average number of new infections caused by a single infected individual, taking into account public health interventions and assuming that everyone in the population is susceptible.
Modeling of virus transmission and time
The team collected specific temperature and humidity data for 913 U.S. counties, specific humidity is defined as the mass of water vapor in a unit mass of humid air. They also collected other data for the counties, such as geographic location, population demographics, socioeconomic factors, and air pollution.
They estimated the daily spawn number using a metapopulation model in which they considered two types of movements: daily commuting to work and casual movement. They adapted the transmission pattern to county-level deaths and daily cases reported between March 15, 2020 and August 31, 2020.
They found that the average breeding number was between 0.46 and 5.43. The mean daily temperature varied widely between about 14 and 40 ºC and the specific humidity varied between about 1 and 22 g / kg.
The largest number of cumulative cases per 100,000 people occurred in Chattahoochee County, Georgia; in contrast, Taylor County, Florida had the lowest number. Southern counties were generally warmer and wetter than northern U.S. counties, and coastal counties were cooler and wetter than inland counties.
Using this model, the researchers found that lower temperatures were associated with higher transmission of SARS-CoV-2, below an optimal temperature of 32.6ºC. However, there was no association above the optimal temperature.
The relationship between reproduction number and specific humidity was non-linear. There was less transmission at higher specific humidity, except for an increase of between 9 and 15 g / kg.
The authors also estimated the fraction of the reproduction number attributable to specific temperature or humidity. In the 913 counties studied, the fraction attributed to temperature was 5.1% and the fraction attributed to humidity was 14.5%. The fraction attributable to temperature increased from south to north. The moisture attributable fraction increased from south to north in the eastern parts of the country, while in the western parts, the attributable fraction was lower in coastal counties and further inland.
Humidity affects transmission more than temperature
The authors found that lower temperatures and lower specific humidity were associated with an increased number of reproductions or greater viral transmission. Transmission of SARS-CoV-2 was associated more with temperature and humidity in colder, drier counties than in warmer, wetter counties. These results are therefore in line with some previous studies in the area.
Reproduction number distribution map, cumulative cases, 85 air temperature and specific humidity in the study counties.
Laboratory tests of SARS-CoV-2 acclimatization to changes in temperature and humidity have also found that it is less stable at higher temperatures and humidity than at lower temperatures and humidity. Additionally, the SARS-CoV-2 virus half-life decreased when the relative humidity was increased from 40% to 65% at 22ºC and 27ºC, but increased when the humidity increased from 65% to 85ºC. %. This is similar to the nonlinear association found using the model of the present study.
Therefore, specific humidity contributed more to virus transmission than temperature, but it was unclear whether humidity was a cause or indicator of virus transmission. Further studies in this area will need to be carried out to clarify this issue.
*Important Notice
medRxiv publishes preliminary scientific reports that are not peer-reviewed and therefore should not be considered conclusive, guide clinical practice / health-related behaviors, or treated as consolidated information.
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