How Weather Affects SARS-CoV-2 Infection: US-Based Case Study

How weather changes affect Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has been a topic of some research. Researchers’ interest in this aspect of the 2019 coronavirus disease (COVID-19) pandemic is influenza virus and other Coronavirus Follow the seasonal pattern. Therefore, understanding how the infectivity of SARS-CoV-2 is affected by seasonal changes will help develop more informed precautions against its spread.

Laboratory studies have shown that SARS-CoV-2, the causative agent of COVID-19, is not very stable in hot and humid conditions. Other studies have found both positive and negative associations between weather and viral infections, based on daily case statistics.

However, the use of daily positive cases may not be very helpful in studying the effects of climate on SARS-CoV-2 infection. For example, this data does not include many undocumented cases, delays between infection and onset of symptoms, and delays between testing and reporting. In addition, many studies do not manage other confounding factors such as the implementation of health interventions, socio-economic factors and other environmental factors.

To illustrate these issues, the author used average reproduction numbers to take into account delayed reporting, unreported infections, and migration. The authors also managed other factors such as smoking, air pollution, and obesity, making the model more powerful.

Researchers at Yale University and Columbia University in the United States medRxiv* A preprint server that further investigates the relationship between SAR-CoV-2 infectivity and weather. The team investigated the relationship between temperature and humidity for SARS-CoV-2 infection in the United States based on the number of virus reproductions. This is an indicator used to calculate the infection rate of pathogens within a population. They consider public health interventions and define the reproduction number as the average number of new infections caused by a single infected individual, assuming that everyone in the population is susceptible to infection.

Virus infection and weather modeling

The team collected 913 US county temperature and specific humidity data. Specific humidity is defined as the mass of water vapor in a unit mass of moist air. We also collected other data about the county, such as geographic location, demographics, socio-economic factors, and air pollution.

They used a metapopulation model to estimate daily reproduction numbers. The metapopulation model considered two types of movements: daily commuting and random movements. They adapted the infection model to county-level deaths and daily cases reported between March 15, 2020 and August 31, 2020.

They found that the average reproduction number was between 0.46 and 5.43. Average daily temperatures fluctuated significantly between about 14-40 ° C and specific humidity fluctuated between about 1-22 g / kg.

Chattahoochee County, Georgia, had the highest cumulative number of cases per 100,000. Conversely, Taylor County, Florida had the lowest number. Southern counties were generally hotter and humider than northern US counties, and coastal counties were cooler and humider than inland counties.

Using this model, researchers found that temperatures below the optimal temperature of 32.6 ºC were associated with higher infections with SARS-CoV-2. However, it was not relevant above the optimum temperature.

The relationship between the number of reproductions and the specific humidity was non-linear. There was lower transmission at higher specific humidity, except for an increase between 9 and 15 g / kg.

The authors also estimated the percentage of reproductions due to temperature or specific humidity. In all 913 counties surveyed, the rate due to temperature was 5.1% and the rate due to humidity was 14.5%. The rate due to temperature increased from south to north. In the eastern part of the country, the proportion caused by humidity increased from south to north, while in the western part, the proportion caused by coastal counties and inland areas decreased.

Humidity affects transmittance more than temperature

The authors found that lower temperatures and lower specific humidity were associated with increased regenerations or increased viral infections. SARS-CoV-2 infection was more associated with temperature and humidity in cold and dry counties than in warm and humid counties. Therefore, these results are consistent with some previous studies in the region.

Laboratory tests of SARS-CoV-2’s adaptability to changes in temperature and humidity also showed that it was less stable at high temperatures and humidity than at low temperatures and humidity. In addition, the half-life of SARS-CoV-2 virus decreased when relative humidity increased from 40% to 65% at 22ºC and 27ºC, but increased when humidity increased from 65% to 85%. This is similar to the non-linear association found using the model in this study.

Therefore, although certain humidity contributed more to the virus infection than temperature, it was not clear whether humidity was the cause or indicator of the virus infection. Further research is needed in this area to clarify this issue.

*Important Notices

medRxiv Publish preliminary scientific reports that should not be considered definitive as they are not peer-reviewed, guide clinical / health-related behaviors, and should not be treated as established information.