A correlation exists between the weather and humidity that patients are exposed to during different stages of COVID-19 infection and the probability of death, according to Dr Sean Yuan Hsiang-yu, Assistant Professor in the Department of Biomedical Sciences at City University of Hong Kong (CityU), and PhD student Liang Jing-bo.

Using mathematical models combined with statistical analysis, the CityU team has estimated the correlation between COVID-19 case fatality rates and temperature and relative humidity based on i) the number of confirmed cases and deaths in eight European countries (UK, Italy, France, Spain, Germany, Netherlands, Sweden and Romania); and ii) data for temperature and relative humidity.

The researchers collected the data between February 2020 and July 2020, which was the first wave of the pandemic, taking into account delays in infection confirmation and death reporting, and then calculating the daily case fatality rates. Afterward, the distributed lag nonlinear model (DLNM), which was commonly used to calculate an exposure-time-response function when the exposure effect is likely to be nonlinear, was used to estimate the relationships of temperature and relative humidity on case fatality rates. 

Daily relative humidity values for five countries (Italy, Spain, Germany, Netherlands, and Sweden) were collected from the European Climate Assessment and Dataset. For those countries (UK, France, Romania) lacking the relevant data, the team calculated relative humidity values using the ratio of the actual water vapour pressure divided by saturation water vapour pressure.

“Starting from the outbreak of the pandemic until July 2021, the number of deaths in Europe was over 1.11 million, which accounts for approximately 27% of total COVID-19 deaths. Although the association between weather conditions and the transmissibility of COVID-19 in Europe has been reported before, the relationship between these exposures and fatality rates has not been well studied. Furthermore, the relationships between these factors at different stages of infection on the probability of death has not been discussed yet,” Dr Yuan said.

The results show that weather conditions were associated with fatality rates among COVID-19 cases differently, i.e. during different stages of infection. During the early stages, warmer temperatures (higher than 20°C) and low relative humidity (lower than 50%) were associated with a lower death risk; after the onset of symptoms, warmer temperatures (higher than 20°C) and high relative humidity (higher than 85%) were associated with a lower death risk. For instance, high relative humidity (89%) during the first few days after symptoms appear can reduce the fatality rate by 31%, compared to the 62% relative humidity.

The team also successfully captured the statistical correlation between temperature, relative humidity and total deaths in eight European cities during the first epidemic wave. Warm conditions were associated with a reduction in the risk of deaths, especially when the temperature was higher than 15°C. For example, Romania showed a warmer temperature for a long period of time, while Sweden and Netherlands also showed a warmer temperature for several days. The risk of death was low when the relative humidity was below 50% as well, such as in Germany and Romania, which both had humidity below 50% for a longer period of time, which may explain why these countries experienced lower mortality rates.

“The weather in Hong Kong has been extreme recently. By studying the data in Europe, we found that temperature and humidity were associated with fatality rates for COVID-19 cases. On the other hand, it is worth to note that due to data availability, the study used highly aggregated data and therefore underestimated within-country variability with respect to all these factors. It also did not take account of variation in case ascertainment within and between countries. More studies are needed to confirm the findings,” Dr Yuan said.