While much remains unknown about the novel coronavirus disease 2019 (COVID-19) that is afflicting the world, preliminary research is starting to suggest that, similar to influenza and diseases caused by existing coronaviruses, the highly contagious respiratory infection is influenced by climate and environmental factors. Because of the urgent need for information in the early phases of the pandemic, many early studies receiving attention are preprints that have not yet undergone the peer review process. However, making available potentially valuable findings with the wider scientific community even without formal peer review is important for sharing knowledge, promoting critical discussion, and informing additional studies. Although it is widely known that infectious diseases and the environment interact in significant ways, understanding this relationship in the context of COVID-19 may help guide public health officials toward more effective interventions.
The role of air pollution exposure on respiratory illnesses is a key question because many areas of the world, particularly low- and middle-income countries, experience consistent poor air quality. In fact, data from the World Health Organization (WHO) show that 9 out of 10 people are exposed to air contaminated with high levels of pollutants. Many of the underlying chronic conditions that are associated with exposure to air pollution over time, such as chronic lung disease and heart disease, are the same conditions that render people more vulnerable to the ravages of COVID-19.
Francesca Dominici, Ph.D., led a team at the Harvard T.H. Chan School of Public Health that examined 3,080 U.S. counties to explore the potential association between historical long-term exposure to air pollution and COVID-19 mortality. In their updated study distributed as a preprint on April 24, 2020, they analyzed the relationship between county-level COVID-19 deaths and particulate matter (PM2.5) concentration averaged across the years 2000-2016. They controlled for 20 potential confounding variables that addressed demographic characteristics of the population, socioeconomic factors, behavioral variables including smoking and obesity, weather variables such as summer and winter average daily temperature and relative humidity, hospital capacity, and stay-at-home policies. They found that an increase of one microgram (µg) per cubic meter (m3) in PM2.5 exposure over a 17-year period of time was associated with an 8% increase in the rate of COVID-19 deaths. Although still to be peer-reviewed, this ecologic study offers early indications that long-term exposure to air pollution may contribute to increased vulnerability to severe COVID-19 outcomes.
As a comparison, in 2017, the team conducted a study of 60 million Americans in the Medicare population over the age of 65 and discovered an association between long-term PM2.5 and a 0.73% increase in all-cause mortality rate. Therefore, for the same unit increase in PM2.5 exposure, the increase in the COVID-19 death rate is 11 times greater than that observed for all-cause mortality. These findings may have important health implications for communities, both domestic and international, that face a higher burden of air pollution.
The question of potential seasonality of COVID-19 has also been raised because of comparisons to influenza, which exhibits a seasonal pattern of disease transmission with lower rates observed during warmer and more humid months. Several preliminary studies have looked at temperature and humidity in particular and the relation of these variables to COVID-19 infection. In a recently published study, Mohammad Sajadi, M.D., and colleagues modeled the potential seasonal pattern of COIVD-19 infection by comparing climate data of cities with and without significant COVID-19 infection in the community. They found communities with significant COVID-19 spread mostly fell along a specific east-west geographic distribution that experienced comparable weather patterns of average temperatures between 5 to 11 degrees Celsius and absolute humidity between 4 to 7 g/m3. Released as a preprint, a study led by Jingyuan Wang, Ph.D., and colleagues examined the relationship between temperature and humidity and the daily reproduction number (R0), the expected number of secondary infections arising from a primary case, for COVID-19 in China. They found that a 1°C rise in temperature and a 1% increase in relative humidity were associated with a reduction in R0 by 0.0225 and 0.0158, respectively, indicating a decrease in transmission of the virus.
Research published in Science of the Total Environment by Yu Wu, et. al., examined temperature and relative humidity across 166 countries to determine whether these climate variables influenced daily new COVID-19 cases and deaths, controlling for several potential confounders. Results indicated a temperature increase of 1°C was associated with a 3% reduction in daily new cases and a 1% decrease in daily new deaths, while a 1% increase in relative humidity was associated with a 0.8% reduction in daily new cases and a 0.5% decrease in daily new deaths. Though preliminary, these example research studies on COVID-19 seasonality appear to suggest that higher temperatures and humidity levels are climate factors that may help reduce, though not suppress, transmission of the virus.
While these studies imply a reduction in transmissibility related to climate factors, a critical question for public health officials is how great the influence of climate is relative to other factors in the transmission, including the lack of immunity in a population. That question has been examined in a recent study by Baker et. al., peer reviewed and published in Science on May 18, 2020. Using a “Susceptible-Infected-Recovered-Susceptible,” or SIRS transmission dynamic model, and historical data from influenza and two other betacoronaviruses already circulating in the U.S. (HCoV-HKU1 and HCoV-OC43), the authors assessed the sensitivity of the peak pandemic incidence and other pandemic descriptors to a range of absolute humidity. They found that in the initial phases of a pandemic, when all or the vast majority of the population is susceptible to infection, the impact of absolute humidity on the spread and extent of disease is relatively minimal. Once the disease transitions from pandemic to an endemic phase, in which a higher proportion of the population has immunity, meteorologic influences on transmission play a much greater role in incidence and timing of the disease.
Although most have yet to undergo the peer review process, these preliminary studies raise important issues for public health officials to consider as they struggle to control the novel virus. With the global research community continuing to study the driving factors that influence COVID-19 infection, there remain many opportunities to better understand the potential role that climate and environmental factors play in influencing disease transmission and disease severity. The NIEHS Notice of Special Interest offers funding for studies on this topic area and is open through May 4, 2021. Newly developed data resources, such as the portal of environmental data sets for infectious disease modeling hosted by the National Oceanic and Atmospheric Administration (NOAA) or the National Aeronautics and Space Administration (NASA) COVID-19 Data Pathfinder for Earth science data sets, contain a wealth of information that can be used to support studies on this topic.
Baker RE, Yang W, Vecchi GA, Metcalf CJE, Grenfell BT. 2020. Susceptible supply limits the role of climate in the COVID-19 pandemic. Science. doi: https://doi.org/10.1101/2020.04.03.20052787.
Sajadi MM, Habibzadeh P, Vintzileos A, Shokouhi S, Miralles-Wilhelm F, Amoroso A. 2020. Temperature, humidity and latitude analysis to predict potential spread and seasonality for COVID-19. SSRN. 10.2139/ssrn.3550308.
Wang J, Tang K, Feng, K, Lin X, Lv W, Chen K, Wang F. 2020. High temperature and high humidity reduce the transmission of COVID-19. SSRN. https://dx.doi.org/10.2139/ssrn.3551767.
Wu Y, Jing W, Liu J, Ma Q, Yuan J, Wang Y, Du M, Liu M. 2020. Effects of temperature and humidity on the daily new cases and new deaths of COVID-19 in 166 countries. Science of The Total Environment, Volume 729. https://doi.org/10.1016/j.scitotenv.2020.139051.
Wu X, Nethery RC, Sabath BM, Braun D, Dominici F. 2020. Exposure to air pollution and COVID-19 mortality in the United States: A nationwide cross-sectional study. medRxiv. doi: https://doi.org/10.1101/2020.04.05.20054502.