Winter Is Coming: A Southern Hemisphere Perspective of the Environmental Drivers of SARS-CoV-2 and the Potential Seasonality of COVID-19

Abstract

SARS-CoV-2 virus infections in humans were first reported in December 2019, the boreal winter. The resulting COVID-19 pandemic was declared by the WHO in March 2020. By July 2020, COVID-19 was present in 213 countries and territories, with over 12 million confirmed cases and over half a million attributed deaths. Knowledge of other viral respiratory diseases suggests that the transmission of SARS-CoV-2 could be modulated by seasonally varying environmental factors such as temperature and humidity. Many studies on the environmental sensitivity of COVID-19 are appearing online, and some have been published in peer-reviewed journals. Initially, these studies raised the hypothesis that climatic conditions would subdue the viral transmission rate in places entering the boreal summer, and that southern hemisphere countries would experience enhanced disease spread. For the latter, the COVID-19 peak would coincide with the peak of the influenza season, increasing misdiagnosis and placing an additional burden on health systems. In this review, we assess the evidence that environmental drivers are a significant factor in the trajectory of the COVID-19 pandemic, globally and regionally. We critically assessed 42 peer-reviewed and 80 preprint publications that met qualifying criteria. Since the disease has been prevalent for only half a year in the northern, and one-quarter of a year in the southern hemisphere, datasets capturing a full seasonal cycle in one locality are not yet available. Analyses based on space-for-time substitutions, i.e., using data from climatically distinct locations as a surrogate for seasonal progression, have been inconclusive. The reported studies present a strong northern bias. Socio-economic conditions peculiar to the ‘Global South’ have been omitted as confounding variables, thereby weakening evidence of environmental signals. We explore why research to date has failed to show convincing evidence for environmental modulation of COVID-19, and discuss directions for future research. We conclude that the evidence thus far suggests a weak modulation effect, currently overwhelmed by the scale and rate of the spread of COVID-19. Seasonally modulated transmission, if it exists, will be more evident in 2021 and subsequent years.

Environment influences SARS-CoV-2 transmission in the absence of non-pharmaceutical interventions

“…The copyright holder for this this version posted September 14, 2020. ; https://doi.org/10.1101/2020.09.12.20193250 doi: medRxiv preprint have already reported effects of environmental variables such as temperature, humidity, UV levels, and wind-speed on the transmission of SARS-CoV-2 [17][18][19][20][21][22][23][24] . While, in general, most studies appear to support increased transmission rates under cool, dry conditions 18 , conflicting results have been observed 21,25 and collectively the environmental signal appears to be weak 5 . Much of the variability in these early results is likely due to the use of inappropriate response variables (such as cases or fatalities) which fail to capture the intrinsic variations in transmission intensity driven by the effects of non-pharmaceutical intervention measures 5 .…”

“…While, in general, most studies appear to support increased transmission rates under cool, dry conditions 18 , conflicting results have been observed 21,25 and collectively the environmental signal appears to be weak 5 . Much of the variability in these early results is likely due to the use of inappropriate response variables (such as cases or fatalities) which fail to capture the intrinsic variations in transmission intensity driven by the effects of non-pharmaceutical intervention measures 5 . Furthermore, COVID-19 has taken hold in many places with diverse climates and there are obvious examples of high transmission rates under hot, humid conditions, e.g.…”

“…Other coronaviruses display seasonal cycles of transmission and up to 30% of seasonal 'colds' are caused by coronaviruses 4 . Thus, as many Northern-hemisphere countries relax the non-pharmaceutical interventions initially imposed to control COVID- 19, there is a pressing need to understand whether seasonality will enhance or drive a 'second wave' of COVID-19 outbreaks as they move into autumn or winter 5 . SARS-CoV-2 is an enveloped RNA virus which is structurally (if not phylogenetically) similar to other RNA viruses such as influenza, MERS, and HcoV-NL63 6 , that are known to display seasonal dynamics due to their physical properties.…”

Investigation of the Importance of Climatic Factors in COVID-19 Worldwide Intensity

“…Some of these studies have focused on specific regions and countries, such as 122 cities in China [10], Wuhan, China [11], Mainland China [12], Jakarta, Indonesia [13], India [14,15], Iran [16], Brazil [17], Europe and United States [18], and Russia [19], while others have examined the effect of weather conditions on the pandemic at a global scale . Smit et al [42], who presented a comprehensive critical review of the relevant literature published in the first half of 2020, concluded that it is important for studies to include the effects of socio-economic factors and non-pharmaceutical interventions (such as government response measures) in their analysis.…”

Assessment of effective mitigation and prediction of the spread of SARS-CoV-2 in Germany using demographic information and spatial resolution

“…As Covid-19 raged all over the world and did not disappear in the summer months, only reduced seasonality can be expected. While [24] found that multiple meteorological aspects such as temperature, humidity, and wind speed influence the spread of the disease, the analysis of over 100 articles in [81] found only a "weak modulation effect" so far, with more evidence only to be expected in 2021. However, another important aspect of seasons is the transition from outdoor to indoor contacts.…”