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

Abstract

As COVID-19 continues to spread across the world, it is increasingly important to understand the factors that influence its transmission. Seasonal variation driven by responses to changing environment has been shown to affect the transmission intensity of several coronaviruses. However, the impact of the environment on SARS-CoV-2 remains largely unknown, and thus seasonal variation remains a source of uncertainty in forecasts of SARS-CoV-2 transmission. Here we address this issue by assessing the association of temperature, humidity, UV radiation, and population density with estimates of transmission rate (R). Using data from the United States of America, we explore correlates of transmission across USA states using comparative regression and integrative epidemiological modelling. We find that policy intervention (`lockdown') and reductions in individuals' mobility are the major predictors of SARS-CoV-2 transmission rates, but in their absence lower temperatures and higher population densities are correlated with increased SARS-CoV-2 transmission. Our results show that summer weather cannot be considered a substitute for mitigation policies, but that lower autumn and winter temperatures may lead to an increase in transmission intensity in the absence of policy interventions or behavioural changes. We outline how this information may improve the forecasting of SARS-CoV-2, its future seasonal dynamics, and inform intervention policies.

Environmental drivers of SARS-CoV-2 lineage B.1.1.7 transmission intensity

“…We find that the impact of the environment on transmission is mediated by VOC frequency, and it is only at higher VOC frequencies that the effects of temperature and population density are pronounced (Table 2 and Figure 1). Furthermore, we find a greater environmental effect after the UK moved from full lockdown to a (less strict) tiered system, confirming our previous observations that the effect of temperature becomes pronounced when NPIs are relaxed (Smith et al, 2020). Thus, the effects of environmental drivers, and even those of the increased transmissibility of the VOC, are secondary to differences in human behaviour driven by differences in NPIs.…”

“…Population density and daily temperature data were computed for LTLAs and STPs using the same methods as Smith et al (2020). Briefly, we collected global population density data from the Center for International Earth Science Information Network (CIESIN) (2018), and hourly temperature ( T ) estimates from the Copernicus Climate Change Service (2020) at a 0.25×0.25° spatial resolution.…”

“…A year into the COVID-19 pandemic, quantifying the factors driving SARS-CoV-2 transmission is still key for the optimal implementation of control strategies. SARS-CoV-2 transmission is influenced by local environmental conditions (Poirier et al, 2020), and we have previously shown that colder temperatures and higher population densities explain spatial and temporal variation in transmission intensity (Smith et al, 2020). A critical insight from previous work has been that human behaviour drives SARS-CoV-2 transmission, and that environment only plays a marginal effect when effective non-pharmaceutical interventions (NPIs) are in place (Poirier et al, 2020, Smith et al, 2020).…”

“…SARS-CoV-2 transmission is influenced by local environmental conditions (Poirier et al, 2020), and we have previously shown that colder temperatures and higher population densities explain spatial and temporal variation in transmission intensity (Smith et al, 2020). A critical insight from previous work has been that human behaviour drives SARS-CoV-2 transmission, and that environment only plays a marginal effect when effective non-pharmaceutical interventions (NPIs) are in place (Poirier et al, 2020, Smith et al, 2020). While several studies have investigated the impact of environmental factors on SARS-CoV-2 transmission during the early stages of the pandemic, numerous new variants have recently emerged (in, for example, the UK; Rambaut et al 2020, Brazil; Voloch et al 2020, and South Africa; Tegally et al 2020) with greater transmissibility than strains present during the early stages of the pandemic (Faria et al, 2021, Volz et al, 2021).…”

Research Agenda of Climate Change during and after the Coronavirus Disease 2019 (COVID-19) Pandemic

“…First, we have begun to understand how temperature levels could have a direct impact on human-to-human transmission of COVID-19 and how cold weather increases secondary transmission of SARS-CoV-2. At the moment, it is very likely that cold weather could increase the spread of COVID-19 [3]. This epidemiological finding is consistent with current experimental evidence indicating lower levels of transmission in warmer and more humid environments [4].…”