Environmental Weather Conditions and Influence on Transmission of SARS-CoV-2

July 3, 2020


Spencer EA, Brassey J, Jefferson T, Heneghan C.
Environmental Weather Conditions and Influence on Transmission of SARS-CoV-2
Included in  Analysis of the Transmission Dynamics of COVID-19: An Open Evidence Review

Weather conditions appear to influence transmission of SARS-CoV-2, although evidence is not sufficient nor consistent enough to allow causation to be definitely inferred. Available studies, of low to moderate quality, tend to report lower transmission at warmer temperatures, and higher transmission in colder temperatures typical of the winter season, along with exacerbating effects of humidity, high levels of pollution, and low wind speed.

In March 2020, we published an overview of how weather conditions may affect the transmission of SARS-CoV-2. The report concluded that weather conditions may influence the transmission, but the evidence was low quality and much of it not yet peer-reviewed. We now update this overview.

We are undertaking an open evidence review investigating factors that impact on the transmission of SARS-CoV-2, based on our published protocol

In brief, this review aims to identify and evaluate relevant articles (peer-reviewed or awaiting peer review) that examine the mode of viral transmission and ecological variables influencing the mode of transmission. Studies with modelling are only included if they report transmission outcome data, not predicted outcomes. We assess study quality and report important findings on an ongoing basis. 

Evidence update
We currently include 14 studies examining the role of ambient conditions on the transmission of SARS-CoV-2 (see Table). 

Weather conditions appear to influence the transmission of SARS-CoV-2, although evidence is not sufficient nor sufficiently consistent to allow causation to be inferred. 

  • Higher temperatures are associated with fewer cases;
  • Higher relative or absolute humidity is associated with fewer cases;
  • Dry conditions seem to favour viral spread; and  
  • Exacerbating effects include high levels of pollution and low wind speed. 
Main findings of 14 studies examining the role of ambient conditions on the transmission of SARS-CoV-2 Study (link to summary)
Estimated the relationship between local temperature and transmission, using 166,686 confirmed new COVID-19 cases from 134 countries from January to March, controlling for local public health interventions, UV exposure, and population densities. Found that temperature was associated with COVID-19 transmission globally, with a 1°C increase in local temperature associated with 13% fewer cases. Carleton 2020
Examined environmental factors in the spread of COVID-19 in Italy up until April 2020. The spread was faster in the north and much slower in the centre and south of the country. The accelerated and vast diffusion of COVID-19 in Northern Italy was associated with the duration of cities’ exposure to polluted air. Coccia 2020
Looking at global cases and meteorological conditions, COVID-19 growth rates peaked in the Northern Hemisphere with a mean temperature of ~5°C, and specific humidity of 4-6 g/m3. Growth rates were lower both in warmer/wetter and colder/drier regions. The relationships remained after controlling for air pollution, population size, population density and health expenditure. Ficetola 2020
Investigated meteorological conditions globally in relation to transmission and found that calm, cold, dry and overcast conditions were favourable to the transmission of COVID-19. Islam 2020
Looked at weather conditions and public health interventions across 144 global areas (excluding China) during one week in March: lower transmission of COVID-19 was associated with higher public health interventions; weakly associated with relative or absolute humidity and not associated with latitude and temperature.  Juni 2020
Collected cumulative incidence data for 23rd January (day of Wuhan lockdown) to 10th February 2020 and compared data from Thailand, Singapore, Japan, South Korea, Hong Kong, Taiwan and other regions of China. There was a positive correlation between absolute humidity and case increase and a weak negative correlation between weather temperature and case increase. Luo W 2020
Briefly reviewed published and unpublished laboratory and ecological studies, and concluded that some evidence indicates reduced transmission in high ambient temperatures and humidity, but it is limited and inconsistent. National Academies 2020
Analysed case data from 110 countries and the relationship with average temperature and absolute humidity values, calendar date, and population density. 

Weather variables were significant in some of the single-variable models but had opposite associations with infection rate for two groups of regions, US states vs countries. With timeline and/or demographic factors introduced as covariables, weather variables lost statistical significance and population density and timeline emerged as more important effects on the infection rate.

Pedrosa 2020
Analysing data for China from December 2019 to February 2020, i.e winter season, found that every 1°C increase in the daily average temperature led to a decrease in the daily confirmed cases by 36% to 57% when relative humidity was in the range of 67% to 86%. Qi 2020
Analysing global meteorological data, reported that: as of 10th March 2020, all the cities with significant outbreaks of COVID-19 had low average temperatures around 5 to 11° C and specific humidity compared to other cities without COVID-19 cases. Cities with consistent transmission had varying relative humidity (44-84%), but consistently low specific (3-6 g/kg) and absolute humidity (4-7 g/cubic mt). The authors pointed out that the meteorological conditions had been stable for at least a month before the outbreaks and speculate that the conditions may favour the stabilization of the droplet and enhanced viral propagation in the nasal mucosa. Sajadi 2020 
Using data for Spain, lower mean temperatures of around 8 to 11ºC and lower specific humidity of <6 g/Kg were related to a higher number of cases and deaths; similar conditions were present in Northern Italy which suffered a dramatic surge of cases. High pollution levels in the Po valley and around Madrid may have contributed to increased transmission. Strong atmospheric stability with dry conditions favoured viral spread by short-range droplet transmission. The authors postulated that the anomalous prolonged period of high pressure in Northern Italy and inland Spain in January/February 2020 may have activated COVID-19 which was already present. Sanchez-Lorenzo 2020 
Across China and the US, in late January to mid-February, some of the variations in COVID-19 transmission between cities worldwide can be explained by differences in temperature and relative humidity, with higher temperature and humidity associated with lower transmission rate. Wang J 2020 
Across 27 countries during late January early February 2020, cumulative total confirmed cases were highest at an average temperature of 8.7°C, and less at lower temperatures and more at higher temperatures. Wang M 2020 
Data for 166 countries excluding China showed that temperature and relative humidity were negatively correlated with daily new cases and daily new deaths of COVID-19. Wu Y 2020


How secure is the evidence? 
Study quality was low to moderate, as observational studies are prone to bias and confounding; case ascertainment methods varied, and assessment of meteorological conditions often involved extrapolation from regional data. 

Case sets from these studies are likely to overlap, making meta-analysis impossible with currently available data. Variation in testing protocols, the bias in case ascertainment, and imprecise temperature and humidity data for specific locations prevents accurate quantification of the relationship between meteorological variables and change in case numbers. 

A number of these included studies still await peer review. Study quality was hampered by observational design and data quality e.g. meteorological data only available by city or region. Also, whilst some studies attempted to control for the impact of public health measures including lockdowns, bias may still remain. There may be publication bias as a result of the impetus to urgently identify modifying factors on transmission; neutral results may not be published as readily.  

We will update the open evidence review as the current case data in the Southern hemisphere will shed further light on the effect of seasonal variations in temperature and humidity on the transmission of  COVID-19.

How this evidence fits with what we already know?
Our findings are similar to the evidence for SARS-CoV and MERS-CoV-2 that a cold and dry environment favours virus survival and incubation. Coronaviruses have been observed to be seasonally active, with a peak in cases occurring in winter months. 

Some evidence on mortality from a study of daily deaths from COVID-19 in Wuhan showed that increased diurnal temperature range was associated with higher death rates and increased relative humidity was associated with lower death rates.  

Mechanisms have been proposed to explain the impact of meteorological effects on transmission of SARS-CoV-2. These include differential survival of the virus at varying temperatures in water or air. For effects within the individual, it has been hypothesised that a combination of low temperature and humidity make the nasal mucosa prone to small ruptures, that create an opportunity for the coronaviruses to invade the tissue.

It is probable that at least some part of this association between weather factors and transmission of SARS-CoV-2 is real and important. Unidentified and unaccounted for confounders may account for all or part of the associations reported by these observational studies, but the range of approaches here converge at reasonably consistently similar findings.

At the time of writing of this update, many world regions have recently been under severe movement restrictions to slow transmission. Notably, some European regions are currently emerging from lockdown and are experiencing relatively low transmission, whilst in the summer season. 

If SARS-CoV-2 transmission is truly affected by weather conditions and seasonality, this offers optimism for periods of low transmission during which public health measures to prevent transmission could effectively dampen down outbreaks. It also poses an alert for increased transmission during the winter season. 

Analysis of the Transmission Dynamics of COVID-19: An Open Evidence Review. Jefferson T, Spencer EA, Plüddemann A, Roberts N, Heneghan C.

Disclaimer: The article has not been peer-reviewed; it should not replace individual clinical judgement and the sources cited should be checked. The views expressed in this commentary represent the views of the authors and not necessarily those of the host institution, the NHS, the NIHR, or the Department of Health and Social Care. The views are not a substitute for professional medical advice.

Elizabeth Spencer is Epidemiology and Evidence Synthesis Researcher at the Centre for Evidence-Based Medicine. (Bio and disclosure statement here)
Tom Jefferson is an Epidemiologist. Disclosure statement is here
Jon Brassey is the Director of Trip Database Ltd, Lead for Knowledge Mobilisation at Public Health Wales (NHS) and an Associate Editor at the BMJ Evidence-Based Medicine.
Carl Heneghan is Professor of Evidence-Based Medicine, Director of the Centre for Evidence-Based Medicine and Director of Studies for the Evidence-Based Health Care Programme. (Full bio and disclosure statement here)