Depends on public behavior
Imperial College London
The study, led by Imperial College London researchers and
published today in Proceedings
of the National Academy of Sciences, is the first to incorporate
environmental data into epidemiological models of the transmission of
SARS-CoV-2, the virus behind COVID-19.
The team show that temperature and population density are
the most important factors determining how easily the virus spreads, but only
in the absence of mobility-restricting measures, such as lockdowns.
First author of the study Dr Tom Smith, from the
Department of Life Sciences at Imperial, said: "Our results show that
temperature changes have a much smaller effect on transmission than policy
interventions, so while people remain unvaccinated, governments mustn't drop
policies like lockdowns and social distancing just because a seasonal change
means the weather is warming up.
"However, our work also suggests that lower autumn
and winter temperatures may lead to the virus spreading more easily in the
absence of policy interventions or behavioural changes."
Quantifying seasonal variation
Seasonal variation has been a source of uncertainty in
forecasts of SARS-CoV-2 transmission. Other viruses, like flu viruses and other
coronaviruses, are known to be affected by environmental factors. For example,
high temperatures and low humidity reduce the transmission of respiratory
droplets, preventing the spread of flu. High temperatures are also known to
inactivate other coronaviruses in the air and on surfaces.
However, quantifying the effects of environmental factors
including temperature, humidity, and UV radiation (sunshine) on SARS-CoV-2
transmission has been difficult during the pandemic, since human factors like
population density and behaviour have been the main drivers of transmission.
The differences in interventions and case-counting
between countries and regions also makes comparing environmental factors on a
global scale difficult, especially as some countries, like Brazil, India and
Iran, have high transmission despite having warmer climates.
As a result, few epidemiological models have included
environmental data, and those that do assume the response of SARS-CoV-2 is
identical to other coronaviruses, as there is a lack of SARS-CoV-2-specific
data.
To fill this gap, the team, from the Departments of Life
Sciences and Mathematics at Imperial, as well as Imperial's MRC Centre for
Global Infectious Disease Analysis, and Utah State University, compared
transmission across the USA. The country has a large range of climates with
comparable policies and case numbers, allowing the impact of environmental
factors to be teased out.
They found strong evidence that lower temperature and
higher population density are both associated with higher SARS-CoV-2
transmission.
The effect of temperature, although significant, was
small. In the team's models, each degree Celsius of increase in temperature
decreased the R number by approximately 0.04. That means that a 20ºC
difference, such as the difference between winter and summer temperatures,
could equate to a difference in R of around 0.8.
However, any impact of weather can be negated by
interventions like lockdown.
Policy and behaviour
Lead researcher Dr Will Pearse, from the Department of
Life Sciences at Imperial, said: "While temperature and population density
do influence SARS-CoV-2 transmission, our findings re-confirm that the most
important drivers are public policy and individual behaviour. For example,
during lockdowns, there was no meaningful signature of temperature influencing
transmission.
"This means, for example, that warmer regions should
not expect to ease mobility restrictions before colder regions. This is
especially true as warmer regions tend to have higher population densities --
for example, the population in Florida is more densely packed than in
Minnesota."
The researchers are now extending their study to new
variants, and say their environmental results should be incorporated into
future forecasts to enhance predictions of disease spread.
Study co-author Dr Ilaria Dorigatti, from the MRC Centre
for Global Infectious Disease Analysis at Imperial, said: "We found
evidence that, in the early phases of the pandemic, places with colder
temperatures were associated with higher SARS-CoV-2 transmission intensities.
However, the effect of climatic seasonality on SARS-CoV-2 transmission is
weaker than the effect of population density and in turn, of policy
interventions.
"This implies that, as we move towards summer in the
Northern Hemisphere, public health policy decisions remain of critical
importance for epidemic control and adherence to recommendations will continue
to play a key role against SARS-CoV-2 transmission."
