8 | New Scientist | 12 September 2020WITH hindsight, we may come to
see late summer in the northern
hemisphere as the calm before
the storm. While many countries
in the north have suppressed the
spread of covid-19 for now, there is
growing evidence warning us that
winter could undo that progress.
Researchers are racing to
pinpoint what role temperature
and humidity may have in
the spread and severity of the
illness. They are exploring how
SARS-CoV-2, the virus that causes
covid-19, will interact with otherseasonal respiratory viruses. And
people are scouring data from
winter in the southern hemisphere
to see what the north might face.
These questions are a matter
of life and death. In the UK alone,
a reasonable worst-case scenario
in the event of a resurgence
of the coronavirus this winter
estimates that there could be
as many as 251,000 deaths in
hospitals – although the total
under the range of possibilities
within this scenario is more
likely to be less than half that.
Whatever the outcome, UK
scientific advisers expect cases
to increase this winter.
The weather is the obvious
change coming. To predict how
this will influence the virus’s
spread, we need to disentangle
the effects of weather from other
confounding factors, such as
how countries have responded,
different demographics and
variation in testing rates.
“Some of our motivation
for studying this is, early in the
pandemic, [US] President Trump
and other global leaders were
assuming, not backed up byevidence, that this would just go
away in hot weather. That could
have led to complacency,” says
Rachel Lowe at the London School
of Hygiene & Tropical Medicine.
Trump’s claim that the virus
would be halted by summer
appears to have been proven
wrong. A study by Lowe’s team
found the virus spreading in
different countries across the
globe regardless of temperature.
Yet despite a flood of research
into links between temperature,
humidity and various other
environmental factors on the
transmission of the coronavirus
and the severity of covid-19, the
results are inconclusive so far.
An analysis of 42 peer-reviewed
papers and 80 preprints led by
Albertus Smit at the University
of the Western Cape, South Africa,
found only five that could offer
“credible insight” and deemed it
too early to say if there is a link.
Similarly, Francois Cohen at
the University of Oxford and
his colleagues concluded in a
paper in July that not enough
testing is going on to provide
the required data. Analysing
weather means looking at
day-to-day changes, but we have
insufficient data on positive
coronavirus cases on individual
days. “The inference you can
make is restricted,” says Cohen.
If it is too soon to tease out
a link, what are the plausible
ways winter weather could affect
respiratory viruses like influenza
and SARS-CoV-2?
“There have been studies
showing warmer temperatures
and humid climates prohibit
these kinds of viruses. That’s
why, in temperate regions,
we tend to see them peaking in
winter. But the mechanisms are
not well understood,” says Lowe.
Colder temperatures could
directly affect the survival of the“Some leaders assumed
that this would go away
in hot weather. That could
have led to complacency”
TransmissionVIOLA
KAM/
SOPA^IMAGE
S/SHUTT
ER
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OCKWhat winter holds for covid-
Will the spread of the coronavirus accelerate in the northern hemisphere
this winter? Adam Vaughan investigatesA worker shovels snow
during the winter months
in Tokyo, Japan251,
Maximum UK hospital deaths this
winter in a worst-case scenarioNews Coronavirus
coronavirus itself. We know that
flu viruses are more stable on
surfaces in the cold, and lower
temperatures are associated
with increased shedding of flu
virus by infected people. With
winter sun delivering less virus-
destroying ultraviolet light, the
coronavirus may be able to survive
on surfaces for longer. The winter
peak in respiratory viruses might
also be because colder weather
hampers how our immune system
functions – through a lack of
vitamin D, for example. Or it could
simply be about how we behave,
crowding indoors in winter.
Indoor humidity, which is
usually at its lowest in winter
because of domestic heating,
could affect things too. Gordan
Lauc at the University of Zagreb in
Croatia and his colleagues looked
at 7000 people with covid-