DISCOVERIES
WHAT DOES SEASONALITY OF A VIRUS MEAN?
We are all familiar with viruses that
routinely circulate, such as influenza,
the common cold and chickenpox,
which is caused by the varicella-zoster
virus, VZV. These viruses are ‘endemic’,
which means they are regularly found
among particular people, or in a certain
area. Some of them, such as VZV,
circulate all year round, while others,
such as influenza, cause more infections
in the winter. We say that these virus
infections show ‘seasonality’.
In some seasons, the peaks become
so high that they are called ‘epidemics’,
meaning a widespread occurrence of
an infectious disease in a community
at a particular time. COVID-19 is not
yet endemic – having only emerged
last year. However, it is an epidemic, in
that it is still spreading rapidly. It can
therefore only display seasonality if, or
when, it becomes endemic.WHY DO VIRUSES SHOW SEASONALITY?
It’s still not fully established, but we
believe a number of parameters come
into play. Not all viruses show higherEverything from temperature differences
to changes in social behaviour can affect
the way a virus spreads, but what does
the future hold for COVID-19?How might
seasonal
changes affect
the coronavirus
pandemic?
Dr Mike Skinner virologist
“WHO has
emphasised
that COVID-19
is transmitting
everywhere,
so seasonality
may not have a
huge impact”
Horizons
ALAMY
peaks of infection in the winter. For
respiratory viruses, which do peak
in winter, an important factor is the
ability of the actual virus to resist
environmental stresses. These include
heat, humidity and UV light.
Another important factor that affects
seasonal virus transmission is our
behaviour. In winter, we tend to be
more closely crowded together, and
create higher relative humidities in our
homes because we remain inside and
turn up the heating.HOW DOES THAT APPLY TO COVID-19?
There was some hope that if we could
delay the peak of this epidemic into
the late spring and summer, that
we’d reduce transmission. But it’s
hard to know how far that would go
with a novel virus like this. It’s worth
pointing out that when it’s summer in
the northern hemisphere, it’s winter
in the southern hemisphere. So at the
same time as influenza infections are
low during our summer, they’re high
in Australasia and South America.
In tropical and subtropical regions
you don’t really get the seasons we’re
used to, and influenza infection rates
stay pretty level. This suggests that
relative humidity is probably the most
important factor. The World Health
Organization has emphasised that
COVID-19 is transmitting everywhere,
so the suspicions are that seasonality
may not have a huge impact on
reducing the spread.WHAT OTHER FACTORS ARE INVOLVED IN
HOW A VIRUS SPREADS?
What we anticipate for this virus is
based mainly on our experience with
flu, particularly back in 1918 and even
in 2009. But this is a different virus and
may behave in subtly different ways.