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1296 20 MARCH 2020 • VOL 367 ISSUE 6484 SCIENCE
dence from June through October. Syphilis
does well in winter in China, but typhoid fe-
ver spikes there in July. Hepatitis C peaks in
winter in India but in spring or summer in
Egypt, China, and Mexico. Dry seasons are
linked to Guinea worm disease and Lassa
fever in Nigeria and hepatitis A in Brazil.
Seasonality is easiest to understand for
diseases spread by insects that thrive dur-
ing rainy seasons, such as African sleeping
sickness, chikungunya, dengue, and river
blindness. For most other infections, there’s
little rhyme or reason to the timing. “What’s
really amazing to me is that you can find
a virus that peaks in almost every month
of the year in the same environment in
the same location,” says Neal Nathanson,
an emeritus virologist at the University of
Pennsylvania Perelman School of Medicine.
“That’s really crazy if you think about it.”
To Nathanson, this variation suggests hu-
man activity—such as children returning to
school or people huddling indoors in cold
weather—doesn’t drive seasonality. “Most
viruses get transmitted between kids, and
under those circumstances, you’d expect
most of the viruses to be in sync,” he says.
Nathanson suspects that, at least for vi-
ruses, their viability outside the human
body is more important. The genetic mate-
rial of some viruses is packaged not only in
a capsid protein, but also in a membrane
called an envelope, which is typically made
of lipids. It interacts with host cells during
the infection process and helps dodge im-
mune attacks. Viruses with envelopes are
more fragile and vulnerable to adverse con-
ditions, Nathanson says, including, for ex-
ample, summertime heat and dryness.
A 2018 study in Scientific Reports
supports the idea. Virologist Sandeep
Ramalingam at the University of Edinburgh
and his colleagues analyzed the presence
and seasonality of nine viruses—some en-
veloped, some not—in more than 36,000 re-
spiratory samples taken over 6.5 years from
people who sought medical care in their re-
gion. “Enveloped viruses have a very, verydefinite seasonality,” Ramalingam says.
RSV and human metapneumovirus both
have an envelope, like the flu, and peak
during the winter months. None of the
three is present for more than one-third
of the year. Rhinoviruses, the best-known
cause of the common cold, lack an enve-
lope and—ironically—have no particular
affinity for cold weather: The study found
them in respiratory samples on 84.7%
of the days of the year and showed that
they peak when children return to school
from summer and spring holidays. Adeno-
viruses, another set of cold viruses, also lack
an envelope and had a similar, nonseasonal
pattern, circulating over half the year.
Ramalingam’s team also studied the rela-
tionship between viral abundance and dailyweather changes. Influenza and RSV both
did best when the change in relative humid-
ity over a 24-hour period was lower than the
average (a 25% difference). “There’s some-
thing about the lipid envelope that’s more
fragile” when the humidity changes sharply,
Ramalingam concludes.
Jeffrey Shaman, a climate geophysicist at
Columbia, contends that what matters most
for the influenza virus is absolute humid-
ity—the total amount of water vapor in a
given volume of air—and not relative humid-
ity, which measures how close the air is to
saturation. In a 2010 paper in PLOS Biology,
Shaman and epidemiologist Marc Lipsitch of
the Harvard T.H. Chan School
of Public Health reported that
drops in absolute humidity
better explained the onset of
influenza epidemics in the con-
tinental United States than rela-
tive humidity or temperature.
And absolute humidity drops
sharply in winter, because cold
air holds less water vapor.
Why lower absolute humid-
ity might favor some viruses
remains unclear, however. Vari-
ables that could affect the viabil-
ity of the viral membrane could
include changes in osmotic
pressure, evaporation rates, and
pH, Shaman says. “Once you get
down to the brass tacks of it, we
don’t have an answer.”
Will SARS-CoV-2, which has
an envelope, prove fragile in
spring and summer, when ab-
solute and relative humidity
climb? The most notorious of
the other coronavirus diseases,
severe acute respiratory syn-
drome (SARS) and Middle East
respiratory syndrome (MERS),
offer no clues. SARS emerged
in late 2002 and was driven out
of the human population in the summer of
2003 through intensive containment efforts.
MERS sporadically jumps from camels to hu-
mans and has caused outbreaks in hospitals,
but has never shown widespread human-to-
human transmission. Neither virus circulated
for long enough, on a wide enough scale, for
any seasonal cycle to emerge.
Four human coronaviruses that cause colds
and other respiratory diseases are more re-
vealing. Three have “marked winter seasonal-
ity,” with few or no detections in the summer,
molecular biologist Kate Templeton, also at
the University of Edinburgh, concluded in a
2010 analysis of 11,661 respiratory samples
collected between 2006 and 2009. These
three viruses essentially behave like the flu.
That does not mean COVID-19 will as well. PHOTO: MIKE GRIPPIIn a study in New York and New Jersey, Micaela Martinez hopes to find out how artificial lighting affects the immune system.