New Scientist - USA (2020-09-12)

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12 September 2020 | New Scientist | 11

There are four of these seasonal
coronaviruses in general circulation.
There is also the original SARS
virus, which caused an epidemic
from 2002 to 2004. It is no
longer in general circulation but
virologists worry that it could make
a comeback. Finally, there is the
MERS coronavirus, which emerged
in 2012 and still circulates at low
levels. The fear is that a human
co-infected with SARS-CoV-2 and
a second coronavirus could be a
crucible for the creation of a new
virus, and even another pandemic.
A caveat is needed, however.
In medical circles, co-infection
means simultaneous infection of
an individual host by two or more
pathogens, not necessarily of the
same cell within that host. We
don’t yet know whether cellular
co-infection occurs with SARS-
CoV-2, says Díaz-Muñoz. But from
what is known about coronaviruses,
it is likely that it does.
“We worry about flu, but we
should be paying more attention
to the seasonal coronaviruses,”
says Díaz-Muñoz. “They are flying
under the radar. Looking at what
is going to happen with seasonal
corona is one of the most important
questions right now.”
It isn’t known whether SARS-
CoV-2 could recombine with a

common coronavirus, but we
can’t rule it out. “To effectively
combine the genomic material
of two viruses is more complicated
if the viruses are very different,”
says Raúl Rabadán at Columbia
University in New York. “If there
is a co-infection between a
common coronavirus and SARS-
CoV-2 it is unlikely, although not
impossible, that the potential
recombinants will be viable.”
Yi told New Scientist that he
isn’t especially worried about
a recombination event between

SARS-CoV-2 and a cold-causing
seasonal coronavirus, because
their genomes aren’t closely related
enough. However, the SARS and
MERS viruses, or as yet unknown
closely related coronaviruses,
are a different story. SARS is
the most genomically similar and
hence the most dangerous, he says.
That sounds worrying, but a
hybrid virus wouldn’t necessarily
be worse than SARS-CoV-2, says
Díaz-Muñoz. It could combine
less virulent features of the two
parent viruses, and become a mild
respiratory virus like the common
cold. “That may be evolutionarily
advantageous to the virus. If
people are no longer sheltering in
place, it could spread more easily.”
Even if a mash-up is unlikely,
recombination in SARS-CoV-
is still a huge concern, says Yi.
It generates genetic variation
that could make the virus more
virulent, drug-resistant or better
able to evade our immune system.
It could also render diagnostics
based on RNA assays useless.
Doctors and disease control
specialists should be concerned
about these possibilities, he says. ❚

“ We worry about flu, but
we should be paying more
attention to the seasonal
coronaviruses”

Coronavirus particles show
up blue in this electron
micrograph image

A high mutation rate can allow
a virus to rapidly evolve resistance
to the host’s immune response.
Coronaviruses are the exception,
because their replicase enzymes
do proofread. SARS-CoV-2 has
proved very resistant to mutation.
According to Díaz-Muñoz, only six
mutants have emerged thus far.
The variability in coronaviruses
comes from something else:
recombination. Their replicase
enzymes frequently jump from one
part of the RNA template to another.
This makes them adept at remixing
their own genomes to create
variation, and also allows them
to steal genetic material from
other closely related coronaviruses.
“It is one of the things that
facilitates jumping from one
species to another. I think there’s
no doubt that recombination in
a bat was involved in the emergence
of SARS-CoV-2,” says Díaz-Muñoz.
The fear is that it could now


happen again, inside a human.
Research by Huiguang Yi at the
Southern University of Science and
Technology in Shenzhen, China,
implies that SARS-CoV-2 is
recombining its genome inside
human cells. He has analysed
84 published SARS-CoV-2 genome
sequences from human cases and
found some that could only have
formed through recombination.
There are also many documented
cases of co-infection with SARS-
CoV-2 and other respiratory viruses
including influenza, rhinovirus,
respiratory syncytial virus and
the seasonal coronaviruses
that cause common colds.
This latter class are especially
concerning, because they are quite
closely related to SARS-CoV-2 and
could potentially hybridise with it.

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New York during a protest
over lives lost to covid-

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