10 | New Scientist | 20 February 2021
COULD the virus that caused a
worldwide pandemic have made
the jump to humans via frozen
food? That was one hypothesis
put forward on 9 February by a
joint World Health Organization
and Chinese investigation into
the origins of SARS-CoV-2.
Frozen animals were found on
sale at Huanan market in Wuhan,
China, the place where the virus
was initially detected, the team
behind the inquiry said. In a press
conference, Peter Ben Embarek,
the head of the investigation, said:
“We know the virus can survive in
conditions that are found in these
cold, frozen environments, but we
don’t really understand if the virus
can transmit to humans.”
The idea that the coronavirus
was carried inside or on the
surface of frozen food, which
has been advanced by Chinese
state media, could place the
source of the virus beyond
China, in an animal imported
from another country.
Yet it is far from clear whether
the virus could survive in an
infectious form in frozen food.
“I would say it’s extremely,
extremely unlikely the virus
would have spread through
that type of route,” says Lawrence
Young at the University of
Warwick, UK, who specialises
in human virology.
The reason why, according
to Young, is that SARS-CoV-2 is
an enveloped virus, meaning
it is covered with a fatty, lipid
membrane that is integral to
its ability to infect human cells.
This membrane is very vulnerable
to cycles of freezing and thawing,
as can happen during the transit
and sale of frozen food. Stripped
of this envelope, such viruses
can’t infect people.
A review by Jie Han and
Zue Zhang at Xi’an Jiaotong
University in Shaanxi, China,and their colleagues of evidence
on spreading the coronavirus
via food concluded that “major
knowledge gaps exist” on the role
that frozen food plays. “Data are
lacking on the long-term survival
of SARS-CoV-2 under freezing
temperatures (-10°C to -20°C)
that are frequently encountered
on the storage and transport of
frozen foods,” the team wrote.
Just one study, which hasn’t
been peer reviewed, has tried to
obtain that data. Dale Fisher at the
National University of Singapore
and his colleagues put the virus
into cubes of pork, chicken and
salmon, finding no decline in the
viral load after 21 days in a lab at
a refrigeration temperature of
4°C or at a standard freezing
temperature of -20°C.However, from this experiment,
it isn’t clear whether the viral load
was still infectious to humans.
In addition, the experimental
parameters may not reflect
real-world viral loads or conditions
in food supply chains.
SARS-CoV-2 may have a
rough time when frozen food
is transported. During air travel,
for instance, temperatures dropto between -20°C and -30°C
in cargo holds when planes
are airborne, then rise much
higher after landing.
By ship, the virus could
suffer from the “salty air issue”,
in which salt levels in the air can
affect the number of viruses that
can survive, says Julian Tang atthe University of Leicester, UK.
Changes in humidity on frozen
food’s journey may also negatively
affect SARS-CoV-2, as the lipid
membrane can be disrupted
by taking on fluid from the air.
We do know that the virus
has been found to persist on
the packaging of frozen food.
After two workers at Qingdao
Port in China tested positive
in September 2020, SARS-CoV-
was found on 50 of 421 samples
of frozen cod packaging.
However, Rodney Rohde at
Texas State University says that
despite the virus being found on
packaging, it doesn’t mean it is in
a viable state that could infect our
cells. “One must remember that
any viral genetic material may
be found on all types of surfaces,
including frozen surfaces. But
molecular PCR tests [that identify
the presence of the virus] do
not differentiate ‘viable’ from
‘non-viable’ virus.”
The other possible route is
if the virus is transported inside
the frozen meat or fish itself.
“If it’s part of the meat, it has
more protection,” says Tang.
Even if the virus was still in
an infectious form by the time
it reached a person in Huanan
market, there are still questions
over how they were actually
infected. Cooking the meat would
kill the virus, as would the gastric
acid in our stomachs, says Young.
However, if the food was raw or
not properly cooked, people could
potentially be infected from
surfaces during food preparation
or via the upper respiratory tract
while chewing the food.
“Overall, the probability [of
infection via frozen food] is low.
[But] if it happens once – a one-in-
a-million event – and it’s enough
to seed the virus in the human
population, you might get that
spread,” says Tang. ❚ST
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VIAGETT
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ESWorkers package frozen
steak at a beef-processing
plant in Binzhou, China“I would say it’s extremely,
extremely unlikely the
virus would have spread
via frozen food”News Coronavirus
OriginsAdam VaughanDid the virus come from frozen food?
The idea is being investigated, but the science is far from clear