猜测：新冠肺炎可能几年前就发生了，起源于越南、老挝、缅甸。
2020-07-25 21:26阅读：3,873
英国经济学家杂志发表文章：有个专家Peter Daszak, head of EcoHealth Alliance
说，越南一亿人中只有300个感染，无死亡。可能他们早就免疫了。这次的新冠肺炎可能已经在东南亚传播了几年，但是，只要人口的50%（甚至更低的比例）有免疫力，它就传播不开，只有到了中国，它才找到了大量无免疫力的人们。
全文如下。
Bat signal.
The hunt for the origins of SARS-CoV-2 will
look beyond China.
The virus may have been born in South-East Asia.
July 22nd 2020.
One of the great questions of the past six months is where
sars-cov-2, the virus that causes covid-19, came from. It is
thought the answer involves bats, because they harbour a variety of
sars-like viruses. Yunnan, one of China’s southernmost provinces,
has drawn the attention of virus hunters, as the closest-known
relatives of sars-cov-2 are found there. But some think the origins
of the virus are not to be found in China at all, but rather just
across the border in Myanmar, Laos or Vietnam.
This is the hunch of Peter Daszak, head of EcoHealth Alliance, an
organisation which researches animals that harbour diseases that
move into pe
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ople. Since the outbreak, in 2003, of the original sars (now known
as sars-cov), scientists have paid close attention to
coronaviruses. Dr Daszak says that around 16,000 bats have been
sampled and around 100 new sars-like viruses discovered. In
particular, some bats found in China are now known to harbour
coronaviruses that seem pre-adapted to infect people. The
chiropteran hosts of these viruses have versions of a protein
called ace2 that closely resemble the equivalent in people. This
molecule is used by sars-like viruses as a point of entry into a
cell.
That such virological diversity has so far been found only in China
is because few people have looked at bats in countries on the other
side of the border. Yet these places are likely to be an
evolutionary hotspot for coronaviruses—one that mirrors bat
diversity. The horseshoe bats in Yunnan which harbour close
relatives of sars-cov-2 are found across the region. Other
countries are thus likely to have bats with similar viral building
blocks. Dr Daszak believes it is “quite likely that bats in
Myanmar, Laos and Vietnam carry similar sars-related coronaviruses,
maybe a huge diversity of them, and that some of them could be
close to sars-cov-2”.
None of this, though, explains how a virus whose ancestor may be
found in South-East Asian bats went on to start a pandemic from
central China. China’s government has agreed that a mission led by
the World Health Organisation (who) can visit later this year to
help answer this question. There is particular interest in how much
sampling has been conducted to look for the missing link in places
like the wildlife market in Wuhan (the first known centre of the
outbreak) and more generally in farmers, traders and possible
intermediate or host species.
Jeremy Farrar, the head of the Wellcome Trust, a large
medical-research charity, and a former professor of tropical
medicine, says his guess is that either sars-cov-2 or something
similar to it has been circulating in people in parts of South-East
Asia and southern China, probably for many years, and that
intermediate hosts have not yet been identified. Dr Farrar spent 18
years working in Vietnam as the head of an Oxford University
research unit. He says people go searching for bats for food and
sell them in markets in what is a sophisticated trade that can end
up in big cities like Wuhan. Bats are able to carry a huge
diversity of viruses without getting sick, and are also more mobile
than people realise. As he puts it, bats “congregate in huge
colonies, and poo everywhere. And then other mammals live off that
poo and then act as a mixing vessel for these sorts of
viruses.”
Support for the idea that something resembling sars-cov-2 might
have been circulating in the region before the pandemic began also
comes from another intriguing observation: the low incidence of
covid-19 in South-East Asia, particularly in Vietnam. John Bell, a
professor of medicine at the University of Oxford, says everyone
thought there would be a flood of cases in Vietnam because the
country is right across the border from China. Yet Vietnam has
reported only 300 in a population of 100m, and no deaths. The
country did not have a great lockdown either, he adds. Nobody could
work out what was going on.
One explanation, he suggests, is that Vietnam’s population is not
as immunologically “naive” as has been assumed. The circulation of
other sars-like viruses could have conferred a generalised immunity
to such pathogens. So, if a new one emerged in the region, it was
able to take hold in the human population only when it travelled
all the way to central China—where people did not have this natural
resistance.
This would tie in with the idea that infection with one coronavirus
can provide protection against others, and that even in countries
away from the evolutionary cauldron of South-East Asia part of the
population may have some protection against the current pandemic.
In particular, there are suggestions that protection might be
conferred mainly via part of the immune system called t-cells
(which work by killing virus-infected cells) rather than via
antibodies (which work by gumming up pathogens). If that is the
case, then serological studies which look at antibodies may be
underestimating natural immunity.
Sunetra Gupta, an epidemiologist at Oxford, argues that natural
immunity to covid-19 is conferred by infections with seasonal
coronaviruses. If correct, this has implications for the level of
vaccination needed to reach herd immunity. It is widely assumed
that over 50% of people need to be vaccinated to prevent a
resurgence of sars-cov-2. In a preprint released on July 15th Dr
Gupta says this figure could be much lower if a significant part of
the population is already resistant to infection.
As for the mystery of the origin of covid-19, more answers will
come when the who mission takes place, perhaps in August. The
critical steps that led a South-East Asian bat virus to start a
pandemic could have happened inside or outside of China—whether in
wild-animal markets or farms, or in traders or hunters. The virus
may have jumped directly from bats into people, or come via an
intermediate species. The story is waiting to be told.
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