3 ways to end a virus

翻译人员: Yip Yan Yeung 校对人员: Grace Man

It’s spring 2021.

2021 年春。

The Alpha variant of the coronavirus has spread rapidly,

新冠病毒阿尔法 （Alpha）变异株火速蔓延，

becoming the dominant variant worldwide.

一跃成为了全球主要流行毒株。

But another, more transmissible variant is about to appear—

但是另一种传染性更强的变异株 即将横空出世——

Delta.

德尔塔（Delta）毒株。

What happens when two variants clash?

两种毒株相遇会发生什么情况？

Let’s do a thought experiment.

我们来做一个思想实验。

Suppose that the variants reach a hypothetical isolated city

假设变异株传播到了 一个与世隔绝的虚构城市，

of 1 million people who are completely susceptible to both viruses

该市一百万人口在同一天 对这两种毒株都是易感的。

on the same day.

When a person here is infected with Alpha, they transmit it to,

假设有个人感染了阿尔法毒株，

on average, 5 close contacts,

他/她平均会传染给 5 个密接人员，

then begin to feel sick and immediately isolate themselves

这些人感觉身体不舒服， 立即采取了隔离措施，

for the rest of the simulation.

直至这场模拟结束。

The same thing happens with Delta,

我们也对德尔塔毒株 做出以上假设，

except that an infected person transmits it to, on average, 7.5 close contacts.

除了一个区别就是感染者平均 会传染给 7.5 个密接人员。

What would you guess happens next?

你猜接下来会发生什么？

After six days, Alpha will have infected 15,625 people.

六天后，阿尔法毒株 感染了 15625 人。

Delta will have infected more than 10 times as many.

德尔塔毒株的感染量 较此超过十倍。

Just 20 hours later, Delta will have infected the rest of the population—

仅仅 20 小时之后， 德尔塔就能感染剩下的全部人口，

all before Alpha could infect 6% of it.

而阿尔法只能感染 6% 的人口。

With no one left to infect, Alpha dies out.

没人可感染了， 阿尔法就消失了。

This model is drastically simplified,

这个模型是过度简化的，

but it accurately reflects one thing that did happen in real life:

但是它准确地反映了 真实生活中会发生的一件事：

when both variants competed,

两个毒株面临竞争时，

Delta drove Alpha towards extinction in a matter of weeks.

德尔塔能在几周内 把阿尔法逼向灭亡。

Viruses are wildly successful organisms.

病毒是非常成功的有机体。

There are about 100 million times as many virus particles on Earth

地球上的病毒颗粒量

大约是可见宇宙中 恒星数量的 1 亿倍。

as there are stars in the observable universe.

Even so, viruses can and do go extinct.

即便如此，病毒还是有可能， 也确实会灭绝。

There are three main ways that can happen.

病毒消失主要有三种可能。

First, a virus could run out of hosts.

第一，病毒没有了宿主。

This might have happened in early 2020 to a flu lineage known as B/Yamagata.

这也许就是 2020 年初

乙型流感病毒 B/Yamagata 谱系毒株的情况。

When much of the world shut down, social distanced, and wore masks

全球大部分地区都停止运转、 社交隔离、佩戴口罩，

to slow the spread of COVID 19,

减缓新冠肺炎的传播，

that dramatically reduced the number of hosts available for B/Yamagata to infect.

也就让 B/Yamagata 可以感染的宿主数量骤减了。

It’ll take a few more flu seasons to know for sure

我们还需要几个流感季才能确认

if it’s truly extinct or just hiding out in an animal reservoir.

这个毒株确实消失了 还是藏匿于动物宿主身上。

Many viruses, as part of their life cycle,

很多病毒， 作为它们生命周期里的一环，

cause diseases severe enough to kill their hosts.

造成的疾病严重到足以杀死宿主。

This can be a problem because if a virus kills all its hosts,

这可能会造成一个问题， 因为如果病毒杀死了全部宿主，

it could— in theory— run out of hosts to infect and go extinct.

理论上它就没有任何可感染的宿主， 只能走向灭亡。

This almost happened back in 1950s Australia.

上世纪 50 年代的澳大利亚 就差点发生了这样的情况。

At the time, Australia was overrun by the European rabbit— an invasive species—

当时，澳大利亚的欧洲野兔—— 一种入侵物种，泛滥成灾，

so, in an attempt to control the population,

所以为了控制野兔的数量，

scientists released a virus called myxoma,

科学家们投放了一种称为 “粘液瘤病毒”（myxoma）的病毒，

which had been previously shown to be almost 100% lethal to European rabbits.

根据以往研究，这种病毒对欧洲野兔 几乎有 100% 的致死能力。

During the initial outbreak,

病毒投放早期，

as planned, tens, perhaps hundreds, of millions of European rabbits died.

按照计划杀死了数千万， 也许有数亿只欧洲野兔。

But as the virus spread, it evolved a series of mutations

但是随着病毒蔓延， 进化出了许多变异体，

that happened to make it less deadly,

这些变异体没有那么强的致死能力，

killing rabbits more slowly and killing fewer rabbits overall.

杀死野兔的速度下降了， 杀死野兔的总体数量也减少了。

With more infected hosts hopping around,

这些已感染的宿主跳来跳去，

this strain of the virus was more likely to spread than its deadlier cousin.

导致了这种毒株比 致死力更强的近亲毒株更容易扩散。

And of course, rabbits evolved too, to mount better immune responses.

当然野兔也会进化， 逐渐具备更强的免疫应答能力。

Overall, instead of killing every single rabbit,

总而言之，这种病毒 没有杀死每一只兔子，

the virus evolved, the rabbit population bounced back,

而是进化了，野兔数量也回升了，

and both survived.

野兔和病毒都活了下来。

The second way a virus could go extinct

第二个让病毒消失的方式是

is if humans fight back with an effective vaccine—

人类通过有效疫苗反击……

and win.

而且打赢了。

Vaccination campaigns have driven two viruses essentially to extinction

自从 19 世纪疫苗被研发出来，

疫苗接种运动已经 让两种病毒走向了灭亡：

since vaccines were invented in the 1800s:

smallpox and rinderpest, which kills cattle.

天花和牛瘟，即杀死牛的病毒。

More on vaccination later.

之后再聊疫苗接种。

The third way a virus can go extinct is if it’s outcompeted

第三个让病毒走向灭亡的方式是

它被另一种病毒或毒株战胜，

by another virus or strain,

like we saw earlier with Delta and Alpha.

如同我们之前看到的 德尔塔和阿尔法毒株。

By the way, viruses don't always compete with each other.

顺带提一句， 病毒未必总是会互相竞争。

A viral species can carve out its own distinct niche—

病毒会占据自己 与众不同的生态位——

for example, influenza infects your respiratory tract,

比如，流感病毒感染呼吸道，

and norovirus infects cells in your intestine,

诺如病毒感染肠道细胞，

so both of these viruses can co-exist.

所以这两种病毒可以共存。

A virus’ ecological niche can be tiny:

病毒的生态位 可以是很小的一个部位：

hepatitis B and hepatitis C viruses can infect the same cell—

乙肝和丙肝病毒 可以感染同一个细胞，

hep B occupies the nucleus, and hep C occupies the cytoplasm.

乙肝侵占细胞核，丙肝侵占细胞质。

In fact, epidemiologists estimate that 2 to 10% of people with hep C

实际上流行病学家估计 有 2% 到 10% 的丙肝患者

are also infected with hep B.

同时患有乙肝。

So, will SARS-CoV-2—

所以，SARS-CoV-2（严重急性 呼吸系统综合征冠状病毒 2 型）——

the species of virus that causes COVID 19—

造成新冠疫情的病毒，

ever go extinct?

会消失吗？

Variants within the species will continue to arise.

这种病毒的变异毒株 还会层出不穷。

Those variants might drive prior ones to extinction, or not.

这些变异毒株可能会让 以前的毒株消失，也有可能不会。

Regardless of how the variants compete (or don’t),

无论这些毒株如何竞争 （也有可能没有竞争），

the species itself— to which all the variants belong—

这种病毒本身—— 所有变异毒株所属的病毒种类，

is pretty firmly established among humans.

已经在人类身上站稳了脚跟。

If we managed to vaccinate enough people, could we drive SARS-CoV-2 to extinction?

如果让足量的人接种疫苗， 我们可以让 SARS-CoV-2 消失吗？

Our vaccination campaign against smallpox worked

我们针对天花的 疫苗接种运动卓有成效，

because the vaccine was highly protective against infection

这是因为疫苗对于感染 有很强的保护能力，

and smallpox had no close animal reservoir in which it could hide.

而且人类周围 没有供天花藏匿的动物宿主。

But SARS-CoV-2 can hide out in animals,

但是 SARS-CoV-2 可以 藏于动物身上，

and our current vaccines—

而我们目前已有的疫苗——

while they provide excellent protection against severe illness and death—

虽然它们能够针对 预防重症和死亡提供有效保护，

don't prevent all infections.

但是无法预防所有感染。

So, conceivably there are two ways that SARS-CoV-2—

所以可以这么说， 让 SARS-CoV-2

the entire species— could go extinct:

整个病毒家族消失 只有两种方式：

a cataclysmic disaster could kill us all.

一场把我们都杀光的大灾难。

Or...

或者……

We could invent a universal vaccine that prevents all SARS-CoV-2 infections—

我们研制出一种通用的疫苗，

预防所有 SARS-CoV-2 的感染，

those caused by all the variants that currently exist and those that don’t.

源自现有的各种毒株和 尚未出现的毒株。

Let's work toward that second option.

我们一起在第二条路上努力吧。