3 ways to end a virus

譯者: Lilian Chiu 審譯者: Helen Chang

It’s spring 2021.

時值 2021 年春天。

The Alpha variant of the coronavirus has spread rapidly,

新冠肺炎病毒的 Alpha 變異株已經快速擴散，

becoming the dominant variant worldwide.

變成全世界的主流病毒株。

But another, more transmissible variant is about to appear—

但，另一種更容易傳染的 變異株即將出現——Delta。

Delta.

What happens when two variants clash?

當兩種變異株碰撞時，會怎麼樣？

Let’s do a thought experiment.

咱們來做個思想實驗。

Suppose that the variants reach a hypothetical isolated city

倘若這些變異株到了 一個完全孤立的假想城市，

of 1 million people who are completely susceptible to both viruses

城市內有一百萬居民，全部 都很容易感染這兩種病毒，

on the same day.

兩種病毒同一天到。

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

當這裡的一個人感染到 Alpha，

平均就會傳染給 5 名親密接觸者，

on average, 5 close contacts,

then begin to feel sick and immediately isolate themselves

接著，他開始不舒服，

便馬上自我隔離一直到模擬結束。

for the rest of the simulation.

The same thing happens with Delta,

Delta 的狀況也一樣，

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

差別只在於被感染的人平均會 傳染給 7.5 個親密接觸者。

What would you guess happens next?

你猜接下來會如何？

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

6 天後，

會有 15625 個人感染 Alpha。

Delta will have infected more than 10 times as many.

感染 Delta 的人數會超過十倍。

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

光是 20 小時後，

剩下的人都會感染到 Delta，

all before Alpha could infect 6% of it.

此時感染 Alpha 的人數 都還沒到總人口的 6%。

With no one left to infect, Alpha dies out.

沒有剩下任何人可以感染了， Alpha 便滅絕了。

This model is drastically simplified,

這個模型極度簡化，

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

但它確實能精確反映出 現實中發生的一個現象：

when both variants competed,

當兩種變異株競爭時，

Delta drove Alpha towards extinction in a matter of weeks.

Delta 只花了幾個星期 就讓 Alpha 絕種。

Viruses are wildly successful organisms.

病毒是非常成功的生物。

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

在地球上，病毒顆粒的數目是宇宙中

as there are stars in the observable universe.

可觀察到的星星的數目乘以一億倍。

Even so, viruses can and do go extinct.

就算如此，病毒仍然 有可能／確實會絕種。

There are three main ways that can happen.

主要有三種方式，會讓病毒絕種：

First, a virus could run out of hosts.

第一，病毒可能會用光宿主。

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

這個狀況有可能 在 2020 年初發生在

流感的 B/Yamagata 病毒種系上。

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

當世界很多地方都關閉、 保持社交距離、戴口罩，

to slow the spread of COVID 19,

以減緩新冠肺炎的散播，

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

能讓 B/Yamagata 感染的宿主也大大減少，

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

還需要再觀察幾個流感季節 才能肯定它是否真正絕種了，

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

或者只是躲藏在動物宿主身上。

Many viruses, as part of their life cycle,

許多病毒在生命週期中都會

cause diseases severe enough to kill their hosts.

造成嚴重的疾病，足以讓宿主死亡。

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

這可能是個問題，因為如果 病毒殺光了所有的宿主，

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

理論上，它可能會沒有剩下 任何宿主可以感染而絕種。

This almost happened back in 1950s Australia.

1950 年代在澳洲 就差點發生了這個狀況。

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

當時澳大利亞被歐洲兔子侵占——

此兔是一種入侵物種——

so, in an attempt to control the population,

所以，為了試圖控制歐洲兔數量，

scientists released a virus called myxoma,

科學家釋放了一種病毒， 叫做黏液瘤病毒，

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

已知歐洲兔感染這種病毒的 致死率幾乎是 100%。

During the initial outbreak,

在疾病初次爆發時，如計畫，

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

歐洲兔的死亡數達數千萬， 甚至可能有數億隻。

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

但，隨著病毒散播，

病毒演化出了一系列突變種， 剛好變得沒有那麼致命。

that happened to make it less deadly,

killing rabbits more slowly and killing fewer rabbits overall.

殺死兔子的速度變慢了， 殺死的兔子總數也變少了。

With more infected hosts hopping around,

有更多被感染的宿主到處跳來跳去，

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

這種病毒株比它的致命表親 更可能散播出去。

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

當然，兔子也會演化，

開始有更好的免疫反應。

Overall, instead of killing every single rabbit,

總的來說，這種病毒沒有殺光 每一隻兔子，反而演化了，

the virus evolved, the rabbit population bounced back,

兔子數量又再回升，

and both survived.

兩者都活了下來。

The second way a virus could go extinct

可能會讓病毒絕種的第二種方式

is if humans fight back with an effective vaccine—

就是人類用有效的疫苗 反擊——且獲勝。

and win.

Vaccination campaigns have driven two viruses essentially to extinction

1800 年代疫苗發明之後，

推行疫苗接種的活動就讓 兩種病毒真的絕種了：

since vaccines were invented in the 1800s:

smallpox and rinderpest, which kills cattle.

天花和會殺死牛的牛疫。

More on vaccination later.

等下再回來談疫苗接種。

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

會讓病毒絕種的第三種方式

就是它被另一種病毒或病毒株打敗，

by another virus or strain,

like we saw earlier with Delta and Alpha.

就如同先前 Delta 和 Alpha 的例子。

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

順便一提，病毒不見得 一定會彼此競爭。

A viral species can carve out its own distinct niche—

病毒物種能夠開拓出 它自己的利基——

for example, influenza infects your respiratory tract,

比如，流感病毒會感染你的呼吸道，

and norovirus infects cells in your intestine,

而諾羅病毒會感染 你的腸道內的細胞，

so both of these viruses can co-exist.

所以這兩種病毒可以共存。

A virus’ ecological niche can be tiny:

病毒的生態利基有可能很小：

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

B 型和 C 型肝炎病毒

可以感染同一個細胞——

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

B 型肝炎病毒佔領細胞核，

C 型肝炎病毒佔領細胞質。

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

事實上，流行病學家估計

C 型肝炎病毒帶原者有 2% 到 10%

are also infected with hep B.

也有感染到 B 型肝炎病毒。

So, will SARS-CoV-2—

所以，造成新冠肺炎的 病毒物種 SARS-CoV-2

the species of virus that causes COVID 19—

ever go extinct?

有可能絕種嗎？

Variants within the species will continue to arise.

在這個物種之內還會 持續出現新變異株。

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

這些變異株有可能會讓 舊的變異株絕種，或者不會。

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

不論變異株如何競爭，或不競爭，

the species itself— to which all the variants belong—

所有這些變異株所屬的這個物種

is pretty firmly established among humans.

已經穩穩地在人類之中立基了。

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

如果我們有辦法 讓足夠的人接種疫苗，

有可能讓 SARS-CoV-2 絕種嗎？

Our vaccination campaign against smallpox worked

我們針對天花做的 疫苗接種活動奏效了，

because the vaccine was highly protective against infection

因為那種疫苗的保護力 強到能讓接種者不被感染，

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

而且天花病毒找不到接近的 動物宿主讓它可以躲藏。

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

但 SARS-CoV-2 可以躲在動物身上，

and our current vaccines—

且我們目前的疫苗——

while they provide excellent protection against severe illness and death—

雖然在避免重症和死亡的 保護力上很出色——

don't prevent all infections.

卻無法預防所有的感染。

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

所以，可以想見，還有兩種 方式可以讓 SARS-CoV-2

the entire species— could go extinct:

整個物種有可能完全絕種。

a cataclysmic disaster could kill us all.

一場重大的災難 有可能讓大家同歸於盡。

Or...

或者，我們可能會發明出 一種通用的疫苗，

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

能預防所有的 SARS-CoV-2 感染，

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

目前既有的以及將來出現的 各種變異株所引起的感染。

Let's work toward that second option.

咱們朝第二個選項努力吧。