3 ways to end a virus

Translator: Chung Hei Franco Cheng Reviewer: Thomas Tam

It’s spring 2021.

喺2021年春天，

The Alpha variant of the coronavirus has spread rapidly,

新冠病毒Alpha變異株 已經快速擴散，

becoming the dominant variant worldwide.

成為全世界主流嘅病毒株。

But another, more transmissible variant is about to appear—

但另一種更加容易傳染嘅 變異株即將出現——

Delta.

Delta。

What happens when two variants clash?

當兩種變異株碰撞嘅時候 會點樣呢？

Let’s do a thought experiment.

不如做個假想實驗。

Suppose that the variants reach a hypothetical isolated city

假如呢啲變異株同一日去到 一個完全孤立嘅假想城市，

城市入面有一百萬名居民， 全部都好容易感染呢兩種病毒。

of 1 million people who are completely susceptible to both viruses

on the same day.

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

當呢度其中一個人 感染到Alpha，

on average, 5 close contacts,

佢就會平均傳染畀5名密切接觸者，

then begin to feel sick and immediately isolate themselves

跟住開始覺得唔舒服，

佢會即刻自我隔離 一直到成個模擬結束。

for the rest of the simulation.

The same thing happens with Delta,

Delta嘅狀況都一樣，

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

差別只係在於被感染嘅人 平均會傳染畀7.5個親密接觸者。

What would you guess happens next?

估吓跟住會點樣？

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

6日之後，會有 15625個人感染Alpha。

Delta will have infected more than 10 times as many.

感染Delta嘅人數 會超過十倍。

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

淨係過咗20個鐘之後， 剩低嘅人都會感染Delta，

all before Alpha could infect 6% of it.

呢個時候感染Alpha嘅人數 都仲未到總人口嘅6%。

With no one left to infect, Alpha dies out.

冇剩低任何人可以感染喇， Alpha就會滅絕。

This model is drastically simplified,

呢個模型極度簡化，

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

但它好精確咁反映出 現實中發生嘅一個現象：

when both variants competed,

當兩種變異株競爭嘅時候，

Delta drove Alpha towards extinction in a matter of weeks.

Delta喺幾個星期裡面 就令到Alpha絕種。

Viruses are wildly successful organisms.

病毒係非常成功嘅有機體。

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

喺地球病毒粒子嘅數目 係宇宙

as there are stars in the observable universe.

觀測到恆星嘅數目1億倍。

Even so, viruses can and do go extinct.

就算係咁，病毒仍然 有可能、確實會絕種。

There are three main ways that can happen.

主要有三種方式會令病毒絕種：

First, a virus could run out of hosts.

首先，病毒可能會耗盡宿主；

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

呢個狀況有可能喺2020年初發生 喺B/Yamagata嘅流感譜系裡面。

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

當世界大部分地方都封關、 保持社交距離、戴口罩，

to slow the spread of COVID 19,

去減慢新冠肺炎嘅散播，

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

咁樣可以大大減少咗B/Yamagata 可以感染嘅宿主。

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

我哋仲需要觀察多幾個流感季節

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

至確定佢係滅絕咗， 定係匿埋存儲喺動物身上。

Many viruses, as part of their life cycle,

好多病毒喺佢咃嘅生命週期裡面

cause diseases severe enough to kill their hosts.

都會造成嚴重嘅疾病， 足以令到宿主死亡。

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

咁樣係一個問題， 因為如果病毒殺死哂所有宿主，

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

理論上佢會因為剩低 冇任何宿主感染而絕種。

This almost happened back in 1950s Australia.

澳洲喺1950年代 就差啲發生呢種狀況。

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

當時澳洲畀歐洲兔侵襲—— 歐洲兔係一種入侵性物種。

so, in an attempt to control the population,

所以，為咗嘗試控制歐洲兔嘅數量，

scientists released a virus called myxoma,

科學家釋放咗一種叫黏液瘤嘅病毒，

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

歐洲兔預計感染咗呢種病毒 之後嘅死亡率幾乎係100%。

During the initial outbreak,

喺初次爆發嗰陣，

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

正如預期，歐洲兔嘅死亡數目 有幾千萬隻，甚至可能高達幾億隻。

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

但隨住病毒傳播， 病毒演化出一系列基因突變，

that happened to make it less deadly,

啱啱好變得冇咁致命，

killing rabbits more slowly and killing fewer rabbits overall.

殺死兔仔嘅速度變慢咗， 殺死嘅兔子整體都變少咗。

With more infected hosts hopping around,

隨住更多被感染嘅宿主 四圍跳嚟跳去，

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

呢種病毒株比佢嘅致命表親 更有可能傳播出去。

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

當然，兔仔都會演化， 開始有更好嘅免疫反應。

Overall, instead of killing every single rabbit,

總之，呢種病毒 冇殺死哂啲歐洲兔，

the virus evolved, the rabbit population bounced back,

反而演化到 令兔仔嘅數量又再回升，

and both survived.

兩者同時都生存落嚟。

The second way a virus could go extinct

第二種會令病毒絕種嘅方式，

is if humans fight back with an effective vaccine—

就係人類利用有效嘅疫苗反擊，

and win.

並獲得勝利。

Vaccination campaigns have driven two viruses essentially to extinction

自從19世紀發明疫苗之後，

since vaccines were invented in the 1800s:

疫苗接種運動 就令兩種病毒真係絕咗種：

smallpox and rinderpest, which kills cattle.

天花同埋會殺死牛嘅牛疫。

More on vaccination later.

陣間翻轉頭再講疫苗接種。

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

會令到病毒絕種嘅第三種方式，

就係佢畀另一種病毒 或者病毒株打敗，

by another virus or strain,

like we saw earlier with Delta and Alpha.

就好似之前Delta 同Alpha嘅例子一樣。

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

話時話，病毒並唔係成日都互相競爭。

A viral species can carve out its own distinct niche—

病毒物種能夠開拓出 屬於佢自己嘅小天地，

for example, influenza infects your respiratory tract,

例如流感病毒會感染你嘅呼吸道，

and norovirus infects cells in your intestine,

而諾如病毒會感染你腸道裡面嘅細胞，

so both of these viruses can co-exist.

所以呢兩種病毒可以共存。

A virus’ ecological niche can be tiny:

病毒的生態位有可能好細：

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

B型同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

實際上，流行病學家估計 有2至10%嘅C型肝炎患者

are also infected with hep B.

都感染咗B型肝炎病毒。

So, will SARS-CoV-2—

所以，SARS-CoV-2

the species of virus that causes COVID 19—

導致新冠肺炎出現嘅物種元兇

ever go extinct?

有可能絕種嘛？

Variants within the species will continue to arise.

喺呢個物種之內仲會 持續出現新嘅變異株。

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

呢啲變異株有可能會令 舊嘅變異株絕種，或者唔會。

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

無論變異株點樣競爭，或者唔競爭，

the species itself— to which all the variants belong—

所有呢啲變異株所屬嘅物種

is pretty firmly established among humans.

已經喺人類之中建立得幾牢固。

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

如果有辦法令足夠嘅人接種疫苗， 有可能令到SARS-CoV-2絕種嘛？

Our vaccination campaign against smallpox worked

我哋天花疫苗接種運動有效，

because the vaccine was highly protective against infection

係因為嗰種疫苗有好高嘅防護性，

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

而且天花病毒搵唔到 接近嘅動物作儲存厙畀佢匿埋。

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

但係SARS-CoV-2 可以匿喺動物身上，

and our current vaccines—

而且目前嘅疫苗——

while they provide excellent protection against severe illness and death—

雖然喺避免重症同死亡方面 做得好出色——

don't prevent all infections.

但係都無辦法預防所有嘅感染。

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

所以，想像下仲有兩種方式 可以令SARS-CoV-2

the entire species— could go extinct:

成個物種有可能完全絕種。

a cataclysmic disaster could kill us all.

一場重大嘅災難 有可能就令大家一鑊熟。

Or...

或者，

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

我哋發明一種通用嘅疫苗， 可以預防所有SARS-CoV-2嘅感染——

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

目前有嘅同埋將來會出現嘅 各種變異株引起嘅感染。

Let's work toward that second option.

我哋一齊為第二個選項努力喇。