How often should you get a flu shot? - Melvin Sanicas

翻译人员: Chien Yet Chong 校对人员: Riley WANG

“疫苗拯救生命，恐惧危害生命。” ——《时代》杂志高级编辑 杰弗里·克鲁格

All year long, researchers at hundreds of hospitals around the world

每年，全球数百家医院的研究人员们 都会从流感患者身上采集样本

collect samples from flu patients

and send them to top virology experts with one goal:

交给顶级病毒学专家，

目的是为下一个 流感发病季研发疫苗。

to design the vaccine for the next flu season.

But why do we need a new one every year?

但为什么我们每年都需要新的疫苗？

Vaccines for diseases like mumps and rubella offer a lifetime of protection

针对腮腺炎和风疹的疫苗 可以提供终身保护，

with two shots early in life.

只要在儿童时期接种两次即可。

What's so special about the flu?

流感有什么特殊之处吗？

Two factors make the flu a tough target.

预防流感之所以困难 有如下两个原因。

First, there are more than 100 subtypes of the influenza virus,

原因一，流感病毒 有一百多种亚型，

and the ones in circulation change from season to season.

每个季节盛行 的流感病毒都有所不同。

And second, the flu's genetic code allows it to mutate more quickly

原因二在于，流感病毒 的基因序列使得它的突变速度

than many other viruses.

比其它病毒更快。

The flu spreads by turning a host's own cells into viral production factories.

流感病毒把宿主细胞变成 病毒生产工厂，从而实现传播。

When the virus is engulfed by a host cell, it expels its genetic material,

当病毒被宿主细胞吞噬后， 会释放遗传物质，

which makes its way to the nucleus.

这些遗传物质会进入细胞核。

There, cellular machinery that normally copies the host's genes

细胞核中本应进行宿主基因的复制，

starts replicating viral genes instead,

现在则转而复制病毒基因，

creating more and more copies of the virus.

从而制造出更多的病毒。

New viruses are repackaged and crammed into the cell until it bursts,

新的病毒将重新组装 充满整个细胞，直至细胞迸裂，

sending freshly minted influenza viruses out to infect additional cells.

新产生的流感病毒得以扩散， 继续感染新的细胞。

Most viruses follow this script.

大部分病毒会遵循这个过程，

The trick with the flu is that its genetic material isn't DNA

但流感病毒的特别之处在于， 其遗传物质并非DNA（脱氧核糖核酸），

but a similar compound called RNA.

而是一种与之类似的化合物， 称为RNA（核糖核酸）。

And RNA viruses can mutate much faster.

RNA病毒突变速度更快。

When cells synthesize DNA,

当细胞合成DNA时，

a built-in proofreader recognizes and corrects mistakes,

细胞本身具有校对机制， 可以辨认和修正错误，

but the RNA synthesis mechanism doesn't have this fail-safe.

但RNA的合成过程 并不具有这种机制。

If errors creep in, they stick around creating new variants of the virus.

如果出现错配 ， 就会制造出新的变异病毒。

为什么这是个麻烦呢？

Why is this a problem?

Because vaccines depend on recognition.

这是因为，疫苗需要辨认病毒特点。

The flu vaccine includes some of the same substances, called antigens,

流感疫苗含有一些与 病毒相同的物质，我们称之为抗原，

found on the surface of the virus itself.

抗原存在于病毒表面。

The body identifies those fragments as foreign

身体把抗原视为外来物质，

and responds by producing compounds called antibodies,

为了应对威胁，身体会释放出 一种化合物，我们称之为抗体，

tailor-made to match the antigens.

抗体与抗原相互匹配。

当一个已接受过疫苗接种的人 被真正的病毒感染时，

When a vaccinated person encounters the actual virus,

the preprogrammed antibodies help the immune system identify the threat

之前已经激活的抗体 会帮助免疫系统识别该种威胁，

and mobilize quickly to prevent an infection.

并迅速出击，避免感染发生。

Those antigens are different for every strain of influenza.

不同流感病毒具有不同的抗原。

If vaccination has prepared the immune system for one strain,

如果疫苗是为单一毒株而设，

a different one may still be able to sneak by.

那么其他病毒依然可以引起感染。

Even within the same strain of flu,

就算流感病毒相同，

those rapid genetic mutations can change the surface compounds enough

迅速的基因突变 也足以改变病毒表面的化合物，

that the antibodies may not recognize them.

以致抗体无法辨认它们。

To make things even more complicated,

让事情更复杂的是，

sometimes two different strains combine to create an entirely new hybrid virus.

有时，两种不同病毒 会结合形成新的混种病毒。

All of this makes vaccinating for the flu

这使得流感疫苗的研发

像是射击一个 不断移动和变化的靶子。

like trying to hit a moving transforming target.

That's why scientists are constantly collecting data

这也就是为什么 科学家夜以继日地工作，

about which strains are circulating

收集当下流行病毒的数据，

and checking to see how much those strains have mutated

并将这些病毒与往年相比，

from previous years' versions.

观察其变异情况。

Twice annually, the World Health Organization pulls together experts

每年，世界卫生组织 都会召集专家

to analyze all that data,

分析这些数据，

holding one meeting for each hemisphere.

南北半球各举行一次会议。

The scientists determine which strains to include in that season's vaccine,

科学家将确定用于 制作当季疫苗的病毒，

picking four for the quadrivalent vaccine in use today.

选出四种， 作为现今所用的四价疫苗。

In spite of the flu's evasive maneuvers,

尽管流感病毒捉摸不定，

in recent years, the group's predictions have been almost always correct.

但近些年来 ，专家团体的预测 几乎都准确无误。

Even when flu strains mutate further, the vaccine is often close enough

就算流感病毒进一步突变， 制作的疫苗仍然奏效，

that a vaccinated person who catches the flu anyway

相比没有进行疫苗接种的人，

will have a milder and shorter illness than they would otherwise.

打过疫苗的人即使感染流感， 病情也更轻微和短暂。

疫苗接种也会保护社区内的其他人，

Vaccination also helps protect other people in the community

who may not be medically eligible for the shot

这些人可能不具备 接种疫苗的医学条件，

by preventing those around them from carrying the virus.

若周边的人接种了疫苗， 他们自己也就不会成为病毒携带者。

This is called herd immunity.

这被称为“群体免疫”。

The flu shot can't give you the flu.

流感疫苗并不会让你感染流感，

It contains an inactivated virus that isn't capable of making you sick.

疫苗包含的是 无法引起病症的灭活病毒。

You might feel tired and achy after getting it,

打完疫苗后 ， 你也许会感觉疲惫或疼痛，

but that's not an infection.

但那并不是病毒感染，

It's your normal immune response to the vaccine.

而是身体对疫苗的正常免疫反应。

Some parts of the world use, instead of a shot, an inhaled vaccine

一些国家使用的不是疫苗针， 而是疫苗吸入器，

that contains a weakened live virus.

其中含有活性较低的活体病毒。

This is also safe for the vast majority of people.

对大多数人来说，这也是很安全的。

可能会面临风险的 只有那些免疫系统存在缺陷的人，

Only those with impaired immune systems would be at risk,

but they're typically not given live vaccines.

但通常也不会 让这些人使用活性疫苗。

Meanwhile, scientists are working to develop a universal flu vaccine

与此同时，科学家正努力 研发一种通用性流感疫苗，

that would protect against any strain, even mutated ones.

可预防任何类型的病毒， 甚至包括变异病毒。

But until then, the hunt for next year's vaccine is on.

但在那之前，研发下一年 疫苗的工作仍在继续。