什么是群体免疫？
昨天刚了解了「熔断机制」，今天我们再来看看英国专家提出的「群体免疫」（Herd Immunity）是什么意思。
What Is 'Herd Immunity' and How Can That Stop the Coronavirus?
By Claire Gillespie, HEALTH
The term has been used a lot since the pandemic began—here's what it means.
With COVID-19 now a global pandemic, and cases of the new coronavirus confirmed in 46 states, the big question is, what happens next?
pandemic
1）作名词，表示“（全国或全球性）流行病；大流行病”，英文解释为“a disease that spreads over a whole country or the whole world”；
2）作形容词，表示“（疾病）大规模流行的，广泛蔓延的”，英文解释为“(of a disease) existing in almost all of an area or in almost all of a group of people, animals, or plants”举个🌰：
In some parts of the world malaria is still pandemic.
在世界上一些地区疟疾仍在大规模流行。
Following a call from the World Health Organization for countries to “take urgent and aggressive action,” world leaders are holding crisis talks with health officials to figure out the best way to protect the public from the coronavirus that’s caused more than 4000 deaths worldwide.
One of the things being talked about is herd immunity, but what does this mean, and how does it relate to COVID-19?
Herd immunity (also known as community immunity) is defined by the Centers for Disease Control and Prevention (CDC) as “a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely.”
据维基百科（Wikipedia），群体免疫指人或动物群体中的很大比例因接种疫苗而获得免疫力，使得其他没有免疫力的个体因此受到保护而不被传染。
immune
1）表示“有免疫力”，英文解释为“that cannot catch or be affected by a particular disease or illness”举个🌰：
Adults are often immune to German measles.
成人往往对风疹有免疫力。
2）表示“不受影响”，英文解释为“not affected by sth”举个🌰：
You'll eventually become immune to criticism.
你终究会不用在乎批评了。
3）表示“受保护；免除；豁免”，英文解释为“protected from sth and therefore able to avoid it”举个🌰：
No one should be immune from prosecution.
任何人都不应免于被起诉。
第三个含义类似的有：exempt，同样作形容词，表示“被免除（义务）的，获豁免的”（not affected by something, or not having to do it or pay it）。举个🌰：
Men in college were exempt from military service.
在校男大学生免服兵役。
作动词，表示“免除，豁免”，英文解释为“to give someone permission not to do or pay something”，exempt sb from sth，举个🌰：
Charities are exempted from paying the tax.
慈善团体免付这项税款。
In other words, where herd immunity exists—when lots of people in an area are vaccinated or have already been infected with a disease—fewer people get sick, and fewer germs are able to spread from person to person.
germ /dʒɜːm/
表示“微生物；细菌；病菌”，英文解释为“a very small living thing that can cause infection and disease”举个🌰：
Disinfectant kills germs.
消毒剂可杀菌。
The CDC adds that even people who are not vaccinated, like newborn babies and individuals with chronic illnesses, have some level of protection because the disease can’t spread within the community.
vaccinate
作动词，表示“给…接种疫苗”，英文解释为“If a person or animal is vaccinated, they are given a vaccine, usually by injection, to prevent them from getting a disease.”举个🌰：
Dogs must be vaccinated against distemper.
犬类必须接种疫苗以防犬热病。
上文出现的是名词形式，vaccination，接种疫苗，Vaccination is the act of getting a shot (a vaccine) to protect you against a certain disease. If you want to protect yourself from catching the mumps, you will need to undergo a vaccination.
对比：vaccine：a substance that is put into the blood and that protects the body from a disease 疫苗；菌苗
chronic illness 慢性病
newborn babies 新生儿
immune systems 免疫系统
The theory behind herd immunity is that when someone gets vaccinated, it’s not only that person who is protected from infection—they can’t transmit the disease to other people. Herd immunity protects people who cannot be vaccinated because their immune systems aren’t strong enough and are therefore the most vulnerable to serious illness.
An example of herd immunity via vaccination is the measles outbreak among preschool-age children in the United States. in the late 1980s. The attack rate decreased faster than coverage increased, and researchers who examined the association between incidence of measles and immunization coverage among preschool-age children concluded that immunization coverage of about 80% may be enough to stop sustained measles outbreaks in an urban community.
measles /ˈmiːzəlz/
上文例句中，出现的German measles/ measles表示“麻疹，风疹”，英文解释为“an infectious disease, especially of children, that causes fever and small red spots that cover the whole body”。
Of course, there’s no vaccine for COVID-19 yet. So the herd immunity situation is a little different. The only option is recovery, which means letting the majority of people catch the virus at some point.
In the UK, which has 596 confirmed infections as of March 12, per the BBC, Prime Minister Boris Johnson is reportedly considering herd immunity as a strategy.
per/as per
此处的per sth=according to sth，实际上常用的是as per sth，表示“按照，根据”，如：as per instructions 按照指示，根据说明，还有另一个短语是“（as） per usual/normal”表示“照常，按惯例”（as usual），举个🌰：
He was late, as per usual.
他照例迟到了。
那么，per和as per有什么区别呢？此处的as是否冗余了呢？
The fact is that both per and as per have existed in English in the sense “according to” for a very long time–since the 15th and 16th centuries, respectively. The choice of which to use (or avoid) is entirely a matter of taste. The more ponderous as per is often found in business and legal prose, or in writing that attempts to adopt a formal tone.
It is not incorrect to use, but some find it overly legalistic and counsel avoiding it for that reason. On the other hand, it has been used to good effect in facetious mock-business-English (“as per the President's shiny new Environmental Policy Act”). As in so many matters of diction, the tonal needs of a particular passage should guide your choice. (Merriam-Webster)
The COVID-19 situation is changing rapidly day by day, and the only thing we know for sure is that we have a long road ahead of us. So we can expect conversations—about herd immunity and various other measures—to continue for the foreseeable future.
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