Understanding Vaccines
认识疫苗
林晓洁 供稿
As COVID-19 continues to spread around the world, people are all waiting desperately for an effective vaccine. Let’s learn more about vaccines and how they work.
随着新冠病毒在世界范围的持续肆虐，人们正迫切地期待着有效的疫苗。让我们来进一步认识疫苗，了解它是如何保护人们的吧！
The Immune System
免疫系统
Credited for causing drastic reductions in dangerous diseases like measles and polio, vaccines are widely heralded as one of the greatest public health achievements in modern history. To understand how they work, it helps to first look at how our body fights illness.
疫苗大幅降低了麻疹和脊髓灰质炎这类危险疾病的发病率，因而被誉为现代历史上最伟大的公共卫生成就之一。要了解疫苗是如何起作用的，首先要了解人体是如何对抗疾病的。
When germs, such as bacteria or viruses, invade the body, they cause illness. The immune system uses several tools to fight infection. Blood contains red blood cells, for carrying oxygen to tissues and organs, and white or immune cells, for fighting infection. These white cells consist primarily of macrophages, B-lymphocytes and T-lymphocytes:
细菌和病毒等微生物是导致疾病形成的原因。免疫系统通过几种工具来对抗感染，例如血液里的红细胞和白细胞。红细胞能向组织和器官输送氧气，而白细胞能够抗击感染。白细胞又称免疫细胞，主要由巨噬细胞、B细胞和T细胞组成：
Macrophages are cells that swallow up and digest germs, plus dead or dying cells. The macrophages leave behind parts of the invading germs called antigens. The body identifies antigens as dangerous and stimulates antibodies to attack them.
巨噬细胞能够吞噬和消化细菌以及死亡或垂死的细胞，并留下一部分称为抗原的入侵细菌。人体会识别出抗原的危险性，并刺激抗体攻击它们。
B-lymphocytes are defensive white blood cells. They produce antibodies that attack the antigens left behind by the macrophages.
B细胞是防御性的白细胞。它们能够产生抗体，攻击巨噬细胞留下的抗原。
T-lymphocytes are another type of defensive white blood cell. They attack cells in the body that have already been infected.
T细胞是另一种防御性白细胞，能够攻击人体内已被感染的细胞。
The first time the body encounters a germ, it can take several days to make and use all the germ-fighting tools needed to get over the infection. After the infection, the immune system remembers what it learned about how to protect the body against that disease. The body keeps a few T-lymphocytes, called memory cells, which go into action quickly if the body encounters the same germ again. When the familiar antigens are detected, B-lymphocytes produce antibodies to attack them.
当人体第一次接触到某种细菌时，需要几天时间制造和使用抗菌工具来克服感染。免疫系统会在感染后记住如何保护人体免受疾病再次侵扰。一些被称为记忆细胞的T细胞在遇到同样细菌时会迅速行动起来。一旦检测到熟悉的抗原时，B细胞也会产生抗体攻击它们。
How Vaccines Work
疫苗是如何起作用的
Vaccines help develop immunity by imitating an infection. This type of infection, however, almost never causes illness, but it does cause the immune system to produce T-lymphocytes and antibodies. Sometimes, after getting a vaccine, the imitation infection can cause minor symptoms, such as fever. Such minor symptoms are normal and should be expected as the body builds immunity.
疫苗通过模仿感染来增强人体免疫力。这种感染很少真正导致疾病，但能使免疫系统产生T细胞和抗体。在接种疫苗后，有时会产生发烧等轻微症状。鉴于人体正在建立免疫，因此这些轻微症状是正常现象，在所难免。
Once the imitation infection goes away, the body is left with a supply of “memory” T-lymphocytes, as well as B-lymphocytes that will remember how to fight that disease in the future. However, it typically takes a few weeks for the body to produce T-lymphocytes and B-lymphocytes after vaccination. Therefore, it is possible that a person infected with a disease just after vaccination could develop symptoms and get a disease, because the vaccine has not had enough time to provide protection.
一旦模仿感染结束，人体内会留下大量记忆细胞和B细胞，它们会记住如何对抗这种疾病。但接种疫苗后，人体通常需要几周的时间来产生T细胞和B细胞。因此，在刚刚接种后感染了疾病的人会出现一些症状和患病，这是因为疫苗还没有足够的时间给人体提供保护。
Types of Vaccines
疫苗的类型
Scientists take many approaches to develop vaccines. Today there are five main types of vaccines:
科学家采用多种方法来研制疫苗。目前主要有五种疫苗：
Live, attenuated vaccines fight viruses and bacteria. These vaccines contain a version of the living virus or bacteria that has been weakened so that it does not cause serious disease in people with healthy immune systems, because live, attenuated vaccines are the closest thing to a natural infection, they are good teachers for the immune system. Even though they are very effective, not everyone can receive these vaccines. Children with weakened immune systems cannot get live vaccines.
减毒活疫苗针对病毒和细菌。该疫苗含有已被削弱的活性病毒或细菌，不会对免疫系统健康的人造成严重疾病。由于减毒活疫苗最接近自然感染，它们是免疫系统的好老师。尽管这类疫苗非常有效，但并不适合所有人。免疫系统薄弱的儿童就无法接种减毒活疫苗。
Inactivated vaccines also fight viruses and bacteria. These vaccines are made by inactivating, or killing the germ during the process of making the vaccine. The inactivated polio vaccine is an example of this type of vaccine. Inactivated vaccines produce immune responses in different ways than live, attenuated vaccines. Often, multiple doses are necessary to build up and maintain immunity.
灭活疫苗也针对病毒和细菌。该疫苗由灭活或死亡的细菌制成。脊髓灰质炎疫苗就是一种灭活疫苗。灭活疫苗与减毒活疫苗产生免疫反应的方式不同。为了建立和维持免疫力，通常需要多次注射灭活疫苗。
Toxoid vaccines prevent diseases caused by bacteria that produce toxins in the body. In the process of making these vaccines, the toxins are weakened so they cannot cause illness. Weakened toxins are called toxoids. When the immune system receives a vaccine containing a toxoid, it learns how to fight off the natural toxin.
类毒素疫苗针对由细菌引起的疾病，这些细菌会在人体内产生毒素。在研发疫苗的过程中，这些疫苗含有被削弱的毒素，并不会引起疾病。减弱的毒素被称为类毒素。当免疫系统接触到含有类毒素的疫苗时，就会懂得如何对抗同类型的天然毒素。
Subunit vaccines include only parts of the virus or bacteria, or subunits, instead of the entire germ. Because these vaccines contain only the essential antigens and not all the other molecules that make up the germ, side effects are less common.
亚单位疫苗只包括病毒或细菌的部分或亚基。由于该疫苗只含有基本抗原，没有构成细菌的其他分子，因此这种疫苗少有副作用产生。
Conjugate vaccines fight a different type of bacteria. These bacteria have antigens with an outer coating of sugar-like substances called polysaccharides. Conjugate vaccines are effective for these types of bacteria because they conjugate the polysaccharides to antigens that the immune system responds to very well. This linkage helps the immature immune system react to the coating and develop an immune response.
结合疫苗针对不同类型的细菌。这些细菌的抗原有着被称为多糖的类糖物质外层。结合疫苗对多种细菌起效的原因在于它将多糖与抗原结合，使得免疫系统对抗原反应良好。这种结合能够帮助不成熟的免疫系统对多糖外层做出反应，并产生免疫反应。
Vaccines Require More Than One Dose
疫苗需要不止一剂
Studies have shown that more than one dose is needed for everyone to develop the best immune response. In the case of flu vaccines, people need to get a dose every year, because the flu viruses causing disease may be different from season to season. Also, the immunity people get from a flu vaccination wears off over time.
研究表明，要产生最佳的免疫反应，人们需要不止一剂疫苗。就流感疫苗而言，每年都需要接种一次，因为引起疾病的流感病毒可能因季节而异。此外，人体因流感疫苗而产生的免疫力会随着时间逐渐消失。
Some people believe that naturally acquired immunity—immunity from having the disease itself—is better than the immunity provided by vaccines. However, natural infections can cause severe complications and be deadly. Even with an advanced health care system, the diseases can still be very serious, and vaccination is the best way to prevent them.
有些人认为，自然获取的免疫力，即通过生病而产生的免疫力，比疫苗提供的免疫力更好。然而，自然感染会导致严重甚至致命的并发症。即使拥有先进的医疗系统，疾病的后果也可能非常严重，而接种疫苗则是预防这些疾病的最佳方式。
【VOCABULARY】
1. herald v. 预示……的来临；宣布
2. B-lymphocyte n. B淋巴细胞
3. T-lymphocyte n. T淋巴细胞
4. antigen n. （免疫）抗原
5. attenuated adj. （力量或效果）减弱的
6. polio n. 脊髓灰质炎；小儿麻痹症
7. molecule n. （化学）分子；微粒
8. conjugate v. （细菌，单细胞生物）结合；接合
9. polysaccharide n. 多糖类
10. wear off v. 逐渐消失或除去