最近，很多人说接种疫苗（如麻疹疫苗）后，疾病暴发时的人数依然居高不下，这是真的吗？
保罗Offit，传染病专家，说：
这是事实，但是，这并不意味着疫苗不起作用。它只是意味着它们不完美。在疾病暴发时，极少数量的接种疫苗的人会生病。并且，因为疫苗接种者比疫苗未接种者的人数多很多，因此得病的接种人数量就显得高于未接种人的数量了。
想知道这是如何发生的，想象一下，一所学校有1000名学生， 995人已接种麻疹疫苗，5人未接种。暴发麻疹时有10个接种疫苗的儿童被感染，而所有5个未接种疫苗的儿童都被感染。因此，尽管接种疫苗的学生感染人数更多，但接种疫苗的人只有1%发病，而未接种疫苗的人100%发病了，接种疫苗发病率要低得多。
没有疫苗可以保证每个接种者都不得病。原因是这样的，有些人接种了疫苗的人也不会产生免疫保护，并且依然保持易患病的状态，这其中的机理我们至今也不明白。另外，随着时间的推移有些人不接受加强免疫，可能疫苗就会失去免疫效果，但疫苗仍然尽可能地防止你被传染这种疾病。
1999年7月在《Journal of the American Medical Association（美国医学协会杂志）》发表的一项研究表明，在一次麻疹爆发过程中，未接种疫苗的人感染麻疹的几率为接种疫苗者的35倍。也有其他一些研究证实了这个结果。
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原文如下：
Do vaccines really work?
Is it true that these days, most people who come down with a disease for which there's a vaccine (such as measles) have been vaccinated against it?
Paul Offit
infectious disease expert
It is true, but that doesn't mean vaccines don't work. It just means they're not perfect. In the rare case of an outbreak, a certain number of vaccinated people will get sick. And because there are so many more vaccinated than unvaccinated people in this country, the immunized people who get sick are likely to outnumber the ones who aren't immunized.
To see how this works, imagine that measles strikes a school of 1,000 students. Of the 995 who have been vaccinated, ten become infected, while all five of the unvaccinated children in the school get sick. So although more vaccinated students got measles, the chances of coming down with the disease were much lower for people who got the vaccine (about 1 percent) than for people who didn't (100 percent).
No vaccine guarantees immunity to everyone who's vaccinated. For reasons we don't understand, some people who are vaccinated don't become immune and remain susceptible to the disease. And some people lose their immunity over time if they don't receive the recommended booster shots. But vaccination still offers the best chance of protection from infectious disease.
A study published in the Journal of the American Medical Association in July 1999 found that during a measles outbreak, unvaccinated people were 35 times more likely to get measles than vaccinated people. Several studies have since confirmed these findings.
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