字幕列表 影片播放 列印英文字幕 When a new disease emerges and starts infecting people in a population, one of the things we really want to know is whether it's going to continue to spread and infect more and more people, or whether it's eventually going to die out Some diseases like measles are highly infectious In a fully susceptible population, each measles case will on average infect about 16 to 18 additional people Something like flu, however, is less transmissible each case, on average, would infect between 2 to 3 people The number of cases that each infectious person generates can vary for different diseases and we call this number the Basic Reproduction Number, or R0 for short R0 doesn't depend on how severe the symptoms are, rather it's a measure of how transmissible the infection is and as a result we can use it to work out what's required to stop an epidemic One of the ways we can stop epidemics is using vaccination Now you might think that to stop a disease you need to vaccinate the entire population but actually this isn't the case Of course if you vaccinate someone it protects them, and stops them from getting infected But because people who are vaccinated can't pass it on, vaccination also stops the chains of transmission and that means that this can create a protective barrier which actually stops the epidemic spreading within a population Say the basic reproduction number of an infection is 2 This means that in a fully susceptible population, each infected person will on average give the disease to 2 other people But if 50% of the population are vaccinated, each infectious person on average will only be able to give it to 1 of these 2 people and this means that the epidemic wouldn't be expected to grow over time This is known as herd immunity Herd immunity means that long as a certain proportion of the population is vaccinated, the disease won't be able to transmit within that population We've seen that if the basic reproduction number, R0, is 2, we need to vaccinate half the population to stop disease transmission By the same logic, if R0 is 3, we need to vaccinate two-thirds of the population If we keep going, for highly infectious diseases like measles, we need to vaccinate 17 out of every 18 people or 94% of the population to stop transmission Herd immunity is especially useful for protecting members of the population who can't be vaccinated perhaps because they're too old, too young, or have weak immune systems If they're surrounded by people who have received the vaccine then that can protect them from infection If however people forego vaccination then the herd can no longer protect these people This means that the population as a whole can be vulnerable to outbreaks So far, we've been talking about averages, one of the big challenges in my work is incorporating some of the complexities of reality into these mathematical models Vaccines are an incredibly powerful public health tool and the best way to protect populations against disease is to make sure as many people are vaccinated as possible so we don't run the risk of slipping below this vaccine threshold for herd immunity

B2 中高級 美國腔 武漢肺炎 新型冠狀病毒 新冠肺炎 COVID-19 什麼是群體免疫力？ (What is Herd Immunity?) 564 21 Sophie 發佈於 2021 年 01 月 14 日 更多分享 分享 收藏 回報 影片單字