Some experts predict that giving a vaccine to just two-thirds of a population could protect whole communities or nations. But the effectiveness of such efforts is highly dependent on what the vaccines will be able to prevent.
有专家预测，仅需要2/3的人口接种疫苗就能保护整个社区或国家。但是这种措施的有效性很大程度上取决于疫苗能够预防什么。
What is herd immunity?
什么是群体免疫？
Herd immunity happens when a large percentage "of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely. As a result, the whole community becomes protected — not just those who are immune," the Mayo Clinic notes on its website.
梅奥诊所在其网站上表示，当社区中很大一部分人口对某种疾病免疫，使得这种疾病无法在人与人之间传播，这就形成了群体免疫。于是整个社区得到了保护，而不仅仅是获得免疫的人。
But there are still many open questions about herd immunity and COVID-19 vaccines.
但是关于群体免疫和新冠疫苗仍存在许多悬而未决的问题。
For example, will the first vaccines be able to stop the spread of the virus, or just stop people from getting sick? How many people in a population will accept a vaccine? Will vaccines offer the same protection to everyone?
例如，第一批疫苗是否能够阻止这种病毒的传播？还是只能阻止人们患病？有多少人会接种疫苗？疫苗是否会给每个人提供相同的保护？
Josep Jansa is an expert at the Stockholm-based European Centre for Disease Prevention and Control (ECDC).
何塞普·扬绍是位于斯德哥尔摩的欧洲疾病预防和控制中心的专家。
"Herd immunity is sometimes wrongly understood as individual protection," Jansa told the Reuters news agency. He added, "Herd immunity refers to community protection, not to how an individual is protected."
扬绍对路透社表示：“群体免疫有时候被错误理解为个体防护。”他还表示：“群体免疫是指社区防护，而不是指个体如何得到防护。”
R Value and disease spread
R值和疾病传播
One of the central questions about herd immunity and COVID-19 is the reproduction rate, or R value, of the virus that causes it. R value is a measure of how many people get the virus from an average infected individual in environments where no restrictions are in place. For example, an R value of 2 means an infected person would be expected to pass the virus on to 2 other people.
有关群体免疫和新冠肺炎的核心问题之一是这种病毒导致的传染指数或R值。R值是用于衡量在没有任何限制的环境中，有多少人会从普通患者身上感染病毒。例如，R值为2就意味着一名感染者会传染另外两个人。
Winfried Pickl is a professor at the Medical University of Vienna.
Winfried Pickl是维也纳医科大学教授。
"The problem is that for now we don't know exactly how fast the virus spreads without any precautions and with the normal travel and social activities we had a year ago," Pickl told Reuters.
Pickl对路透社表示：“问题是，目前我们不知道该病毒在没有任何预防措施的情况下，它在我们一年前这种正常的旅行和社交活动中的传播速度。”
Health experts have noted that anything less than 100 percent vaccine efficacy would require an increase in vaccinations to reach herd immunity. Early data on the leading COVID-19 vaccines – from Pfizer-BioNTech and Moderna - suggests only about 90 percent efficacy.
卫生专家指出，若疫苗效果低于100%，则需要增加疫苗接种量才能达到群体免疫。辉瑞公司联合BioNTech公司以及莫德纳公司提供的主要新冠疫苗的早期数据表明，其有效率仅为90%左右。
Amesh Adalja is with the Johns Hopkins Center for Health Security. He said a good target for immunity in the United States would be to have more than 70 percent of the population vaccinated. But the number could go up if vaccines are less effective, he added.
Amesh Adalja任职于约翰霍普金斯大学健康安全中心。他说，美国实现群体免疫的目标是让70%以上的人口接种疫苗。但他还说，如果疫苗效果不好，这一数字可能还会上升。
The ECDC uses an estimated herd immunity level of 67 percent for its models. In Germany, Chancellor Angela Merkel said this month that COVID-19 restrictions there could be lifted once 60 to 70 percent of the population reached immunity.
欧洲疾病预防和控制中心在其模型中采用了67%的群体免疫率。德国总理安哥拉·默克尔本月表示，一旦有60%到70%的人口获得免疫，就可以取消新冠肺炎限制措施。
World Health Organization experts have also suggested a 65 to 70 percent vaccine coverage rate as a way to reach population immunity through vaccination.
世卫组织专家还建议疫苗覆盖率达到65%到70%，以此作为通过疫苗接种获得群体免疫的途径。
Vaccines and virus transmission
疫苗和病毒传播
Experts say another important question is whether vaccines can stop the spread of the coronavirus, which causes the disease COVID-19.
专家表示，另一个重要问题是这些疫苗是否能阻止导致新冠肺炎的新冠病毒传播。
Evidence so far suggests the first COVID-19 vaccines will at least stop people from developing the disease. But it cannot be ruled out that people will still catch the virus and pass it on to others.
迄今为止的证据表明，首批新冠疫苗将至少能阻止人们患病。但是不能排除人们仍然会感染并病毒并传播给他人。
Bodo Plachter is a professor and deputy director of the Institute of Virology at Germany's Mainz University teaching hospital. Plachter said that respiratory infections can be hard to block completely with vaccines - although the shots are expected to reduce the amount of virus that is moving around.
博多·普拉希特是德国美因茨大学教学医院病毒研究所的教授兼副所长。普拉希特表示，用疫苗很难完全阻断呼吸道感染，尽管注射疫苗可以减少正在传播的病毒数量。
"It may well be that vaccinated people will shed fewer viruses," he said. "But it would be a mistake," he added, to think that "vaccination alone can suppress a pandemic."
他说：“接种疫苗者很可能会降低病毒的传播。”他还表示：“但是认为仅仅接种疫苗就能抑制大流行的观点是错误的。”