随着冠状病毒病例在美国各地的热点地区继续攀升，几项药物试验第一阶段的积极结果带来了希望，即疫苗将很快帮助美国人恢复正常生活。
但是专家强调，即使疫苗不是100%有效，它仍然是对抗病毒的一个安全和重要的工具。
事实上，“疫苗从来都不是100%有效的，”世界卫生组织卫生应急计划执行主任迈克尔·瑞安博士在周三的问答环节中说。
Ryan说:“我们将在2-3个月内有一个疫苗，然后这种病毒会突然消失的想法-我很想在这里对你说，但这是不现实的。”
世卫组织称，总有一小部分人在接种疫苗后得不到保护，而对其他人来说，这种保护可能会随着时间的推移而减弱。
在评估疫苗在普通人群中的有效性时，专家们致力于估计接种疫苗的人群中有多少人会受到疫苗的保护。例如，根据疾病预防控制中心的数据，每年流感疫苗的有效性一般在40-60%之间。因此，平均而言，流感疫苗将一个人患流感的风险降低了60%。
有几个因素会影响疫苗的有效性，如个体的年龄、共病和以前接触过的疾病。根据美国微生物学会发表的一项研究，疫苗自身的特性——它的成分，它的传递方式，不管它是活的还是不活的——也决定了它的有效性。
瑞安说，疫苗在“大多数”人身上产生免疫力，他指出麻疹疫苗是“高效”疫苗的一个例子，可以为大约95%的人提供免疫力。
护士凯瑟琳·奥姆斯特德准备了一个镜头，作为世界上最大的一个可能的COVID-19疫苗的研究，由美国国立卫生研究院和现代公司开发，开始于2020年7月27日，在纽约宾汉姆顿
在疫苗据美国疾病控制与预防中心称，目前作为美国儿童免疫计划的一部分，三剂脊髓灰质炎疫苗的有效性为99-100%，两剂水痘疫苗的有效性约为90%，一剂麻疹减毒活疫苗的有效性约为97%。
然而，科学家们仍然不知道COVID-19疫苗会有多有效，瑞安说。截至7月28日，世界卫生组织报告有25种候选疫苗正在临床评估中。
美国食品和药物管理局宣布，该疫苗的目标是至少有50%的有效性，它可能需要助推器。
但是专家说，仅仅因为疫苗不完美，并不意味着它不能挽救生命。
普渡大学的里夏德·库恩、特伦特和朱迪丝·安德森杰出的科学教授告诉美国广播公司新闻:“有一些病原体在那里，我们得到加强注射。”“你每10年打一次破伤风针。你得到了保护，但不会是永远。”
库恩说:“我们没有一个完美的世界。”“你需要疫苗，你需要接种疫苗。”
如果疫苗不是100%有效，它是安全的吗?
“大多数人认为疫苗是针对个人的——他们接种疫苗是因为他们不想生病。但是疫苗接种不是针对个人的...华盛顿大学微生物学教授黛博拉·富勒博士在接受美国广播公司采访时说:“这是为了保护我们的社区。”
“病毒是机会均等的入侵者。他们会感染任何他们能感染的人。”
事实上，它们可以使整个国家丧失能力，正如COVID-19在美国和许多其他国家所做的那样。
但是随着越来越多的人接种疫苗，许多人将获得对感染的免疫力——因此病毒将有更少的宿主入侵。
2020年7月27日，在纽约宾汉姆顿，美国国立卫生研究院和现代公司开发的世界上最大规模的疫苗研究正在进行，一张海报展示在入口处寻找志愿者
富勒解释说:“当你达到约50%的人口免疫，这是一个支点，你将开始看到感染人数下降。”
哥伦比亚大学研究高致病性病毒的病毒学家安吉拉·拉斯穆森博士说，一种至少50%有效的疫苗还有其他好处。
拉斯穆森说，这将“极大地帮助降低疾病的严重程度，并可能不仅降低死亡的可能性，而且降低长期存在健康问题的病例数。”这对于减轻COVID-19给卫生保健系统带来的负担也是非常有益的。”
有多少人需要接种疫苗才能减少感染?
Fuller说:“这个问题直接关系到群体免疫的核心，它的不同取决于病毒的传播程度、最易感染的人群以及疫苗的有效性。”
她解释说:“麻疹病毒是高度传播的，你需要93-95%的人口接种疫苗。”“就流感而言，70%的疫苗接种率是必需的，但每年我们都达不到这一目标——这就是为什么即使在典型的流感季节，每年仍有30-60，000人死于流感。”
富勒说:“我们对COVID-19还有很多不了解的地方，比如它的传染性和疫苗的有效性，以便确定需要接种疫苗的人口比例。”
然而，富勒说，美国科学进步协会发表的一项新研究估计，只有大约43%的人口需要免疫才能获得针对COVID-19的群体免疫。
她说:“如果是真的，那真是个好消息。”
专家表示，公众对疫苗接种的不信任度最近特别高，官员们担心大量美国人会拒绝接种疫苗——这本身可能会延长大流行的持续时间。
尽管如此，富勒说:“我们必须记住，真正的敌人是病毒，病毒不关心你的政治，你的种族，你的性别或你的财富。”
她说:“真正的战争是人与病毒的战争，疫苗和口罩是我们的武器。”“我们需要找到一种方法，让整个社区、整个人口团结起来，用这些武器来对抗这种病毒。否则我们就冒着让这种病毒得逞的风险。”
《美国广播公司新闻》的克里斯汀·泽奥多洛对此报道有所贡献。
A COVID-19 vaccine will still save lives even if it's not 100% effective, experts say
As coronavirus cases continue to climb in hot spots across the U.S., positive results from the first phase of several drug trials have raised hopes that a vaccine will soon help Americans return to a normal life.
But experts are stressing that even if the vaccine is not 100% effective, it will still be a safe and important tool in the fight against the virus.
In fact, "vaccines are never 100% effective," said Dr. Michael Ryan, Executive Director of the World Health Organization's Health Emergencies Program, during a question and answer session on Wednesday.
"The idea that we're going to have a vaccine in 2-3 months, and then all of a sudden this virus is going to go away -- I would love to be here saying that to you, but that's just not realistic," Ryan said.
There is always a small percentage of people who are not protected after vaccination, while for others, the protection may diminish over time, according to the WHO.
In assessing a vaccine's effectiveness within the general population, experts work to estimate the percentage of vaccinated people that would be expected to be protected by the vaccine. The annual flu shot's effectiveness, for example, generally ranges between 40-60%, according to the CDC. Hence, on average, the flu vaccine reduces a person's risk of a flu outcome by 60%.
There are several factors that impact a vaccine's effectiveness, such as an individual's age, comorbidity, and prior exposure to the disease. The vaccine's own characteristics -- its composition, its mode of delivery, whether it's live or inactive -- also determine its effectiveness, according to a study published by the American Society for Microbiology.
Vaccines generate immunity in "most" people, said Ryan, pointing to the measles vaccine as an example of a "highly effective" vaccine that provides immunity to approximately 95% of people.
Among thevaccinescurrently recommended as part of the U.S. Childhood Immunization Schedule, three doses of the polio vaccine are 99-100% effective, two doses of the chickenpox vaccine are about 90% effective, and one dose of the MMR vaccine is about 97% effective at preventing rubella, according to the CDC.
However, scientists still do not know how effective a COVID-19 vaccine will be, said Ryan. As of July 28, the WHO reports there are 25 vaccine candidates in clinical evaluation.
The FDA announced it is aiming for the vaccine to have at least 50% effectiveness, and it may require boosters.
But experts say just because a vaccine is not perfect, it doesn't mean it's not life-saving.
"There are pathogens out there in which we get booster shots," Richard Kuhn, Trent and Judith Anderson Distinguished Professor in Science at Purdue University, told ABC News. "You get a tetanus shot every 10 years. You are getting protected, but it's not going to be forever."
"We don't have a perfect world," Kuhn said. "You need a vaccine, and you need to get vaccinated."
Is a vaccine safe if it is not 100% effective?
"Most people think about vaccines as something personal -- they take a vaccine because they don't want to get sick. But vaccination is not about the individual ... it's about protecting your community," Dr. Deborah Fuller, professor of microbiology at the University of Washington, told ABC News.
"Viruses are an equal opportunity invader. They will infect anyone they can," Fuller said.
They can, in fact, incapacitate an entire nation, as COVID-19 has done in the United States and many other countries.
But as more people are vaccinated, many will acquire immunity to infection -- and thus the virus will have fewer hosts to invade.
"When you hit about 50% immunity in the population, that's the pivot point where you will start to see a decline in the number of people getting infected," explained Fuller.
A vaccine that is at least 50% effective will have other benefits, according to Angela Rasmussen, Ph.D., a virologist at Columbia University who studies highly pathogenic viruses.
Rasmussen said it would "help tremendously by reducing disease severity and possibly decrease not only the likelihood of death, but also the number of cases with lasting long-term health problems. This would also be hugely beneficial in relieving the burden on the health care system caused by COVID-19."
How many people will need to be vaccinated to see a decline in infections?
"This question goes to the heart of herd immunity and is different depending on how transmissible the virus is, what demographics are most susceptible, and how efficacious the vaccine is," said Fuller.
"The measles virus is highly transmissible, and you need 93-95% of the population vaccinated," she explained. "For influenza, 70% vaccination rate is required, yet each year we fall short of that -- which is why even in a typical flu season we still see 30-60,000 people die from flu each year."
There is still much that we do not know about COVID-19 -- such as its transmissibility and how effective a vaccine will be -- in order to determine for sure what percentage of the population will need to be vaccinated, Fuller said.
However, Fuller said, a new study published in the American Association with the Advancement of Science estimates that only about 43% of the population will need to be immune to achieve herd immunity against COVID-19.
"If true, it's really good news," she said.
Experts say that public distrust in vaccinations is particularly high these days, and there is concern among officials that a significant number of Americans will refuse to get the shot -- which itself could prolong the duration of the pandemic.
Despite such concerns, Fuller says that "we have to remember that the real enemy is the virus and the virus doesn't care about your politics, your race, your sex or your wealth."
"The real war is people versus the virus, and vaccines and masks are our weapons," she said. "We need to find a way to band together as a community, as a population, to use these weapons to fight this virus. Or we risk letting this virus win."
ABC News' Christine Theodorou contributed to this report.
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