6月26日，安东尼·福西博士宣布新冠肺炎三分之二的美国人服用70-75%效力的疫苗可以提供群体免疫到非典-CoV-2冠状病毒。
他的声明激起了关于美国反疫苗运动的讨论。
然而，一个至关重要的问题仍未得到解答:COVID-19是否受到群体免疫?从大学到运动队，顶尖专家仍在争论这个问题。
当全世界都在焦急地等待疫苗的时候，它所能提供的保护性免疫的持续时间和持久性却远比人们想象的更令人怀疑。
中国的一项新研究表明，抗体可能在两到三个月内消失。该研究进一步发现，无症状患者的免疫力比有症状的患者短:一个人的症状越少，免疫反应和抗体强度就越弱。犹他大学传染病主任桑卡·斯瓦米纳坦说:“患有轻微疾病或无症状疾病的年轻人，他们的抗体可能永远不会升高。”。“我们甚至不知道这些抗体是否有保护作用。”
群体免疫是如何工作的?
群体免疫当足够比例的人群通过既往疾病或疫苗接种对传染病免疫时发生，因此不太可能在人与人之间传染。
根据约翰·霍普金斯的说法，70%-90%的人口(2.3亿-3亿美国人)需要开发针对COVID-19的保护性抗体来实现群体免疫。
大约274万美国人冠状病毒检测呈阳性，其中超过13万人死亡(病死率4.74%)。相比之下，美国流感的病死率约为0.1%。
如果不存在COVID-19疫苗，任何合理的数据推断——即使是目前病死率的一半，也意味着在我们获得群体免疫之前，我们将看到超过500万人死亡的7位数。
COVID-19疫苗如何影响群体免疫?
有三个因素决定了疫苗是否以及在多大程度上能够安全地将我们带到群体免疫中，而不会使个体暴露于疾病本身的威胁生命的后果中。
首先是疫苗的效力——例如，麻疹疫苗有97-98%的效力。Fauci博士认为，对于COVID-19，我们不可能得到超过75%有效的疫苗。
其次是疫苗的普遍使用——这是福西博士对反疫苗运动的关注发挥作用的地方。
Fauci指出“在这个国家的一些人中间有一种普遍的反科学、反权威、反疫苗的感觉——相对来说，这个比例高得惊人”他说，鉴于反疫苗运动的力量，“我们有很多工作要做”来教育人们关于疫苗。
第三是疫苗诱导的抗体免疫反应的持久性和持久性。这就是抗体的两个月到三个月寿命成为一个问题的地方。
阿斯利康公司是致力于开发有效疫苗的公司之一，该公司的一名高管告诉一家广播电台，他认为他的疫苗只能提供一年的保护。
我们对COVID-19抗体了解多少?
抗体是特异性结合入侵病原体以中和它们的蛋白质，因此它们不会感染宿主细胞。它们引发了一种被称为吞噬作用的机制，这种机制可以摧毁病毒。IgG抗体是最常见的，可以保护我们免受细菌和病毒感染。
抗COVID-19的IgG抗体通过感染SARS-CoV-2或通过产生免疫保护反应的疫苗产生。
这就是事情变得危险的地方。在4月24日的一份科学简报中，世界卫生组织表示，“目前没有证据表明，从COVID-19中康复并有抗体的人可以避免第二次感染。”他们描述了已经感染冠状病毒的人对疾病进一步感染的获得性免疫的概念，这是一个未经证实且不可靠的理论。
福西在接受《美国医学会杂志》总编辑霍华德·鲍彻纳的采访时说:“这并不是一个一致的强有力的抗体反应，这可能是为什么当你查看导致普通感冒的普通冠状病毒的历史时，文献中的报告显示，保护性免疫的持久性从3个月到6个月不等，几乎总是不到一年。”。
2020年5月4日，在巴尔的摩的马里兰大学医学院，第一名患者参加了辉瑞公司的COVID-19冠状病毒疫苗临床试验，并接受了注射。2020年7月1日，星期三，辉瑞公司和它的德国合作伙伴BioNTech对四种实验性COVID-19疫苗进行了测试，其中第一种疫苗在45人的早期测试中显示出令人鼓舞的结果。马里兰大学医学院通过美联社
结果
联合王国、瑞典和巴西都允许群体豁免以这样或那样的方式影响它们对COVID-19的处理方式，并带来严重后果。
3月中旬，英国政府首席科学顾问帕特里克·瓦兰斯(Patrick Vallance)宣布，他们正在对COVID-19采取一种“建立某种群体免疫力”的方法，但由于存在致命风险，这种方法很快就出现了逆转。
同样，根据欧洲委员会的说法，瑞典试图通过一种方法获得群体免疫，这种方法最终使COVID-19的人均死亡率达到世界最高，并且没有可衡量的相关经济收益。截至5月底，只有6.1%的瑞典人产生了冠状病毒抗体——这个数字比预测的要低得多。
作为参考，一项在西班牙进行的大规模研究表明，只有5%的西班牙人患有糖尿病产生抗体。这项研究表明，尽管西班牙是受病毒打击最严重的国家之一，但抗体的存在仍然只有5%左右，这还不足以实现群体免疫。
大多数专家表示，群体免疫要求至少60-70%的人群具有抗体，尽管这个数字因病毒而异。因为这种程度的感染会对健康护理系统和对人群而言，实现群体免疫的最安全的方法是通过广泛可用、广泛使用的有效疫苗。
瑞典有争议的方法的糟糕结果导致了对政府应对公共卫生危机行动的正式调查。
巴西的情况也好不到哪里去，其冠状病毒病例数量位居世界第二。
结论
卫生局局长杰罗姆·亚当斯博士最近建议一个领先的国家医师组织，美国远未达到开始真正讨论群体免疫所需的70%以上的感染率。作为参考，亚当斯提到他的家乡印第安纳州目前的感染率约为3%。
亚当斯博士进一步重申了福西博士的担忧，即抗体的鲁棒性似乎有限，因此尚不清楚对COVID-19的获得性免疫有多重要或有效。虽然还不清楚一个人是否会不止一次感染COVID-19，但免疫保护性IgG抗体的有限持久性可能暗示了一个世界不愿听到的答案。
Is COVID-19 even subject to herd immunity?
On June 26, Dr. Anthony Fauci announced it's "unlikely" that aCOVID-19vaccine with 70-75% efficacy taken by two-thirds of Americans can provideherd immunityto the SARS-CoV-2coronavirus.
His statement has since stirred discussion about America's anti-vaccine movement.
A crucial question remains unanswered, however: Is COVID-19 even subject to herd immunity? From universities to sports teams, top experts are still debating this issue.
While the world anxiously awaits a vaccine, the length and durability of the protective immunity it would provide is far more in doubt than one might think.
A new study from China shows that antibodies can disappear in two to three months. The study further found that immunity is shorter for asymptomatic patients than symptomatic ones: The less symptomatic a person is, the weaker the immune response and antibody strength. "Young people who have mild disease or asymptomatic disease, their antibodies may never rise very high," said Sankar Swaminathan, chief of infectious diseases at the University of Utah. "We don't even know if those antibodies are protective."
How does herd immunity work?
Herd immunityoccurs when a sufficient proportion of a population is immune to an infectious disease -- either through prior illness or vaccination -- so that contagion from person to person is unlikely.
According to Johns Hopkins, 70%-90% of the population (230-300 million Americans) needs to develop protective antibodies to COVID-19 to achieve herd immunity.
Approximately 2.74 million Americans have tested positive for the coronavirus, over 130,000 of which have died (case fatality of 4.74%). By contrast, the case fatality of the flu in the U.S. is roughly 0.1%.
Absent the existence of a COVID-19 vaccine, any reasonable extrapolation of the data -- even at half the current case fatality rate, means we will see a seven-figure body count that exceeds 5 million deaths before we can attain herd immunity.
How does a COVID-19 vaccine impact herd immunity?
There are three factors that determine if and how well a vaccine can safely bring us along to herd immunity without exposing individuals to the life-threatening consequences of the disease itself.
First is the vaccine's efficacy -- for example, the measles vaccine is 97-98% effective. Dr. Fauci believes that for COVID-19, we are unlikely to get a vaccine that is more than 75% effective.
Second is the vaccine's prevalence of use -- this is where Dr. Fauci's concern about the anti-vaccine movement comes into play.
Fauci noted that "there is a general anti-science, anti-authority, anti-vaccine feeling among some people in this country -- an alarmingly large percentage of people, relatively speaking." He said given the power of the anti-vaccine movement, "we have a lot of work to do" to educate people on the truth aboutvaccines.
Third is the durability and longevity of the vaccine's induced antibody immuno-response. This is where the two-month to three-month life span of antibodies becomes a concern.
An executive at AstraZeneca, one of the companies working to develop an effective vaccine,told a radio stationthat he thinks his vaccine might only offer protection for one year.
What do we know about COVID-19 antibodies?
Antibodiesare proteins that specifically bind to invading pathogens to neutralize them so they cannot infect the host cell. They trigger a mechanism known as phagocytosis which destroys the virus. IgG antibodies are the most common and can protect us against bacterial and viral infections.
IgG antibody immunity to COVID-19 occurs through contracting SARS-CoV-2, or through a vaccine that produces an immuno-protective response.
Here's where things get dicey. In a scientific brief from April 24, the World Health Organization said, "there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection." They described the notion of an acquired immunity from further infections of the disease for those who had already contracted the coronavirus an unproven and unreliable theory.
"It isn't a uniformly robust antibody response, which may be a reason why, when you look at the history of the common coronaviruses that cause the common cold, the reports in the literature are that the durability of immunity that's protective ranges from three to six months, to almost always less than a year," Fauci said in an interview with JAMA Editor-in-Chief Howard Bauchner.
Consequences
The United Kingdom, Sweden and Brazil have each allowed herd immunity to inform their approaches to COVID-19 in one way or another -- with severe consequences.
In mid-March, Patrick Vallance, the British government's chief scientific adviser, announced they were taking an approach to COVID-19 that would "build up some kind of herd immunity," but quickly reversed course due to fatal risks.
Similarly, Sweden attempted to attain herd immunity through an approach that ultimately yielded the highest per capita death rates from COVID-19 in the world, with no measurable associated economic gain, according to the European Commisison. Only 6.1% of Sweden's population developed coronavirus antibodies by late May -- a number much lower than predicted.
For reference, a large-scale study out of Spain indicates that just5% of its population hasdeveloped antibodies. This study shows that even thoughSpainwas one of the countries hardest hit by the virus, the presence of antibodies is still only around 5%, which is not high enough to achieve herd immunity.
Most experts say that herd immunity requires at least 60-70% of the population to have antibodies, though this number varies depending on the virus.Because of the toll that this degree of infection would take on thehealthcare system and on the population, the safest way to achieve herd immunity would be through a widely available, widely utilized, effective vaccine.
The poor results from Sweden's controversial approach have led to a formal investigation into the government's actions in response to the public health crisis.
Brazil has fared no better with the second highest number of coronavirus cases in the world.
Conclusions
Surgeon General Dr. Jerome Adams recently advised a leading national physicians' organization that the U.S. is far from reaching the 70%+ infection rate needed to begin having a real discussion about herd immunity. For reference, Adams mentioned his home state of Indiana was currently at an approximate 3% infection rate.
Dr. Adams further reiterated Dr. Fauci's concerns that antibodies appear to be limited in robustness, so it is unknown how significant or effective acquired immunity to COVID-19 may be. While it's still unclear whether or not a person can get infected with COVID-19 more than once, the limited durability of immuno-protective IgG antibodies may suggest an answer the world does not want to hear.
