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★专业/深度/睿见资讯尽在医疗旅游
世界卫生组织公布了关于调整下一阶段COVID-19应对措施的公共卫生和社会措施的指导意见。它警告说，签发“豁免护照”或“无风险证”以使个人能够旅行是危险的。
一些政府建议，对引发COVID-19疾病的病毒抗体检测，可以作为允许个人旅行或重返工作岗位的“免疫护照”或“无风险证明”的基础，假设他们能受到保护防止再次感染。
世界卫生组织指出，没有证据表明从COVID-19中恢复过来并具有抗体的人可以免受第二次感染。
通过自然感染对病原体产生免疫是一个多步骤过程，通常需要1-2周。人体对病毒感染的反应是一种非特异性先天反应，其中巨噬细胞，中性粒细胞和树突状细胞可以减缓病毒的进程，甚至可以阻止病毒引起症状。这种非特异性反应之后是适应性反应，在这种适应性反应中，人体产生与病毒特异性结合的抗体。这些抗体是称为免疫球蛋白的蛋白质。人体还制造T细胞，从而识别并消除感染病毒的其他细胞。这称为细胞免疫。
这种组合的适应性反应可以清除体内病毒，如果反应足够强烈，可以防止进展为严重疾病或再次感染同一病毒。这个过程通常是通过血液中抗体的存在来衡量的。
世卫组织继续审查有关抗体反应的证据，大多数研究表明，从感染中恢复过来的人都具有该病毒的抗体。但是，其中一些人的血液中的中和抗体水平很低，这表明细胞免疫对于恢复也可能至关重要。
目前还没有研究评估这种病毒的抗体，即严重急性呼吸综合征冠状病毒2（SARS-CoV-2）的存在是否会对这种病毒随后在人体内的感染产生免疫力。
检测人体中SARS-CoV-2抗体的实验室检测，包括快速免疫诊断试验，需要进一步验证，以确定其是否准确可靠。不准确的免疫诊断试验可能会以两种方式对人进行错误的分类。一是可能错误地将感染者标记为阴性，二是未感染者被错误地标记为阳性。这两个错误都有严重后果，并将影响控制努力。
这些测试还需要准确区分过去的SARS-CoV-2感染和已知的6种人类冠状病毒引起的感染。这6中病毒中有4种会引起普通感冒并广泛传播。其余2种是引起中东呼吸综合症和严重急性呼吸综合症的病毒。被这些病毒中的任何一种感染的人所产生的抗体可能会与针对SARS-CoV-2感染而产生的抗体发生交叉反应。
许多国家/地区正在基础人群或特定人群中（如卫生工作者，已知病例的亲密接触者或家庭内部）检测SARS-CoV-2抗体。世卫组织支持这些研究，因为它们对于了解感染的程度和危险因素至关重要。这些研究将提供可检测到COVID-19抗体的人的百分比数据，但大多数研究并不是为了确定这些人是否对继发感染免疫。
在大流行的这一点上，没有足够的证据证明抗体介导免疫的有效性，以保证“免疫护照”或“无风险证明”的准确性。由于检测结果呈阳性而认为自己对第二次感染免疫的人，可能会忽视公共卫生建议。因此，使用这种证书可能会增加继续传播的风险。＊
WHO WARNS AGAINST ‘FREE OF COVID-19’ CERTIFICATES
The World Health Organization (WHO) has published guidance on adjusting public health and social measures for the next phase of the COVID-19 response. It warns that issuing an ‘immunity passport’ or ‘risk-free’ certificates to enable individuals to travel are dangerous.
Some governments have suggested that the detection of antibodies to the virus that causes the disease COVID-19, could serve as the basis for an “immunity passport” or “risk-free certificate” that would allow individuals to travel or to return to work, on the assumption that they are protected against re-infection.
The WHO states that there is no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection.
The development of immunity to a pathogen through natural infection is a multi-step process that typically takes place over 1-2 weeks. The body responds to a viral infection immediately with a non-specific innate response in which macrophages, neutrophils, and dendritic cells slow the progress of virus and may even prevent it from causing symptoms. This non-specific response is followed by an adaptive response where the body makes antibodies that specifically bind to the virus. These antibodies are proteins called immunoglobulins. The body also makes T-cells that recognise and eliminate other cells infected with the virus. This is called cellular immunity. This combined adaptive response may clear the virus from the body, and if the response is strong enough, may prevent progression to severe illness or re-infection by the same virus. This process is often measured by the presence of antibodies in blood.
The WHO continues to review the evidence on antibody responses and most studies show that people who have recovered from infection have antibodies to the virus. However, some of these people have very low levels of neutralising antibodies in their blood, suggesting that cellular immunity may also be critical for recovery.
No study has evaluated whether the presence of antibodies to the virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), confers immunity to subsequent infection by this virus in humans.
Laboratory tests that detect antibodies to SARS-CoV-2 in people, including rapid immunodiagnostic tests, need further validation to determine their accuracy and reliability. Inaccurate immunodiagnostic tests may falsely categorise people in two ways. The first is that they may falsely label people who have been infected as negative, and the second is that people who have not been infected are falsely labelled as positive. Both errors have serious consequences and will affect control efforts.
These tests also need to accurately distinguish between past infections from SARS-CoV-2 and those caused by the known set of six human coronaviruses. Four of these viruses cause the common cold and circulate widely. The remaining two are the viruses that cause Middle East Respiratory Syndrome and Severe Acute Respiratory Syndrome. People infected by any one of these viruses may produce antibodies that cross-react with antibodies produced in response to infection with SARS-CoV-2.
Many countries are testing for SARS-CoV-2 antibodies at the population level or in specific groups, such as health workers, close contacts of known cases, or within households. The WHO supports these studies, as they are critical for understanding the extent of and risk factors associated with infection. These studies will provide data on the percentage of people with detectable COVID-19 antibodies, but most are not designed to determine whether those people are immune to secondary infections.
At this point in the pandemic, there is not enough evidence about the effectiveness of antibody-mediated immunity to guarantee the accuracy of an “immunity passport” or “risk-free certificate.” People who assume that they are immune to a second infection because they have received a positive test result may ignore public health advice. The use of such certificates may therefore increase the risks of continued transmission.