世界衛生組織25日發佈的每日疫情報告說，迄今尚無證據表明擁有新冠抗體的康復者可免於“二次感染”。
世衛組織指出，通過自然感染對病原體產生免疫是一個多步驟過程，通常需要1至2周。感染病毒時，人體首先出現非特異性的固有免疫應答，巨噬細胞、嗜中性粒細胞和樹突狀細胞可減慢病毒感染進程，甚至阻止症狀出現。在此之後，人體出現適應性免疫應答，產生與病毒特異性結合的抗體，即免疫球蛋白；還會產生T細胞，識別和清除受感染細胞，即細胞免疫。
世衛組織說，如果適應性免疫應答足夠強，則可以阻止病情發展為重症，或防止同一病毒的“二次感染”。迄今大多數研究表明，新冠康復者體內擁有新冠病毒抗體，但其中一些人血液中的抗體水平非常低。由此推斷，細胞免疫可能對病人的康復也至關重要。
世衛組織強調，截至4月24日,尚無研究對新冠病毒抗體是否賦予人類對該病毒的免疫力進行評估。此外，對人體內新冠病毒抗體的實驗室檢測需要進一步驗證，以確定檢測的準確性和可靠性。
針對一些國家政府建議將抗體檢測作為發放“免疫護照”或“無風險證書”的基礎，從而允許持證人能夠出行或恢復工作，世衛組織認為，沒有足夠證據證明抗體介導免疫的有效性，進而無法保證此類證書的準確性。
世衛組織警告，那些檢測顯示擁有抗體的人們也許自認為可免於“二次感染”，進而忽略公共衛生建議。因此，使用此類證書可能增加病毒繼續傳播的風險。
There is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection, the World Health Organization (WHO) said in a daily situation report on Saturday.
Although many studies show that people who recovered from COVID-19 have antibodies to the novel coronavirus, also known as SARS-CoV-2, the WHO noted, some of these people have very low levels of neutralizing antibodies in their blood.
"As of April 24, 2020, no study has evaluated whether the presence of antibodies to SARS-CoV-2 confers immunity to subsequent infection by this virus in humans," stressed the UN health agency.
Some countries have suggested that the detection of antibodies to the novel coronavirus could serve as the basis for an "immunity passport" or "risk-free certificate", which would enable individuals to travel or return to work, assuming that they are protected against re-infection.
However, there is still a lack of sufficient evidence for the effectiveness of antibody-mediated immunity to guarantee the accuracy of an "immunity passport" or "risk-free certificate, the WHO underlined, warning that "the use of such certificates may therefore increase the risks of continued transmission."
According to the UN agency, laboratory tests that detect antibodies to SARS-CoV-2 in people, including rapid immunodiagnostic tests, need further validation to determine their accuracy and reliability.
