We analyzed prevalence of IgG antibodies to SARS-CoV-2 in hospitalized COVID-19 patients, healthcare providers without a confirmed COVID-19 diagnosis, general workers, and other patients to be admitted to hospital. The most intriguing finding of this study is that only 4% of healthcare providers without confirmed COVID-19 diagnosis had IgG antibodies to SARS-CoV-2 in their blood.

Zhongnan Hospital of Wuhan University, 2.88% (118/4099) healthcare workers were diagnosed with COVID-19 before March 16, 2020. With a moderate estimation, the true infection rate would be ten times that had been confirmed, i.e., >25% of those healthcare providers without diagnosed COVID-19 had been infected. However, only 4% of those infected healthcare workers without confirmed COVID-19 still had IgG antibodies to SARS-CoV-2. They just got infected with SARS-CoV-2 and cleared the virus by their own immune systems. No long-lasting protective antibodies against SARS-CoV-2 were produced in these healthcare providers. Our observed high prevalence of IgG antibodies to SARS-CoV-2 in older groups (60-69 years old and ≥70 years old) among health care workers and general worker in Wuhan also raised the concern that IgG antibodies to SARS-CoV-2 would be lost some time after the infection was cleared

We also found that >10% of confirmed COVID-19 cases had no detectable serum levels of IgG antibodies to SARS-CoV-2 after 21 days post symptom onset. They were unlikely to not produce IgG antibodies to SARS-CoV-2 after being infected with the virus. Long et al reported that after 17-19 days post symptom onset, IgG was positive in all patients with COVID-19. Therefore, >10% patients in our study who had no IgG antibodies after 21 days post symptom onset most likely lost those IgG antibodies after the infection was resolved.

We found mortality risks were similar between hospitalized COVID- 19 patients with IgG antibodies to SARS-CoV-2 and those without, which indicates that absence of IgG antibodies may not affect clinical end outcome and IgG antibodies may even not be part of short-term protective immune response against SARS-CoV-2.

In conclusion, very few healthcare providers without confirmed COVID-19 diagnosis in Wuhan have IgG antibodies to SARS-CoV-2, though a substantial portion of them had been infected with the virus. More than 10% of COVID-19 patients did not have those antibodies after 21 days post symptom onset. After SARS-CoV-2 infection, people are unlikely to produce long-lasting protective antibodies against this virus.