Serological tests facilitate identification of asymptomatic SARS‐CoV‐2 infection in Wuhan, China

The Wuhan City has ended the lockdown and people have been allowed to resume working since April 8 if meeting a set of COVID‐19‐associated tests including SARS‐CoV‐2 nucleic acid test (NAT) of nasopharyngeal swabs, chest CT scan or a SARS‐CoV‐2‐specific serological test. Here, we reported the positive rate of COVID‐19 tests based on NAT, chest CT scan and a serological SARS‐CoV‐2 test, from April 3 to 15 in one hospital in Qingshan Destrict, Wuhan. We observed a ~10% SARS‐CoV‐2‐specific IgG positive rate from 1,402 tests. Combination of SARS‐CoV‐2 NAT and a specific serological test might facilitate the detection of COVID‐19 infection, or the asymptomatic SARS‐CoV‐2‐infected subjects. Large‐scale investigation is required to evaluate the herd immunity of the city, for the resuming people and for the re‐opened city.

Till April 15, 2020, 83,745 confirmed cases and 3,352 deaths related to coronavirus disease 2019 (COVID-19) had been reported in mainland China. Among them, 50,008 confirmed cases and 2,579 deaths were from Wuhan alone. Wuhan has been sealed off since January 23, with 16 of its neighbouring cities in

Hubei province included, to contain the spread of COVID-19. The institution of interventions including cordons sanitaire, traffic restriction, social distancing, home quarantine, centralized quarantine, and universal symptom survey contributed largely to the reduced spread of SARS-CoV-2.1 The effective reproduction number of SARS-CoV-2 fluctuated above 3.0 before January 26, but decreased to less than 0.3 after March 1.

The Wuhan City has lifted lockdown and people have been allowed to resume working since April 8, 2020. A set of COVID-19-associated tests are required before resuming, including SARS-CoV-2 nucleic acid test (NAT) of nasopharyngeal swabs, chest CT scan or a SARS-CoV-2-specific serological test.2, 3 These tests are presumed to identify COVID-19 patients, or asymptomatic SARS-CoV-2 infections.3, 4 Since few confirmed cases were found in Wuhan after April 8, it is essential to investigate the positive rate of this “tests for resume”, which might help to evaluate, in some degree, the herd immunity of the city.

Here, we reported the positive rate of COVID-19 tests based on NAT, chest CT scan and serological SARS-CoV-2 test, from April 3 to 15 in one hospital in Qingshan Destrict, Wuhan. The hospital reopened for non-COVID-19 patients since the end of March. We compared data from two groups of tested people: one was those applying for a permission of resume (n=1,021), another was hospitalized patients during April 3 to 15 (n=381). The NAT tests were performed as described,2 and the serological SARS-CoV-2 test kits were performed according to the manufacturer’s instructions (Beijing Innovita Biological Technology Co., Ltd., 2019-nCoV Ab Test (Colloidal Gold)). This study was approved by the Ethics Committee of the CR & WISCO General Hospital, Wuhan.

There was no SARS-CoV-2 NAT positive from any of the 1,021 subjects in the resuming group. However, in the 381 hospitalized patients group, 1 female was NAT, IgM and IgG positive. She is 55 years old, and was recently discharged from designated hospital because of COVID-19. She was sent back to the designated hospital immediately after the NAT test. The remaining 380 hospitalized patients were all NAT negative.

Interestingly, another 39 (39/380, 10.26%) in hospital patients were IgG positive, but were all IgM and NAT negative. Meanwhile, there were 98 (98/1,021, 9.60%) from resuming group that were IgG positive and IgM and NAT negative. None of these people had a history of COVID-19, or suspect symptoms. These IgG+IgM-NAT- individuals with no history of COVID-19 probably suggested a recovered asymptomatic SARS-CoV-2 infection.

In conclusion, we observed a ~10% SARS-CoV-2-specific IgG positive rate from a single-center investigation. Combination of SARS-CoV-2 NAT and serological tests might facilitate the identification of COVID-19, or the asymptomatic SARS-CoV-2 subjects. Large-scale investigation is required to evaluate the herd immunity of the city, for the resuming people and the re-opened city.

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