key: cord-297470-lx3xwg92 authors: Pan, Yunbao; Li, Xinran; Yang, Gui; Fan, Junli; Tang, Yueting; Hong, Xiaoyue; Guo, Shuang; Li, Jin; Yao, Dongai; Cheng, Zhenshun; Yuan, Yufeng; Li, Yirong; Wang, Xinghuan title: Seroprevalence of SARS-CoV-2 immunoglobulin antibodies in Wuhan, China: part of the city-wide massive testing campaign date: 2020-10-07 journal: Clin Microbiol Infect DOI: 10.1016/j.cmi.2020.09.044 sha: doc_id: 297470 cord_uid: lx3xwg92 OBJECTIVES: The outbreak of 2019 coronavirus disease (COVID-19) pandemic in Wuhan, China, has subsided after a hard hit by the disease and subsequent city lockdown. Information on the number of people involved in Wuhan is still inadequate. This study aimed to describe the screening results of 61,437 community members in Wuchang District, Wuhan. METHODS: In mid-May 2020, Wuhan launched a population-scale city-wide SARS-CoV-2 testing campaign, which aimed to perform nucleic acid and viral antibody testing for citizens in Wuhan. Here we show the screening results of cluster sampled 61,437 residents in Wuchang District, Wuhan, China. RESULTS: A total of 1470 (2.39%, 95% CI: 2.27-2.52) individuals were detected positive for at least one antiviral antibody. Among the positive individuals, 324 (0.53%, 95% CI: 0.47-0.59) and 1200 (1.95%, 95% CI: 1.85-2.07) were positive for immunoglobulin IgM and IgG, respectively, and 54 (0.08%, 95% CI: 0.07-0.12) were positive for both antibodies. The positive rate of female carriers of antibodies were higher than those of male counterparts (male-to-female ratio of 0.75), especially in elderly citizens (ratio of 0.18 in 90+ age subgroup), indicating a sexual discrepancy in seroprevalence. In addition, viral nucleic acid detection using real-time PCR had showed 8 (0.013%, 95% CI: 0.006-0.026) asymptomatic virus carriers. CONCLUSIONS: The seroprevalence of SARS-CoV-2 in Wuhan was low. Most of Wuhan residents are still susceptible to this virus. Precautions, such as wearing mask, frequent hand hygiene, and proper social distance, are necessary before an effective vaccine or antiviral treatments are available. Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) related coronavirus disease (COVID-19) is a respiratory transmissible disease that may cause critical illness to death (1, 2) . Various control measures against SARS-CoV-2 were implemented in Wuhan, the first city hit by the coronavirus. After months of endeavor, the viral transmission was largely contained (3). However, sporadically infected cases and asymptomatic carriers were still detected. Hence, Wuhan launched a population-scale, massive SARS-CoV-2 testing campaign for detecting viral nucleic acid and antibodies in residents to further prevent viral transmission, screen out infected patients who were in the incubation period or were asymptomatic virus carriers, and map the epidemiological sero-distribution of this infectious disease in the epicenter. Wuchang District, one of the 13th administrative divisions in Wuhan, is located in the central urban area and is adjacent to the Yangtze River. According to the Wuhan Statistical Yearbook 2018, the total population of Wuchang District is 1.28 million, accounting for 11.7% of the Wuhan population. The present study described the screening results of 61,437 community members in Wuchang District, Wuhan. J o u r n a l P r e -p r o o f Individual blood samples were collected at community sampling stations. The The graphs of age and sex distribution were depicted for the entire tested population. For the seropositive populations, the positive rates in each age and sex group were calculated by dividing the corresponding entire tested population. The 95% confidence interval (CI) was presented. The male-to-female ratio (MFR) was calculated as the male positivity rate divided by the female positivity rate. Among 61,437 community members included in the study, 30,032 (48.88%) were male. The age range is from several months to 101 years old and the median age was 48 (interquartile range (IQR): 32-64) years. The sex and age distribution is depicted in Figure 1 . As shown in Table 1 Table 1 ). Furthermore, the participants were voluntarily recruited, and hence it was reasonable to speculate that a small proportion of mobility-impaired individuals were reluctant to participate, although they performed fewer activities during the outbreak and had fewer chances to be infected. The present study was conducted in Wuchang District, while data in different districts in Wuhan hit by COVID-19 might vary. However, the observed rate was comparable with that reported in other similar studies. A recent study detected 17,368 individuals from different geographic regions in China, including 1993 residents from different Wuhan sub-cohorts. It suggested a seropositivity rate of 3.8%, 3.2%, and 3.8% in healthcare workers, their family members, and their staff members, respectively, from hotels designated for the accommodation of healthcare workers during city lockdown in Wuhan between March 30 and April 10, 2020 (5) . As the exposure of healthcare workers and their close contacts to SARS-CoV-2 was relatively higher than that of most of the other citizens, it was reasonable that the seropositivity from these J o u r n a l P r e -p r o o f subgroups was higher than that from populations with massive testing. Another study testing 452 asymptomatic Hong Kong residents evacuated from Hubei province in early March 2020 indicated a 3.76% (17/452) seropositivity rate (6) . All these seropositivity rates indicated that the prevalence of the population carrying the antibody in Wuhan was low. Eight (0.013%), 324 (0.53%), and 1200 (1.95%) individuals were detected positive for nucleic acid, IgM and IgG, respectively. As IgM is regarded as the first class of immunoglobulins in response to initial exposure, the presence of IgM antibody represents an early exposure to the antigen (7). The anti-SARS-CoV-2 IgM antibody could be detected in patients after 4 days of onset, peaking at 2-3 weeks after the onset of symptoms before its level started to decline (8, 9) . However, IgM positivity alone may not be a good diagnostic indicator because not all of the people develop a detectable IgM antibody (8) . In addition, the IgM antibody may still be detectable after several months, although it is considered as an "early infection". Most of the recent studies showed detectable SARS-CoV-2 anti-IgM antibodies after one to two months (10, 11) . IgG represents the most robust and long-duration antibody against the virus (12) . Remarkably, the present study detected more female carriers of asymptomatic antibodies compared with male carriers in most age subgroups and a reverse correlation trend of MFR with the increase in age. Another study in Wuhan also reported similar findings (13) . Indeed, it was suggested that SARS-CoV-2 affected women less compared with men, due to different innate immunity, steroid hormones, and factors related to sex chromosomes (14) . However, this female over male trend in asymptomatic carriers was not captured in other areas and countries, such as South Korea, Thailand, Iran, Spain, and California of USA (15) (16) (17) (18) . Whether this trend is observed only in Wuhan or can be observed in other parts of China or other countries too needs further investigation. This study had several limitations. First, several groups of people were not included in the study, which might have had different impacts on detecting real seropositivity. Second, the rates were affected by the quality of the kit. As (19) . These intrinsic shortcomings of the rapid immunochromatographic kit might inevitably cause false-positive and false-negative results. Third, massive tests were conducted within 12 days, and hence the possibility of a more false-positive or false-negative rate due to the labor-intensive work was unavoidable. Fourth, a rapid and ready-to-use method, the immune colloidal gold technique, was adopted for the screening test because a large number of samples were needed for handling and the technicians had some limitations. The test provided only a qualitative positive or negative result. A more quantitative result may be obtained using the chemiluminescence enzyme immunoassay. In summary, the majority of the residents in Wuhan are still immunologically naive to SARS-CoV-2, far from herd immunity. 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