Seroprevalence and epidemiological characteristics of immunoglobulin M and G antibodies against SARS-CoV-2 in asymptomatic people in Wuhan, China


Background: The seroprevalence of immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) may be a more reliable approach to detect true infected population, particularly in asymptomatic persons. The seroprevalence of IgG or IgM in people in general has not been well described. We choose a general hospital in Jianghan District in Wuhan, near the Huanan Seafood Wholesale Market, the epicenter of the COVID-19 pandemic in China, to conduct a serological survey, aimed at assessing asymptomatic infections of COVID-19 compared to epidemiological characteristics of people in Wuhan. Methods: We conducted a serological survey of asymptomatic people who were tested in the general hospital using a validated colloidal gold method for IgM and IgG antibodies against SARS-CoV-2. Demographic, clinical, laboratory data and CT imaging findings from March 25 to April 28, 2020 were collected and compared. A total of 18,712 people mainly met the inclusion criteria to be enrolled (89.4%), with a median age of 40 years (range 4-81 years old), including 11,391 males (60.9%) with a median age of 42 years and 7,321 females (39.1%) with a median age of 37 years. The seroprevalence was estimated adjusting for imperfect diagnostic tests and the demographic structure of the population. Results: During the period from 25 March 2020 to 28 April 2020, the seroprevalence of IgG and IgM standardized for age and sex in Wuhan varied between 7.67% and 1.56% for IgG, and between 0.71% and 0.16% for IgM, and showed a downward trend. No significant correlation was observed between the seroprevalence of IgG and the different age groups, although none of the 26 individuals under the age of 19 years tested positive for IgG. The seroprevalence of IgM in different age groups was correlated with age (x2 = 18.496, p= 0.035), with no IgM positivity detected under the age of 24 years old (n = 679). Accounting for test performance and adjusting for the age and sex of the general population, the seroprevalence of IgG and /or IgM was estimated at 2.72% (95% confidence interval [CI]: 2.49-2.95%), with a seroprevalence of 2.05% (1.79-2.31%) for males and 3.41% (2.99-3.83%) for females. The seroprevalence was significantly higher for females than males (x2 = 35.702, p < 0.001), with an odds ratio of 1.36 (95% CI: 1.24-1.48). Based on the census number of the Wuhan population aged 4-81 years old in 2017, using IgG and/or IgM seroprevalence tests, the number of asymptomatic COVID-19-positive individuals aged 4-81 years old was estimated at 217,332 (95% CI: 198,709-235,955) in Wuhan from March 25 to April 28, 2020. A significant difference was seen in the seroprevalence of IgG among people from different geographic areas and different types of workplaces (respectively, x2 = 42.871, p < 0.001 and x2 = 202.43, p < 0.001). Sixty percent of antibody-positive cases came from the top ten work units out of a total of 154 units. Some professions had a higher risk for positive antibody tests. From CT imaging of 1636 participants, the IgG antibody-positive cases had a greater number of abnormalities in CT imaging than IgG-negative cases (30.7% vs 19.7%). Significant differences were seen between test groups of antibody-positive and negative cases of IgG and /or IgM in the percentage of leucocytes, neutrophilic granulocytes and monocytes. Conclusions: The reported number of confirmed patients in Wuhan only represents a small proportion of the total number of infections, and most of the Wuhan population remains susceptible to COVID-19. There were differences in IgG seroprevalence among geographic areas, which were consistent with the spread of the SARS-CoV-2 coronavirus in Wuhan. There was a significant aggregation of asymptomatic infections in individuals from some occupations, and based on CT and laboratory findings, some damage may have occurred in asymptomatic individuals positive for IgG antibody.