The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has given rise to the ongoing coronavirus disease 2019 (COVID-19) pandemic, is highly contagious and can lead to high morbidity and mortality rates.1,2 A recent study published in Nature Medicine used data of host serological response, measured by levels of immunoglobin M (IgM) and G (IgG) antibodies, to assess the cumulative prevalence of SARS-CoV-2 infection in different geographic regions in China. A decreasing trend in the seroprevalence of IgG and IgM antibodies was detected as the distance from the epicentre, Wuhan, increased. The results underpin the use of serological surveillance for a more accurate cumulative viral attack rate.1
In December 2019, SARS-CoV-2 was first identified as the pathogen for a number of pneumonia cases with unknown origin in Wuhan, China.3 Within a few months, the virus has spread across the globe and the World Health Organization (WHO) declared this health crisis as a pandemic in March 2020.1 Contributing to the rapid spread of the virus was the asymptomatic carriers, who may or may not develop symptoms later but are nonetheless able to transmit COVID-19 through close contact with others.4 Meta-analyses estimated that asymptomatic infections might constitute 15.6% of all confirmed cases.5 However, many of these cases remain undetected due to insufficient testing frequency and limited testing methods.1 For the diagnosis based on real-time polymerase chain reaction (RT-PCR), the accuracy is usually limited by the assay method, and the timing of specimen collection, transportation and storage.1
To better understand the epidemiology and contagiousness of the disease, serological test for the presence of antibodies (IgM or IgG) against SARS-CoV-2 might be a better option when compared to the viral test as the antibodies against the virus, for example IgG, are likely to persist for a longer period of time after the viral infection is cleared. Hence, a serological survey was carried out to establish the patterns of SARS-CoV-2 prevalence in China.1
A total of 17,368 individuals from four geographical regions were recruited for the survey. To evaluate the host serological response, serum samples were collected from the participants and assayed against recombinant antigens containing the nucleoprotein and a peptide from the spike protein of SARS-CoV-2. The data collected was sorted and grouped according to the individuals’ respective regions and occupations.1
The results showed a decreasing trend in the seroprevalence of IgG and IgM antibodies as the population was located further away from the pandemic’s epicentre. Healthcare workers in Wuhan had the highest seropositive prevalence rate (SPR) at 3.8% (95% CI: 2.6-5.4), while those in Jingzhou and Honghu, cities west of Wuhan in the Hubei Province, had a SPR of 1.3% (95% CI: 1.0-1.8). Further west in Chongqing, the SPR was 3.1% (95% CI: 1.7-5.7); while in Foshan and Guangdong, far south of Wuhan in Guangdong Province, has a SPR of 1.2% (95% CI: 0.4-3.3). Similar trends were observed in outpatients who had frequent hospital visits for maintenance haemodialysis, hotel staff members, and family members of healthcare workers. The SPR was also significantly higher in individuals above the age of 65 (2.0% in those ≥65 years vs. 1.3% in those <65 years, p<0.01), however differences between genders were not statistically significant. This distribution in SPR was consistent in different geographical areas in China during the early spread of the SARS-CoV-2 coronavirus.1
In summary, serological assay has demonstrated an inverse relationship between the seroprevalence of IgG and IgM antibodies and the distance away from Wuhan. Nevertheless, more data will be needed to extrapolate the findings of this survey to other population groups and locations.1
1. Xu X et al. (2020). Seroprevalence of immunoglobulin M and G antibodies against SARS-CoV-2 in China. Nature Medicine.
2. Wang D et al. (2020). Clinical characteristics of 138 hospitalized patients with 2019 novel coronavirus–infected pneumonia in Wuhan, China. JAMA, 323(11), 1061.
3. Guan WJ et al. (2020). Clinical characteristics of coronavirus disease 2019 in China. New England Journal of Medicine, 382(18), 1708-1720.
4. Hu Z et al. (2020). Clinical characteristics of 24 asymptomatic infections with COVID-19 screened among close contacts in Nanjing, China. Science China Life Sciences, 63(5), 706-711.
5. He J et al. (2020). Proportion of asymptomatic coronavirus disease 2019 (COVID‐19): a systematic review and meta‐analysis. Journal of Medical Virology, jmv.26326.