SARS-CoV2 seroprevalence data indicates that asymptomatic transmission is largely underreported. It is also known to play an important role in disease transmission among the general population.
Asymptomatic individuals, defined as persons with no clinical symptoms, no radiological sign of the disease in the lungs, and a positive SARS-CoV2 nucleic acid test result, shed viral particles for a prolonged time. This and other observations suggest they may develop a different humoral immune response. Nucleic acid testing is the method of choice for the detection of viral infection. It typically relies on Real‐time PCR (RT‐qPCR) analysis of SARS‐CoV‐2 genes in nasopharyngeal swabs. The test sensitivity is highly dependent on a number of factors, including the course and the type of the clinical COVID-19 syndrome, correct collection, transportation, and storage of specimens. The reported rate of false negatives associated with nucleic acid testing (about 30%) highlights the need to develop more sensitive and accurate test methods. Serum IgM and IgG profiling can not only provide an alternative method to test asymptomatic individuals early and accurately but can also contribute to clarifying their immune response profile. The authors of this study relied on a SARS-CoV2 proteome microarray to evaluate serum IgM and IgG of asymptomatic patients, comparing their immune response to that of individuals with mild symptoms and healthy controls.
SARS‐CoV‐2 Proteome Microarray Fabrication and Serum Analysis
A panel of 20 recombinant proteins encompassing the whole proteome of SARS-CoV-2 and comprising both commercial proteins as well as recombinant proteins generated in different expression systems were selected for this study. The purified proteins were printed in triplicates on PATH® microarray slides using a Super Marathon printer (Arrayjet). Microarrays were blocked with 3% BSA in PBS with 0.1% tween 20 and incubated with diluted serum samples. After washing, a Cy3-conjugated goat anti-human antibody and an Alexa fluor 647- conjugated donkey antibody were used for secondary detection of serum antibodies.
Nucleic acid testing alone detected only 19% of asymptomatic patients, when used in conjunction with IgM detection 55.5% of total asymptomatic individuals were correctly identified. Asymptomatics mainly evolved IgM and IgG antibodies against S1 and N proteins, with S1‐specific IgM antibodies detected as early as 1 week after exposure disappearing within two months. Viral shedding of SARS‐CoV‐2 in these individuals follows the same pattern. By contrast, stronger N‐specific IgM and IgG responses persist in symptomatic COVID‐19 patients for a longer time. Thus suggesting that S1‐specific IgM antibody response be a hallmark of asymptomatic infection with the potential to assist nucleic acid testing for earlier identification of infectious individuals. These findings highlight the importance of serological analysis for SARS-CoV2 diagnosis and confirm that the microarray format is a robust and reliable technology for the simultaneous analysis of a large number of samples.