Immunoassays allow the determination of antibody responses against the severe acute respiratory syndrome coronavirus (SARS-Cov-2; COVID-19). Specifically developed immunoassays detect immunoglobulin M (IgM) and immunoglobulin G (IgG) in human blood.
In general, after infection, antibodies become detectable one to three weeks after symptom onset. For COVID-19, like other viral diseases, evidence suggests that infectiousness likely is significantly decreased after two to three weeks. However, according to the CDC, presently accurate data sets are not available. The availability of additional, accurate data will allow modifying public health recommendations more accurately based on serologic test results, including decisions on discontinuing physical distancing and the use of personal protective equipment.
[ CDC antibody test guidelines ]
Serologic tests
Serological tests detect and measure the amounts of antibodies in blood made by the immune system. Antibody tests reveal persons infected with a pathogen such as SARS-Cov-2 coronavirus and their resistance to this pathogen. Serological tests are known as the ‘In-Vitro Diagnostic” Test.
[ FDA antibody serological testing covid-19 , EUA medical devices IVD ]
Chemiluminescence immunoassay to assess IgM and IgG antibody levels
Hou et al. (May 2020) used a chemiluminescence immunoassay to assess IgM and IgG antibody levels in 338 COVID‐19 patients. The study found that anti‐SARS‐CoV‐2 antibody levels differ significantly among COVID‐19 patients with different illness severities and outcomes. In most patients, IgM levels increased during the first week after SARS‐CoV‐2 infection, peaked at two weeks, and then fell to near‐background levels. IgG could be detected after one week and remained at a high level for an extended period. IgM and IgG levels did not differ much in both mild and severe cases. However, IgM levels decreased rapidly in recovered patients, whereas in deceased cases, either IgM levels remained high or both IgM and IgG were undetectable during the course of the disease.
{ SARS‐CoV‐2 antibody detection: The IgM and IgG antibodies against SARS‐CoV‐2 in serum specimens were detected using YHLO‐CLIA‐IgG, YHLO‐CLIA‐IgM kits supplied by YHLO (YHLO Biotech Co. Ltd Shenzhen, China), according to the manufacturer's instructions. The recombinant antigens contain nucleoprotein (N) and spike protein (S) of SARS‐CoV‐2. The antibody levels were expressed as arbitrary unit per mL (AU mL−1). The results ≥ 10 AU mL−1 are reactive (positive), and the results < 10 AU mL−1 are nonreactive (negative). }
[ Hou H, Wang T, Zhang B, Luo Y, Mao L, Wang F, Wu S, Sun Z. Detection of IgM and IgG antibodies in patients with coronavirus disease 2019. Clin Transl Immunology. 2020 May 6;9(5):e01136. [PMC] ]
Lateral flow immunoassay test based antibody response
Sood et al., in spring 2020 (May 2020), investigated the prevalence of IgG and IgM antibodies to SARS-CoV-2 in Los Angeles County, California, as a marker of both active and past infections. The study tested a total of 865 (50.9%) invited people. The 863 adults surveyed included 60% women, 55% aged 35 to 54, and 58% white people. Thirteen percent reported fever with a cough, 9% fever with shortness of breath, and 6% reported a loss of smell or taste. Thirty-five individuals (4.06% [exact binomial CI, 2.84%-5.60%]) tested positive. People that tested positively varied by race/ethnicity, sex, and income. The weighted proportion of participants who tested positive was 4.31% (bootstrap CI, 2.59%-6.24%).
After adjusting for test sensitivity and specificity, the unweighted and weighted prevalence of SARS-CoV-2 antibodies was 4.34% (bootstrap CI, 2.76%-6.07%) and 4.65% (bootstrap CI, 2.52%-7.07%), respectively.
Hence, the study found that the prevalence of antibodies to SARS-CoV-2 was approximately 4.6 to 4.7 %.
The researcher concluded that approximately 367 000 adults had SARS-CoV-2 antibodies. This infection rate is substantially more significant than the 8430 cumulative number of confirmed infections in the county on April 10.
Therefore, fatality rates based on confirmed cases may be much higher than rates based on the number of infections.
This survey suggests that the actual infection rate for COVID-19 could be approximately 40 to 50 fold higher than reported.
[ Los Angeles County announces 18 new deaths related to 2019 novel coronavirus (COVID-19)—475 new cases of confirmed COVID-19 in Los Angeles County. News release. Los Angeles County Department of Public Health. April 10, 2020. ]
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