This study provides key data on how long it takes to develop antibodies after symptoms of COVID (5 days for IgM and IgA, and 14 days for IgG) and argues that combining IgM ELISA assay with PCR makes a false negative result really rare.

While testing for COVID is currently being done through RT-PCR, which looks for specific RNA from the virus to detect the virus, we will eventually need to transition to antibody-testing to know who has already been infected and is likely immune. What we don’t know is how long it takes to develop IgA, IgM and IgG antibodies to SARS-CoV-2.

This study examined 208 plasma samples collected from 82 confirmed and 58 probable cases in China. They found that the median time to detection of IgM and IgA antibodies was 5 days (IQR 3-6), and IgG in 14 days (IQR 10-18) after symptom onset.

They also found that the detection efficiency by IgM ELISA was higher than that of qPCR method after 5.5 days of symptom onset. Combining IgM ELISA assay with PCR for a patient resulted in a significantly increased positive detection rate (98.6%) compared to a single qPCR test (51.9%).