During the period March 5th through April 9th, 51 individuals with mild clinical signs of COVID-19 in addition to seven asymptomatic patients were subsequently subjected to antibody assay eight months after diagnosis.

The results varied according to the immunoassay used.  An anti-nucleocapsid (anti-N) panimmunoglobulin electrochemiluminescence assay detected antibody in 91 percent of 53 patients sampled.  The anti-N IgG ELISA detected antibody in 86 percent of 40 patients assayed.  The anti-spike subunit IgG ELISA detected 69 percent of 40 patients and the anti-N IgG ELISA in 25.9 percent of 50 assayed.

Based on the high level of antibody detection using two of the immunoassays, it can be concluded that antibodies persisted for at least eight months after exposure to COVID. The low level of recovery using the anti-N IgG ELISA denotes that different immunoassay techniques will provide potentially conflicting results. Waning immunity as previously documented in published studies is attributed by the group conducting the investigation to the specific assay used to detect IgG.

Given the low frequency of reports of second infections with COVID, it is presumed that an adequate level of IgG antibody following mild exposure, confers protection over a prolonged period.  Correlating IgG antibody level with protection will be important to establish the immune status of a population after widespread administration of vaccines of known immunogenicity.  If protection can be correlated to IgG using a standard immunoassay, it will be possible to determine the time required between initial immunization and administration of a booster vaccine.