Seroconversion assessed by different methods:
90.9% (30 of 33) SARS-CoV-2-positive patients in the convalescent phase (> 21 d after onset of the symptoms) tested had seroconverted at T1 in April and in total, 85.3% of the patients (29 of 34 samples) had seroconverted until September, as determined by neutralization test (table 1).
Overall, 85.3% of SARS-CoV-2-infected persons seroconverted and maintained constant neutralizing antibodies over the study period of five months.
The highest number of positives was found in the IgG IFA one month post infection (32 of 34; 94.1%), the lowest number of seroconversions was found in CLIA IgG (28 of 34; 82.4%). In 11 cases (32.4%), IgM was detectable by IFA 28 – 41 days after onset of the symptoms.
Four patients had no specific antibody response at all and eleven patients developed only weak antibody responses five months post infection. All four negative patients had a mild course of disease; their mean age was 51 (sd = 12.4) years.
Comparison CLIA IgG – ELNA
Both methods showed that specific antibodies stayed constant over the observation period and that a stronger disease severity led to a more stable antibody response.
CLIA IgG recognized two sera from the first time point as positive (5.9%), which turned out as negative in ELNA. This result goes along with the manufacturer´s information concerning 95% specificity and 0% false-negative results. Repeated serologic investigations at T5 exhibited one patient (3.0%) false-negative in CLIA IgG.