The humoral immune response more than one year after SARS-CoV-2 infection: low detection rate of anti-nucleocapsid antibodies via Euroimmun ELISA.

PURPOSE: Antibody assays against SARS-CoV-2 are used in sero-epidemiological studies to estimate the proportion of a population with past infection. IgG antibodies against the spike protein (S-IgG) allow no distinction between infection and vaccination. We evaluated the role of anti-nucleocapsid-IgG (N-IgG) to identify individuals with infection more than one year past infection. METHODS: S- and N-IgG were determined using the Euroimmun enzyme-linked immunosorbent assay (ELISA) in two groups: a randomly selected sample from the population of Stuttgart, Germany, and individuals with PCR-proven SARS-CoV-2 infection. Participants were five years or older. Demographics and comorbidities were registered from participants above 17 years. RESULTS: Between June 15, 2021 and July 14, 2021, 454 individuals from the random sample participated, as well as 217 individuals with past SARS-CoV-2 infection. Mean time from positive PCR test result to antibody testing was 458.7 days (standard deviation 14.6 days) in the past infection group. In unvaccinated individuals, the seroconversion rate for S-IgG was 25.5% in the random sample and 75% in the past infection group (P = < 0.001). In vaccinated individuals, the mean signal ratios for S-IgG were higher in individuals with prior infection (6.9 vs 11.2; P = < 0.001). N-IgG were only detectable in 17.1% of participants with past infection. Predictors for detectable N-IgG were older age, male sex, fever, wheezing and in-hospital treatment for COVID-19 and cardiovascular comorbidities. CONCLUSION: N-IgG is not a reliable marker for SARS-CoV-2 infection after more than one year. In future, other diagnostic tests are needed to identify individuals with past natural infection.

Potential pitfalls of comparative measurements of reticulocytes with flow cytometry and microscopy in prematures and infants.

Reticulocyte counting by flow cytometry (Bayer H*3, ADVIA 120) in blood of prematures, infants and children > 1 year of age was compared with microscopic counting under research conditions (9000 counted red blood cells per slide). While in children >1 year a good concordance of both methods was observed, 2.3-2.4-fold higher values were obtained in neonates by microscopy (Brilliant Cresyl Blue stain, 0.5%). However, another laboratory found good agreement between H*3-counting and microscopy in samples also obtained from neonates using the same methods. Despite very similar results for all age groups in comparative flow cytometry measurements in both laboratories, counting of smears from neonates differed, showing an approximately 2.3-fold larger amount of reticulocytes in our laboratory. The reason for these observations was a greater enlargement (1250-fold) used routinely in our laboratory compared with 800-fold in the other one. Thus very mature reticulocytes frequently found in neonates could only be detected using a 1250-fold enlargement. Similarly, the low concentration of the colouring matter used in the H*3 (0.0005% oxazin or 0.001% ADVIA 120) is obviously not sufficient for detection of mature reticulocytes. Therefore, it is important to consider this phenomenon and to standardise microscopic enlargement, especially for comparisons in multicentre studies.