Since the implementation of newly developed SARS-CoV-2 vaccines in the general population, serological tests are of increasing importance. Because DBS samples can be obtained with a finger prick and can be shipped and stored at room temperature, they are optimal for use in large-scale SARS-CoV-2 serosurveillance or postauthorization vaccination studies, even in an elderly study population.

Seroprevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) IgG antibodies, using dried blood spots, was determined in October-November 2020, among residents and staff randomly selected from 20 nursing homes (NH) geographically distributed in Flanders, Belgium. Sociodemographic and medical data [including coronavirus disease 2019 (COVID-19) symptoms and results of RT-PCR tests] were retrieved using questionnaires. The overall seroprevalence was 17.1% [95% confidence interval (CI) 14.9-19.5], with 18.9% (95% CI 15.9-22.2) of the residents and 14.9% (95% CI 11.9-18.4) of the staff having antibodies, which was higher than the seroprevalence in blood donors. The seroprevalence in the 20 NH varied between 0.0% and 45.0%. Fourteen per cent of the staff with antibodies, reported no typical COVID-19 symptoms, while in residents, 51.0% of those with antibodies had no symptoms. The generalised mixed effect model showed a positive association between COVID-19 symptoms and positive serology, but this relation was weaker in residents compared to staff. This study shows that NH are more affected by SARS-CoV-2 than the general population. The large variation between NH, suggests that some risk factors for the spread among residents and staff may be related to the NH. Further, the results suggest that infected people, without the typical COVID-19 symptoms, might play a role in outbreaks. 8Heidi Janssens et al.

In the current SARS-CoV-2 pandemic, testing for SARS-CoV-2 specific antibodies is paramount to monitor immune responses in post-authorization vaccination and sero-epidemiology studies. However, large scale and iterative serological testing by venipuncture in older persons can be challenging. Capillary blood sampled using a finger prick and collected on protein saver cards, i.e. dried blood spots (DBS), has already proven to be a promising alternative. However, elderly persons have a reduced cutaneous microvasculature, which may affect DBS-based antibody testing. Therefore, we aimed to evaluate the performance of DBS for the detection of SARS-CoV-2 antibodies in nursing homes residents. We collected venous blood and paired Whatman and EUROIMMUN DBS from nursing home residents, and from staff as a reference population. Venous blood samples were analyzed for the presence of SARS-CoV-2 IgG antibodies using the Abbot chemiluminescent microparticle immunoassay (CMIA). DBS were analyzed by the EUROIMMUN enzyme-linked immuno sorbent assay (ELISA) for SARS-CoV-2 IgG antibodies. We performed a statistical assessment to optimize the ELISA cut-off value for the DBS using the Youden's J index. A total of 273 paired DBS-serum samples were analyzed, of which 129 were positive as assessed by the reference test. The sensitivities and specificities of DBS ranged from 95% to 97.1% and from 97.1% to 98.8%, respectively, depending on population (residents or staff) or DBS card type. These results demonstrate that DBS sampling is a valid alternative to venepuncture for the detection of SARS-CoV-2 antibodies in the elderly.