A cheap immunoassay for determination of immune response to SARS-CoV-2 vaccine in capillary blood samples

The coronavirus disease 2019 (COVID-19) caused by SARS-Cov- 2 virus represents public health emergency of international concern. The recent development of SARS-CoV-2 vaccines offer great hope to fight against this worldwide pandemic. However, there are many unknowns moving forward, like how long immunogenicity lasts or how many doses are required to immunize a person. High-quality serology is a key tool in understanding the immunity to viruses, and the relevance of protection. The measurement of specific COVID-19 antibodies can be used and currently very necessary to verify the effectiveness of the different vaccines in the population. Here, we present an optimized ELISA-based serology protocol that can be applied in serum and in capillary blood. Validation was performed using serum collected from 113 hospitalized patients in Principe de Asturias University Hospital. Ninety-eight of them were diagnosed with COVID19 by a positive rRT-PCR result and fifteen samples were negative in rRT-PCR. We used an indirect ELISA assay, using the receptor binding domain (RBD) of the recombinant spike protein of SARS-Cov-2 to cover the plate. In addition, all patients were tested by commercial ELISA to compare with our cheap immunoassay. Results show that our ELISA assay is suitable for capillary blood samples and has a sensibility significantly higher than commercial ELISA. This study demonstrates the validity of our cheap immunoassay for the measurement of specific IgG in the serum of patients, which allows the safe and economic testing of all currently vaccinated people to ensure immunity in the population and the efficacy of vaccination. Surprisingly our assay works on capillary blood samples, which facilitates the process of obtaining the sample and can be carried out quickly and safely by the patients themselves. Therefore, our cheap immunoassay may be useful to provide population-based data on SARS-CoV-2 seropositivity, infection and immunity generated by the vaccines.