SARS-CoV-2 has now infected almost 170 million individuals and caused over 3.5 million deaths around the world. Despite the vaccine administration across the globe, many challenges are presented in overcoming this pandemic. One of the crucial paths to develop an effective solution towards this is to understand the immunological responses generated in the body upon infection and vaccination. SARS-CoV-2 stimulates a rapid antibody response in the infected individuals. While the RBD (receptor binding domain) region of the S1 protein is the prominent antigenic target, it is also highly mutating. Several mutations in this region have produced new virus variants, as previously described: P.1, P.2, B.1.351, B.1.1.7, and B.1.325. Comparatively, the N-protein is observed to have lower mutation levels. Therefore, researchers reason that seroconversion of N-protein antibodies may be an alternative for the vaccine industry to produce efficient vaccines for circulating strains, including those that may arise in the future.

Several vaccines against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with various distinct antigen delivery systems (non-replicating viral vector, protein subunit, inactivated virus, and mRNA) have been developed since the advent of the COVID-19 (coronavirus disease 2019) pandemic. The CoronaVac (Sinovac Life Sciences, Beijing, China) is an inactivated virus. As of April 2021, it is the most administered vaccine in Brazil. Researchers from the Federal University of Paraná, Brazil, looked into the seroconversion rate after vaccination with the CoronaVac. Using immunoassays, they measured the immunoglobulins (IgG) for anti-nucleocapsid protein (anti-N) antibodies and anti-spike-1 (anti-S1) protein antibodies. They found that the CoronaVac has a high seroconversion rate, evaluated in the healthcare workers population.  In the study, the researchers also accounted for the comorbidities of the healthcare workers. Individuals with medical conditions such as cancer, chronic kidney disease, COPD, asthma, etc., are more likely to get severe COVID-19.

The healthcare workers with comorbidities showed similar responses to the vaccine. The study cohort included 133 participants recruited at the Complexo Hospital de Clínicas, UFPR, Clinical Laboratory, Curitiba, Brazil, during the vaccination of healthcare workers in this city. The serum samples were collected on days 0 (first dose application), +10, +20 (second dose application), and +40. While the researchers used the Chemiluminescent Microparticle Immunoassay (CMIA) Architect-I System for anti-nucleocapsid protein (anti-N) IgG, they used the ELSIA assay for IgG anti-S1 spike-protein receptor-binding domain (RBD). In the study, they observed robust antibody production only after day 40 of CoronaVac vaccination. Although the seroconversion to S1 protein on day 0 differed (reactive (n=16) and non-reactive (n=117), on day +40, it averaged the same. While two immunosuppressed workers did not show seroconversion (probably due to rituximab), two others who were not immunosuppressed also did not present seroconversion.

Of the two cases which reported no seroconversion at day +40, one of them presented a delayed response on day +60. The researchers called for further studies to explain the lack of seroconversion to understand what interfered with the immune response. This study also confirms previous reports of research post-vaccination with mRNA vaccine in individuals with and without previous COVID-19: no difference in individual antibody response regardless of previous infection. Indeed, both natural infection and vaccination act on the immune system in complex ways, stimulating the production of non-neutralizing antibodies (with their own specific actions) as well as TCD4+ and TCD8+ T cells, which also act to protect against COVID-19. This study demonstrated high seroconversion in healthcare workers from Brazil, 40 days post-vaccination with CoronaVac. The study provides important data on the seroconversion rate; for CoronaVac it is 97% for the anti-S1 IgG in hearthcare workers, which is similar or lower to the seroconversion rates observed in phase III studies of other vaccine candidates for SARS-CoV-2.

• Edited by Dr. Gianfrancesco Cormaci, PhD, specialist in Clinical Biochemistry.

Scientific references

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