Browsing by Author "Montonen, Reetta"

Since the beginning of the Coronavirus Disease 2019 (COVID-19) pandemic, there has been a need for developing an efficient vaccine against the SARS-CoV-2 virus and its emerging variants. In this thesis, humoral immune responses induced by either two doses of mRNA Pfizer-BioNTech (Pfizer) vaccine or one dose of adenoviral vector-based Oxford-AstraZeneca followed by a second dose of Pfizer were studied. Levels of anti-spike protein (anti-S1) IgG, IgA and IgM antibodies were measured with enzyme-linked immunosorbent assay. In addition, neutralizing antibody titers against the original Wuhan-Hu-1 strain, the Beta variant and the Delta variant were studied with a pseudovirus neutralization assay. The study used 180 serum samples from a cohort of vaccinated Finnish healthcare workers. Sera were collected from vaccinees before their first vaccination, after which vaccinees provided sequential samples at timepoints of three weeks, six weeks, three months, and six months. The results showed that both vaccination regimes produce high levels of anti-S1 IgG antibodies in vaccinees, and the antibodies persist in blood at least for six months. Anti-S1 IgA levels were lower compared to IgG levels, but were long-lasting, as 95% of vaccinees were IgA seropositive six months after the first vaccine dose in both study groups. Anti-S1 IgM levels resembled the levels of IgA, but the IgM seropositivity after six months was only 50% in the Pfizer-only group and 70% in the “mix-and-match” vaccination group. Neutralization assays demonstrated that the heterologous vaccination induces higher neutralizing antibody titers compared to the homologous vaccination. After six months, the neutralization in the homologous vaccination group was 6-fold reduced against Wuhan-Hu-1, 4-fold reduced against the Beta variant and 5-fold reduced against the Delta variant compared to the heterologous regime. The results are in line with previous findings that have proven the “mix-and-match” vaccination to be more immunogenic than the homologous prime-boost vaccination. By combining different booster COVID-19 vaccines to the primer vaccine dose, it would aid in cases of vaccine shortages and provide options to individuals that respond poorly to a specific type of vaccine.