Abstract
Background: In Greece, administration of the AstraZeneca COVID-19 vaccine was discontinued in specific age groups. Individuals who had received the first dose of AstraZeneca subsequently completed their vacci nation with an mRNA vaccine, most commonly Pfizer-BioNTech.
Objective: This study aimed to evaluate the antibody response on the day of administration of the second Pfizer dose in participants previously vaccinated with AstraZeneca, and to compare their antibody titers with those of individuals who received two Pfizer doses.
Materials and Methods: The study included healthy volunteers aged 19–32 years (mean 24.3), without un derlying diseases or prior COVID-19 infection. Antibody titers were determined using the Roche Cobas-Pro analyzer with the same reagent lot. The study group (AstraZeneca first dose – Pfizer second dose) was com pared with the control group (two Pfizer doses).
Results: According to Cobas 801 (Roche) standards (0.8 AU/ml normal range), the maximum quantifica tion capacity after 1:10 dilution was 2,500 AU/ml. The antibody response efficiency of the Pfizer-after-Astra Zeneca scheme was 93.88%, while two Pfizer doses achieved 41.38%. The mean increase in antibody titer following the second Pfizer dose was 30.12-fold higher than after the AstraZeneca dose. Age and gender were not statistically significant factors. Statistical comparison using the two-way Wilcoxon test confirmed significant differences between groups (p < 0.01).
Conclusions: The heterologous (“mix-and-match”) vaccination strategy demonstrated higher immuno genicity, potentially due to a longer dosing interval and complementary activation of immune mechanisms. AstraZeneca appears to elicit stronger T-cell responses, while Pfizer enhances antibody production, suggest ing cumulative immune benefits when combined.
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