TAIPEI (Taiwan News) — A large Phase II study published in The Lancet on Wednesday (Oct. 13) reported a seroconversion rate of nearly 100% for Taiwan's domestic COVID-19 vaccine developed by Medigen Vaccine Biologics Corp. (高端疫苗, MVC).
The higher the seroconversion rate, the higher the number of protective antibodies, meaning the more potent the vaccine. WHO describes it as a kind of "definition of the magnitude of the immune response."
On Wednesday, Taiwanese scientists, including Hsieh Szu-min (謝思民) from National Taiwan University and Liu Ming-che (劉明哲) from Taipei Medical University, released in The Lancet the results of a double-blind, randomized, placebo-controlled Phase II clinical trial of the Medigen jab. The study was carried out on 3,854 participants aged 20 and older at 10 medical centers and one regional hospital in Taiwan.
Of these participants, 3,304 were injected with the vaccine and 550 were given a placebo, with none of the recipients experiencing serious adverse events during the study. The most commonly reported reactions were pain at the injection site at 71.2%, malaise or fatigue at 36%, and fever at an extremely low 0.7%.
Twenty-eight days after receiving the second dose, which was 57 days after the first dose, the seroconversion rate among recipients was 99.8%. However, seroconversion does not always equate to vaccine efficacy, which can only be confirmed through Phase III trials.
Prior to Phase III trials, there are other methods to estimate efficacy such as comparing Geometric Mean Titer (GMT) to recovered COVID patients and the binding antibody units (BAU) conversion model.
Recipients of the Medigen jab were found to have a GMT 1.5 times higher than serum samples from recovered COVID patients. When using the WHO’s reference standard NIBSC 20/148 as a parameter for comparison, Medigen recipients had a GMT that was 1.94 higher than recovered COVID patients.
Researchers at the University of Oxford have devised a correlates of protection (CoP) estimate for the AstraZeneca vaccine based on BAUs called the BAU conversion model. According to this model, an antispike IgG titre between 264 and 899 BAU/mL correlates to a vaccine efficacy of between 80% and 90%.
In the case of the Medigen study, the anti-spike IgG GMT on the 28th day after the second shot was 594.7 BAU/mL in the younger adult group and 354.5 BAU/mL in the older adult group. This places the vaccine well within the 80 to 90 percentile range for efficacy.