Writing in The Lancet Microbe, researchers at The University of Hong Kong described the enrollment of a cohort of 1,442 healthcare workers. The researchers collected blood samples from the subjects to compare the immunogenicity of mRNA and inactivated vaccines against COVID-19.
The analysis presented in correspondence to the journal is based on preliminary laboratory testing data from 93 fully vaccinated participants, around two-thirds of whom received the mRNA product.
Samples were analyzed for antibodies that bind to the virus and antibodies that neutralize the virus. In both cases, the first dose of the Sinovac vaccine was associated with a minimal change in levels of the antibodies.
The second dose of the Sinovac vaccine caused a steeper climb in antibody levels. However, antibody levels after two doses of the Sinovac vaccine were comparable to levels after one dose of the jab from Pfizer and BioNTech. The second dose of the Pfizer-BioNTech vaccine drove antibody levels well above the peak in recipients of the Sinovac product.
A similar pattern is evident in a subset of 12 participants on whom the researchers have additional data from the plaque reduction neutralization test. The researchers assessed the antibody titers that reduced live virus plaque formation by 50% or 90%, assessments known respectively as PRNT50 and PRNT90.
The analysis found a 10-fold difference in the geometric mean PRNT50 titers and a 13-fold difference in geometric mean PRNT90 titers a month after the administration of the second doses of the jabs.
Vaccine effectiveness
While factors other than antibody levels influence whether a person is protected from the virus, the preliminary findings, which are in line with results from a separate study described in the Hong Kong Medical Journal, nonetheless have potential implications for the effectiveness of the vaccines.
A paper published in Nature Medicine in May provided insights into the impact of antibody levels by analyzing the relationship between in vitro neutralization levels and protection from infection with the coronavirus. The analysis suggests the 10- to 13-fold differences in geometric mean titers seen in the Lancet publication will translate into a notable divergence in vaccine effectiveness.