Chinese COVID-19 vaccine trial reports falling antibody titers

Chinese Academy of Sciences
The Chinese Academy of Sciences (Chinese Academy of Sciences/LinkedIn)

The Chinese Academy of Medical Sciences has presented phase 1 data on its inactivated SARS-CoV-2 vaccine. The readout suggests the vaccine is safe and triggers an immune response, although a drop in neutralizing antibody titers from Day 14 to Day 28 is a potential cause for concern.

While vaccines based on largely untested mRNA platforms made the early running in the COVID-19 race, multiple groups including China’s Sinopharm and France’s Valneva are working on traditional inactivated prophylactic candidates that could ultimately prove to be safer and more effective than the novel approaches. The Chinese Academy of Medical Sciences is among the organizations working on an inactivated SARS-CoV-2 vaccine.

In a preprint paper published Tuesday, the academic center shared findings from a phase 1 trial that tested three doses of the vaccine on two dosing schedules. The 192-subject trial was mainly focused on the safety and tolerability of the vaccine.

The vaccine passed the safety and tolerability test. Twenty-three percent of participants experienced an adverse reaction, most commonly mild injection site pain and redness or slight fatigue. The rate of adverse events is slightly higher than the 15% seen by Sinopharm but lower than the percentages reported in trials by developers of mRNA and viral vector vaccines such as Moderna and AstraZeneca. It is unclear whether comparisons of the small trials accurately reflect the effects of the vaccines. 

The preprint also included data on the effect of the vaccine on antibody titers. At Day 14, the seroconversion rate in subjects who received two shots of the middle dose—which generated the strongest response—two weeks apart was 100%, and the geometric mean titer was 54.5. Over the next 14 days, neutralizing antibody titers trended downward. The paper offers no explanation for the decline, and the cutoff means it is unclear whether the downward trend continued.

A clearer picture of the effects of the vaccine may emerge as the investigators continue to gather data. The ClinicalTrials.gov listing for the study states the Chinese Academy of Medical Sciences plans to enroll a further 750 participants for the phase 2a portion of the study. The authors of the preprint said further study is needed to build on the “encouraging” results seen in the phase 1.