Researchers said that the results from early and mid-stage trials of Chinese Academy of Sciences’ (CAS) Covid-19 vaccine candidate, ZF2001, showed that it was safe and triggered immune responses.

ZF2001 is a receptor-binding domain-based protein subunit vaccine, which utilises a harmless fragment of SARS-CoV-2 instead of the complete germ.

The randomised, double-blind, placebo-controlled Phase I and II trials evaluated the safety and immunogenicity of ZF2001 and to find the appropriate dose and schedule for an efficacy study.

Last month, a late-stage trial of the vaccine being developed by CAS with a unit of Chongqing Zhifei Biological Products, started in China, Reuters reported.

The Phase I study enrolled 50 healthy Chinese adults aged 18 to 59 years. They were categorised into three groups to receive three intramuscular doses 25μg or 50μg RBD-dimer, with adjuvant or placebo (adjuvant-only), 30 days apart.

In the Phase II study, 900 healthy adults of the same age group were enrolled and randomised into six groups to receive 25μg or 50 μg RBD-dimer, with adjuvant or placebo.

The former three groups were given two doses and the latter three groups received three doses, 30 days apart.

Data showed that higher dose given through three injections spaced 30 days apart did not trigger an improved immune response versus the lower dose.

In addition, ZF2001 was well-tolerated and immunogenic with no serious adverse events relating to the vaccine candidate observed in both the trials.

After three doses, neutralising antibodies were detected in all subjects who received the treatment in Phase I study. In the Phase II study, 97% in the 25μg group and 93% in the 50μg group had neutralising antibodies.

ZF2001 also triggered moderate cell-based immune responses.

Furthermore, the 50μg group did not show enhanced immunogenicity as compared to the 25μg group.

The safety and immunogenicity data from these trials support the use of 25μg vaccine dose with three-dose regimen to an ongoing Phase III trial for safety and efficacy.