Johnson & Johnson (J&J) has reported positive interim data from an ongoing Phase I/IIa clinical trial of a potential Covid-19 vaccine, JNJ-78436735, developed by its subsidiary Janssen Pharmaceutical.

According to the results, a single dose of the vaccine candidate showed a strong neutralising antibody response in almost all participants aged 18 years and above. The candidate was observed to be generally well-tolerated.

With seroconversion found in 99% of the trial subjects, 98% were positive for neutralising antibodies against the virus at day 29 after vaccination.

Apart from antibody responses, the vaccine candidate led to T cell responses and a Th1 response, which are expected to protect from the risk of vaccine-related enhanced respiratory disease.

Data from the initial 15 participants aged 65 years and above revealed strong humoral and cellular immune responses with a single dose of the vaccine candidate. J&J added that immune responses were similar across the age groups, including older adults.

These findings, published on pre-print server medRxiv, are consistent with preclinical studies, where a single vaccine dose was able to prevent subsequent infection and offered complete protection in the lungs of non-human primates.

The Phase I/IIa study is assessing the safety, reactogenicity and immunogenicity of two dose levels of JNJ-78436735 given intramuscularly as single and two-dose schedules, administered eight weeks apart.

It is being performed at several sites in Belgium and the US.

Based on the interim results, the company has selected the single vaccine dose of 5×1010 virus particles (vp) for further review in the Phase III ENSEMBLE trial.

The ENSEMBLE study was launched last week and will involve subjects aged 18 years and above, with and without comorbidities, in Argentina, Brazil, Chile, Colombia, Mexico, Peru, South Africa and the US.

Besides, J&J intends to conduct another Phase III trial later this year to compare a two-dose regimen of the Covid-19 vaccine candidate to placebo.