A homegrown candidate vaccine developed by the Hyderabad-based Bharat Biotech has generated “robust” immune responses, including high levels of neutralising antibodies, against the novel coronavirus, government and industry researchers said on Wednesday.

In their first scientific paper on human clinical trials of their candidate vaccine, the researchers have also described observing possible hints of immune memory responses critical for long-term efficacy triggered by all three formulations they tested.

These early results do not permit assessment of the vaccine’s efficacy in protecting people from the coronavirus infection but had guided India’s drug regulatory agency’s decision to approve the efficacy trials that started last month and seek to enrol around 26,000 volunteers.

In the safety and immunogenicity study, 375 volunteers were divided into four groups — 100 each in three groups who received different formulations of the candidate vaccine, and 75 in a placebo group who received dummy jabs.

The researchers observed antibody responses in 83 per cent, 88 per cent and 92 per cent of volunteers who had received two doses of the vaccine. Six per cent of those on placebo too developed antibodies likely because they picked up natural infections during the trial.

The study also detected neutralising antibodies that block the virus in significant proportions of the volunteers, the 24-member research team from Bharat Biotech, the

Indian Council of Medical Research and hospitals that participated in the clinical trials said.

“The vaccine was well-tolerated in all the dose groups with no vaccine-related serious adverse events,” the researchers said in their paper posted on an online archive for preprints but not peer-reviewed yet. One volunteer developed coronavirus disease but, the researchers said, the event was analysed and determined as “not causally associated with the vaccine”.

Bharat Biotech is among three vaccine makers that have applied to the Central Drugs Standard Control Organisation (CDSCO), the regulatory agency, for emergency use authorisation of their candidate vaccines. The Serum Institute of India, which is manufacturing the AstraZeneca vaccine in India, and the US-based Pfizer have also applied for emergency use authorisation.

While Bharat Biotech is currently conducting its efficacy trials at multiple sites across India, the AstraZeneca and Pfizer vaccines have been shown as effective in clinical trials in other countries.

A CDSCO panel reviewing the applications had last week asked the Serum Institute to update its immunogenicity data from India and the UK and submit the outcome of AstraZeneca’s application for similar approval with the UK regulatory authority. The panel asked Bharat Biotech to submit safety and efficacy data from its ongoing clinical trial.

Medical experts believe the CDSCO panel is likely to wait for regulatory approval of the AstraZeneca vaccine in the UK, the vaccine’s home country, before granting an approval in India.

Bharat Biotech has a current capacity to produce 150 million doses per year but could increase the capacity to 500 million doses if required. The Serum Institute has a current capacity of 70 million doses per month, of which it can make available half for use within India.