Hyderabad-based firm Bharat Biotech’s indigenous vaccine candidate Covaxin induced a strong immune response, without any serious adverse effects, the interim results of phase 1 clinical trial showed on Wednesday.

The results of the trial were published in medRxiv, an online platform for unpublished medical research manuscripts. The study was conducted on 375 volunteers.

“The vaccine was well tolerated in all dose groups with no vaccine-related serious adverse events,” the firm said in the research document. “Robust humoral and cell-mediated responses were observed in the Algel-IMDG recipients.”

The document added that the most common adverse event was pain at the injection site, which resolved spontaneously. “The overall incidence rate of local and systemic adverse events in this study was 10%-20% in all vaccine treated arms, which is noticeably lower than the rates for other SARS-CoV-2 vaccine platform candidates (11-15) and comparable to the rates for other inactivated SARSCoV-2 vaccine candidates (11, 16),” it said.

The research paper said that one volunteer who was vaccinated on July 30 developed a headache and fever five days later. This was initially reported as a “solicited adverse event”, but the patient was found to be infected with the coronavirus a week later.

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Bharat Biotech’s indigenous vaccine is being developed in collaboration with the National Institute of Virology and the Indian Council of Medical Research.

Earlier this month, Haryana Health Minister Anil Vij, who had received a trial dose of the vaccine on November 20, tested positive for the coronavirus. Bharat Biotech clarified that clinical trials of the vaccine were based on a two-dose schedule, given 28 days apart. It added that the efficacy of its vaccine can only be determined 14 days after a second dose.

“The vaccine efficacy will be determined 14 days post the second dose,” the firm had said in a statement. “Covaxin has been designed to be efficacious when subjects receive both doses. The phase-3 trials are double-blinded and randomized, where 50% of subjects [participants in the trial] receive vaccine and 50% of subjects receive placebo.”

In a double-blind clinical trial, a gold standard for testing new medicines or vaccines, neither the company nor the participants know who receives vaccines and who gets placebos, which may be a similar-looking injection having no medical effect. Only an independent review board has access to that information.

Bharat Biotech, Serum Institute of India and American pharmaceutical firm Pfizer have applied for emergency use authorisation of their vaccines in India.

The government has not signed a deal for a coronavirus vaccine yet so it is unclear when it will be available for use in the country. It is likely that the Centre may approve some coronavirus vaccines over the next few weeks. The final call on the efficacy of a vaccine will be taken by the Drugs Controller General of India.