Both companies had been stuck with their vaccine development as they were unable to get doses from Serum Institute of India’s Covishield and Bharat Biotech’s Covaxin to proceed with their late-stage trials.

According to information put up by the companies in the clinical trial registry, Biological E’s Corbevax covid vaccine has entered phase 3 trials. A total of 2,140 volunteers have been recruited between ages 18 and 80, with one set of participants given Corbevax and the other set Covishield. They will be compared for levels of antibodies produced.

In the case of Gennova, a total of 4,000 subjects will be enrolled in the trial across phases 2 and 3, with 3,000 given HGCO19 (its underdevelopment vaccine candidate) and 1,000 Covishield.

“We had to push the government really hard to get the comparator vaccines as it was getting difficult to access them,” said an official at a vaccine company who did not want to be named due to the sensitivity of the issue.

Biological E and Gennova did not respond to email queries by Mint.

With four approved vaccines in the market, companies that started their trials early this year have been forced to use an approved vaccine in place of a placebo to compare the efficacy of their vaccines as recruiting trial participants was becoming ethically and logistically challenging.

Globally, too, new covid-19 vaccine candidates have hit a roadblock as vaccine makers such as Pfizer, Moderna and J&J are not sharing doses for comparator clinical trials.

“While the number of doses of comparator vaccine needed to support clinical trials is small, covid-19 vaccines remain scarce, and they are not available for purchase in the open market,” said Melanie Saville, director of vaccine research and development at the Norway-based Coalition of Epidemic Preparedness Innovations (CEPI), which funds vaccines for neglected diseases and now also covid.

Saville pointed out that pre-purchase agreements between vaccine makers and countries restrict the latter’s use of vaccines to only mass vaccination campaigns, banning their use for clinical trials.

Vaccine makers, too, are not ready to change this agreement “threatening to bring vital covid-19 vaccine R&D to a standstill,” CEPI said.

It urged governments, developers and vaccine manufacturers to urgently offer solutions to unlock supplies of comparator vaccines in order to speed up trials and end a shortage of vaccines in low- and middle-income countries.

In India, despite the country making progress with its vaccination drive, the erratic supply of vaccines has made room for new vaccine candidates to be included in the immunization drive.

Currently, only SII and Bharat Biotech have continued to supply vaccines en mass in India’s immunization drive. And with only Covishield having approval from the WHO, the supply pressure rests on SII.