World Health Organization’s chief scientist Soumya Swaminathan on Monday advised against mixing and matching coronavirus vaccines from different manufacturers, calling it a “dangerous trend”, reported Reuters.

“We are in a data-free, evidence-free zone as far as mix and match,” Swaminathan said at an online briefing. “It will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, a third and a fourth dose.”

Mixing and matching Covid-19 vaccines is a method of immunisation in which two doses of the shot from different manufacturers are given to a beneficiary. Most vaccines currently in use, including those of Pfizer, AstraZeneca and Bharat Biotech, are all required to be administered in two doses with the prescribed intervals between the shots differing for each vaccine.

“There are studies going on [on mix and watch], we need to wait for that,” Swaminathan said, according to the Hindustan Times. “Maybe it will be a very good approach. But, at the moment, we only have data on the Oxford-AstraZeneca vaccine, followed by Pfizer.”

A study published in medical journal The Lancet had shown that beneficiaries who were given the AstraZeneca vaccine as the first dose and Pfizer’s as the second reported more short-lived side effects. The study, however, did not say if the mixing of vaccines could help protect against the infection.

Another study in Spain found that the two vaccines given in the same order was highly effective. One of the scientists said that 1.7% of participants reported severe side effects such as headaches and muscle pain.

Earlier in June, Swaminathan had said that using two different vaccines seemed to generate a stronger immune response. She had then cited data from the United Kingdom and Germany that showed the “mix-and-match” inoculation regimen caused more minor side effects compared to two doses of the same vaccine.

In May, the Centre had also said that mixing of vaccines was unlikely to cause any significant adverse effects. However, it had added that more scientific data was needed on the immunisation method. In the same month, 20 villagers in Uttar Pradesh’s Siddharthnagar district were administered one dose of Covishield and a second dose of Covaxin during vaccinations against Covid-19.

On the incident, NK Arora, the chairperson of Working Group of the National Technical Advisory Group on Immunisation, had clarified that there were no safety concerns but said that it should not have happened.

Meanwhile, India recorded 37,154 new coronavirus cases and 724 deaths in 24 hours on Monday morning. With this, the country’s tally of infections rose to 3,08,74,376 and the toll went up to 4,08,764 since the pandemic broke out in January last year.