Russia claimed its coronavirus vaccine, the first to be registered worldwide, will soon be tested on 40,000 people in order to test its reliability.
The vaccine, called "Sputnik V," has been hailed by Russian authorities as safe and effective after it received domestic regulatory approval earlier this month. Full-scale production is due to start in September.
Health experts and public officials, however, have expressed concern over the proposed vaccine, citing the speed of its approval and the lack of available data. The results of two months of small-scale human trials in Russia have not yet been made available to the public.
"The so-called conditioned registration certificate means that we are obliged to conduct an additional expanded clinical trial," Denis Logunov, deputy director for scientific work of Gamaleya Research Institute of Epidemiology and Microbiology, said in an online briefing on Thursday, according to a translation.
"And currently we have agreed on a huge protocol for 40,000 participants. The purpose of this protocol is not that much to study the immunogenicity and safety of the vaccine, that goes without saying … But, additionally, we will have to assess the epidemic efficacy of this vaccine," Logunov said.
Russia's vaccine is being developed at the Gamaleya Research Institute in Moscow.
To date, more than 22.4 million people have contracted the Covid-19 infection, with 788,356 related deaths, according to data compiled by Johns Hopkins University.
Concerns over the safety and reliability of Sputnik V stem from the fact that it has only undergone rapid Phase 1 and Phase 2 clinical trials on a relatively small number of people.
Phase 3 trials are to begin shortly, but Russia has said it expects to start producing the vaccine as soon as next month.
When asked about Russia's coronavirus vaccine during a separate online press briefing on Thursday, Hans Kluge, regional director for Europe at the WHO, replied: "Let me say, overall, that any advances in vaccine development is very encouraging news."
The country has a "long tradition" of vaccine development and uptake, he added. "But … every vaccine has to go (through) the same rigorous standards of efficacy and safety. And, ultimately to know there is only one way to do so is clinical trials: Phase 1, Phase 2 and Phase 3 – including Phase 3," Kluge said.
Catherine Smallwood, senior emergency officer at WHO Europe, also confirmed that the United Nations health agency had recently entered into direct discussions with Russia over the potential development of Sputnik V, with updates to follow in due course.
"This concern that we have around safety and efficacy is not specifically for the Russian vaccine, it is for all of the vaccines that are under development," Smallwood said. "It is absolutely essential that we don't cut corners in safety or efficacy, so it is a central concern for all of the vaccines."
"We are not going through a rush job of trying to jump to conclusions here. We want to take our time to really understand where the vaccine is at and to get as full information as possible on the steps that have already been taken," Smallwood said.
— CNBC's Holly Ellyatt contributed to this report.