The break-away group is led by trial volunteer Andrey Krinitsky, who is not a scientist. The group's findings were made public in an open letter on their blog on Wednesday.
EpiVacCorona, Russia's second vaccine to be registered, is a synthetic peptide vaccine which uses a different technology from the better-known Sputnik V shot.
Russia has said it has vaccinated 3.5 million of its 144 million population with both shots of the Sputnik V vaccine since it began in early December.
On Friday, St Petersburg, a city of more than 5 million, received 15,300 doses of the two-component vaccine, the municipal authority said on its website.
"We demand a public apology from EMA's Christa Wirthumer-Hoche for her negative comments..., (which) raise serious questions about possible political interference in the ongoing EMA review," the developers wrote on the official Sputnik V Twitter account.
Kremlin spokesman Dmitry Peskov called Wirthumer-Hoche's statement regrettable and inappropriate.
"Currently no talks are ongoing to integrate the Sputnik vaccine in the portfolio," a Commission spokesman told a news conference.
Earlier on Thursday, the European Medicines Agency (EMA) said it had begun a rolling review of the Sputnik V vaccine, which could lead to its approval for use in all 27 EU countries.
"(A) recent study carried out by the Gamaleya Centre in Russia showed that revaccination with Sputnik V vaccine is working very well against new coronavirus mutations, including the UK and South African strains of coronavirus.
We believe that vector-based vaccines are actually better for future revaccinations than vaccines based on other platforms.
Argentina was one of the first countries in the region to sign an agreement with Russia to buy Sputnik V and has already received 1.22 million doses from the Russian Gamaleya Institute.
This potential development will be carried out with the participation of Hetero Labs Limited, a laboratory established in India, with whom Laboratorios Richmond has had a strategic alliance for more than 25 years.
The approval of the Sputnik V vaccine would bolster India's immunisation campaign, which on Friday crossed the 10 million mark. The country aims to inoculate 300 million people by August.
Shares of the company, which had fallen as much as 1.9%, reversed course to trade 1.4% higher following the news.
Bahrain already uses the Pfizer/BioNTech, vaccine, one manufactured by Chinese state-backed pharmaceutical giant Sinopharm, and the Oxford-AstraZeneca COVID-19 vaccine.
Experts said the Phase III trial results meant the world had another effective weapon to fight the deadly pandemic and justified to some extent Moscow’s decision to roll out the vaccine before final data had been released.
The results were based on data from 19,866 volunteers, of whom a quarter received a placebo, the researchers, led by the Gamaleya Institute’s Denis Logunov, said in The Lancet.
Ferrer said the finishing touches were being put on the contract for the deal, and that it could be signed on Tuesday.
Mexico is trying to secure as much vaccine supply as possible amid delays of some products and has said it plans to administer 7.4 million doses of Sputnik V through March.
Tehran also said it had approved Russia's Sputnik V coronavirus vaccine for domestic use, and that it was trying to buy vaccines from AstraZeneca and other companies.
He also threatened that Iran would block short-notice inspections of Iran's nuclear facilities by the United Nations atomic agency if Washington did not lift sanctions.
The country could start receiving supplies of the vaccine as soon as next week, according to the plan presented during President Andres Manuel Lopez Obrador's daily morning news conference.
Government officials say Mexico is set to purchase 12 million doses of the Russian vaccine.
The data, based on Phase I and II trials, were released before the start of a larger Phase III trial which would normally involve thousands of participants and a placebo group as a comparison.
The effectiveness of the vaccine is made up of its immunological effectiveness and preventative effectiveness.
Anvisa officials had said previously that the Sputnik V vaccine would have to be submitted to Phase III clinical trials in Brazil before its use can be authorized.
Peer-reviewed results of the vaccine would be released shortly and would demonstrate its high efficacy, fund chief Kirill Dmitriev said.
Sputnik V would be produced in seven countries. He added that regulators in nine countries are expected to approve the vaccine for domestic use this month.
Mexico was considering acquiring 24 million doses of Sputnik V.
Separately, Lopez Obrador said his government would not be reserving information on Mexico's vaccine contracts despite officials previously saying they would be withheld for five years.