Russia's coronavirus vaccine produced an antibody response in all participants in two early-stage trials, according to results published by The Lancet.
The medical journal said on Friday the results of the trials - conducted in June-July and involving 76 people - showed 100 percent of participants developing antibodies to the new coronavirus and no serious side-effects.
"The two 42-day trials - including 38 healthy adults each - did not find any serious adverse effects among participants, and confirmed that the vaccine candidates elicit an antibody response," The Lancet said.
"Large, long-term trials including a placebo comparison, and further monitoring are needed to establish the long-term safety and effectiveness of the vaccine for preventing COVID-19 infection," it added.
Russian President Vladimir Putin's announcement on August 13 that Russia had become the first country to grant regulatory approval to a COVID-19 vaccine raised scepticism.
Some health experts warned against its use as the "Sputnik V" vaccine - named in homage to the world's first satellite launched by the Soviet Union - had not completed its phase-three trial, which involves wide-scale testing with thousands of participants.
However, with the results now published for the first time in an international peer-reviewed journal, and with a 40,000-strong later-stage trial launched last week, a senior Russian official said the results answered Moscow's critics abroad.
"With this [publication] we answer all of the questions of the West that were diligently asked over the past three weeks, frankly with the clear goal of tarnishing the Russian vaccine," Kirill Dmitriev, the head of the Russian Direct Investment Fund (RDIF), which has backed the vaccine, told Reuters news agency.
"Now ... we will start asking questions of some of the Western vaccines," he said.
Dmitriev said at least 3,000 people had already been recruited for the large-scale trial of the Sputnik V vaccine launched last week, and initial results were expected in October or November this year.
Commenting on the results of the early-stage trials, lead author Dr Naor Bar-Zeev of the International Vaccine Access Center, Johns Hopkins Bloomberg School of Public Health in the United States, said the studies were "encouraging but small".
Bar-Zeev, who was not involved in the study, said "clinical efficacy for any COVID-19 vaccine has not yet been shown".
Russia has said it expects to produce between 1.5 million and two million doses per month of its potential COVID-19 vaccine by the end of the year, gradually increasing production to six million doses a month.