Russia's "Sputnik-V" COVID-19 vaccine produced an antibody response in all participants in early-stage trials, according to results published on Friday by The Lancet medical journal that were hailed by Moscow as an answer to its critics.
The results of the two trials, conducted in June-July this year and involving 76 participants, showed 100% of participants developing antibodies to the new coronavirus and no serious side effects, The Lancet said.
Russia licensed the two-shot jab for domestic use in August, the first country to do so and before any data had been published or a large-scale trial begun.
"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 said.
The vaccine is named Sputnik-V in homage to the world's first satellite, launched by the Soviet Union. Some Western experts have warned against its use until all internationally approved testing and regulatory steps have been taken.
But 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 Moscow had faced down its critics abroad.
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"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," said Kirill Dmitriev, the head of the Russian Direct Investment Fund (RDIF), Russia's sovereign wealth fund, which has backed the vaccine.
"All of the boxes are checked," he told Reuters. "Now... we will start asking questions of some of the Western vaccines."
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, USA 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."
RACE TO DEVELOP VACCINE
Governments and big pharmaceutical firms are racing to develop a vaccine to end the COVID-19 pandemic, which has killed more than 850,000 people globally and infected around 26 million.
More than half a dozen drugmakers are already conducting advanced clinical trials, each with tens of thousands of participants and several, including Britain's AstraZeneca and U.S. drugmakers Moderna and Pfizer expect to know if their COVID-19 vaccines work and are safe by the end of this year.
The Lancet said the early-stage trials suggested the Sputnik-V vaccine produced a response in a component of the immune system known as T cells.
Scientists have been scrutinising the role played by T cells in battling coronavirus infection, with recent findings showing these cells may provide longer-term protection than antibodies.
The vaccine, developed by Moscow's Gamaleya Institute, is administered in two doses, with each based on a different vector that normally causes the common cold: human adenoviruses Ad5 and Ad26.
Some experts have said that using this delivery mechanism could make a COVID-19 vaccine less effective, since many people have already been exposed to the Ad5 adenovirus and developed immunity to it.
In China and the United States, about 40% of people have high levels of antibodies from prior Ad5 exposure. In Africa, it could be has high as 80%, experts have said.
Denis Logunov, one of the vaccine's developers at the Gamaleya Institute, told Reuters the vaccine uses a strong enough dose of Ad5 to overcome any earlier immunity, without compromising safety.
The booster dose, based on the rarer Ad26 adenovirus, provides further support because the likelihood of widespread immunity to both types in the population is minimal, he said.
Russia has said it expects to produce between 1.5 million and 2 million doses per month of its potential COVID-19 vaccine by the end of the year, gradually increasing production to 6 million doses a month.