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Russia's Sputnik V vaccine is 91.6 percent effective against symptomatic Covid-19, according to results published Tuesday that independent experts said allayed transparency concerns over the jab, which Moscow is already rolling out.
Sputnik V -- named after the Soviet-era satellite -- was approved in Russia months before results from its final-stage clinical trials were published, leading to scepticism among experts.
But the new analysis of data from 20,000 Phase 3 trial participants, published in the medical journal The Lancet, suggests that the two-dose vaccination offers more than 90 percent efficacy against symptomatic Covid-19.
"The development of the Sputnik V vaccine has been criticised for unseemly haste, corner cutting, and an absence of transparency," said a joint independent commentary by Ian Jones of the University of Reading and Polly Roy of the London School of Hygiene and Tropical Medicine.
"But the outcome reported here is clear and the scientific principle of vaccination is demonstrated, which means another vaccine can now join the fight to reduce the incidence of Covid-19."
The results suggest Sputnik V is among the top performers alongside the Pfizer/BioNTech and Moderna jabs, which also reported more than 90 percent efficacy.
- 'Checkmate' -
"There are no arguments left for critics of this vaccine, the article in The Lancet is a checkmate," said Kirill Dmitriev, head of the Russian Direct Investment Fund (RDIF), which was part of Sputnik V's development.
Pre-empting the results of the Phase 3 trials, Russia has already launched a mass inoculation campaign.
The Russian vaccine is also already registered in 16 countries including former Soviet republics as well as allies such as Venezuela and Iran, as well as South Korea, Argentina, Algeria, Tunisia and Pakistan.
Russia applied last month for registration of the Sputnik V jab with the European Union through the European Medicines Agency (EMA).
On Tuesday, German Chancellor Angela Merkel said all vaccines "are welcome" in the EU once they have received regulatory approval, adding that she had recently spoken to President Vladimir Putin about the Sputnik V jab.
The Hungarian government has already given its green light to the Russian vaccine separately.
Sputnik V has the advantage of being relatively cheap and able to be kept at normal refrigerator temperatures rather than far below freezing for some other vaccines.
But the European Commission spokesman for health, Stefan De Keersmaecker, said that a key selection criterion was that the company have production capacity within the bloc.
"The aim is to make sure that we can deliver supplies very quickly, i.e., the day on which the vaccine gets the green light," he told a press conference.
Dmitriev mooted a partnership with other laboratories, while Merkel has proposed German aid for potential "joint production".
- 'No serious adverse events' -
The Lancet report said that during the September-to-November trial across 25 hospitals and polyclinics in Moscow, "no serious adverse events were deemed to be associated with vaccination".
Analysis of some 2,000 participants older than 60 also suggested the vaccine is similarly effective and well tolerated in this group.
The trial is ongoing and plans to recruit a total of 40,000 people.
According to the Lancet report, 14,964 participants in the vaccine group and 4,902 in the placebo group were given two jabs 21 days apart.
From the second dose, 16 cases of symptomatic Covid-19 were confirmed in the vaccine group and 62 cases were reported in the placebo group, giving an efficacy equivalent to 91.6 percent.
The authors noted however that efficacy was calculated only on symptomatic cases and said more research would be needed to assess its effects on asymptomatic patients.
Sputnik V uses different disarmed strains of the adenovirus, a virus that causes the common cold, as vectors to deliver the vaccine dose.
In this respect it is similar to the Oxford/AstraZeneca jab, which reported efficacy of about 60 percent when two full jabs were given in its trials.
But Sputnik V uses two different adenovirus strains -- one for each jab. Developers said this minimises the risk of the immune system developing resistance to the initial vector, which may help create a more powerful response.
Alexander Edwards of the University of Reading said the trial might help provide evidence to this theory of immune response.