The Russian vaccine Gam-COVID-Vac, also known as Sputnik V, is 91.6 % effective against symptomatic COVID-19, a phase III trial has shown. The preliminary results of this crucial final round of testing were published in the prestigious medical journal The Lancet  on Tuesday.

The findings are based on data from nearly 20,000 people who participated in the study in 25 hospitals and clinics in Moscow from September through November 2020. 

Participants received an initial shot of either the vaccine or a placebo and then a booster shot 21 days later. Of the 14,964 people in the vaccine group, only 16 had developed symptomatic cases of COVID-19 after receiving the second shot of the vaccine. This compares to 62 infected people in the 4,902-strong placebo group. PCR tests for the coronavirus were conducted at screening, on the day participants received the booster shot and if they reported symptoms of a respiratory infection.

Researchers at Russia's Gamaleya National Research Centre for Epidemiology and Microbiology, where Sputnik V was developed, say they are happy with the results.

"Our interim analysis of the randomized, controlled, phase III trial of Gam-COVID-Vac in Russia has shown high efficacy, immunogenicity and a good tolerability profile in participants aged 18 years or older," says Gamaleya's Dr. Inna V Dolzhikova, co-lead author of the study published in The Lancet. 

The researchers found no serious negative consequences of vaccination. Reported side effects included flu-like symptoms and pain at the injection site. 

Encouraging result for previously contentious vaccine

Sputnik V was the first coronavirus vaccine to be released worldwide. Russia's move to start using it on a mass scale last year, before all the trials were completed and results analyzed, has been criticized by the international community. Another point of contention: The Gamaleya research center did not just develop the vaccine but was also in charge of authorizing its use in Russia. In the EU or the US, authorization is handled not by the companies who develop the serum, but by separate entities like the European Medicines Agency (EMA) or the US Food and Drug Administration (FDA).

The release of the phase III results is likely to increase confidence in the Russian vaccine.

"The development of the Sputnik V vaccine has been criticized for unseemly haste, corner-cutting and an absence of transparency. But the outcome reported here is clear and the scientific principle of vaccination is demonstrated," Professor Ian Jones, University of Reading, and Professor Polly Roy, London School of Hygiene & Tropical Medicine, who were not involved in the study, wrote in a comment about the new findings. 

Good results with the elderly as well

The most recent Sputnik V trial included 2,144 participants over 60 years of age, and the vaccine efficacy in this group was 91.8 %. A British coronavirus vaccine developed by AstraZeneca had been criticized for not testing a sufficient number of people from this risk group.

The Russian COVID-19 vaccine includes two adenovirus (a common cold virus) vectors that have been modified to carry the gene for the SARS-CoV-2 spike protein, which makes Sputnik V similar to the AstraZeneca vaccine in the way it works.  

The study's authors note that because COVID-19 cases were only detected when participants reported symptoms themselves, the efficacy analysis includes only symptomatic cases of COVID-19. Further research is needed to understand how efficient the vaccine is when it comes to asymptomatic COVID-19 cases and their transmission.