Russian vaccine is 91.6% effective against symptomatic Covid - how it works

Tuesday, 2nd February 2021, 2:42 pm
Updated Tuesday, 2nd February 2021, 3:20 pm
The vaccine is 91.6 per cent effective against symptomatic Covid-19 (Photo: Getty Images)

Russia’s vaccine is 91.6 per cent effective against symptomatic Covid-19, interim trial results show.

The preliminary findings, published in The Lancet, are based on analysis of data from nearly 20,000 participants, three quarters of whom were given the vaccine. One quarter received a placebo.

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Two dose vaccine

Interim data from the phase three trial of the vaccine, Gam-COVID-Vac, or Sputnik V, suggests that two doses of the jab will offer 91.6 per cent efficacy.

The vaccine includes two adenovirus vectors - recombinant human adenovirus type 26 (rAd26-S) and recombinant human adenovirus type 5 (rAd5-S) - which have been modified to express the SARS-CoV-2 spike protein. The adenoviruses are also weakened so that they cannot replicate in human cells and cause disease.

In the trial, participants were given one dose of rAd26-S, followed by a booster dose of rAd5-S 21 days later.

Using a different adenovirus vector for the booster vaccination may help to create a more powerful immune response, compared with using the same vector twice, according to the researchers. They suggest this is because it minimises the risk of the immune system developing resistance to the initial vector.

No serious adverse effects were considered to be associated with the vaccine and most reported side effects were mild, including flu-like symptoms, pain at the injection site and weakness or low energy.

The trial included 2,144 participants more than 60 years old, and vaccine efficacy was 91.8 per cent in this group. The vaccine was well tolerated and safety data from 1,369 of these older adults found that the most common adverse events were flu-like symptoms and local reaction.

Further research on asymptomatic cases

The trial was conducted between 7 September and 24 November last year and saw 14,964 participants given two doses of the vaccine, and 4,902 given the placebo.

From 21 days after receiving the first dose, on the day of the second dose, 16 cases of symptomatic Covid-19 were confirmed in the vaccine group, and 62 cases in the placebo group - equivalent to an efficacy of 91.6 per cent.

Researchers say the vaccine prompted an effective antibody response and T-cell response, but note that because Covid-19 cases were detected only when participants self-reported symptoms, followed by a test, the efficacy analysis only includes symptomatic cases of the virus.

Further research is needed to understand the efficacy of the vaccine on asymptomatic Covid-19 and transmission. The median follow up was 48 days from the first dose, so the study cannot assess the full duration of protection.

Four deaths were recorded during the trial - three in the vaccine group, and one in the placebo group.

In the vaccine group, one death was associated with a fracture, and two had underlying conditions and developed symptoms of the virus four to five days after the first dose of the vaccine. Both participants were deemed to have already been infected before inclusion in the trial.

In the placebo group, the death was associated with a stroke. None of the deaths were deemed to be associated with vaccination.