The levels of Omicron-neutralising antibodies of people vaccinated with two doses of Russia's Sputnik V vaccine did not decline as much as of those who had Pfizer shots, according to a small study.

The joint Russian-Italian study funded by the Russian Direct Investment Fund which markets Sputnik V compared the blood serum samples of people taken three to six months after the second dose of the two vaccines. The yet-to-be peer-reviewed study, posted on the preprint repository MedRxiv on January 19, was conducted by scientists from the Spallanzani Institute in Italy and Gamaleya Institute in Russia, the developer of the vaccine.

The study found that the Sputnik V vaccine demonstrates more than 2 times higher levels of virus neutralising antibodies to Omicron variant than two doses of Pfizer vaccine. The research was conducted on comparable blood samples from 51 people vaccinated with Sputnik V and 17 with the Pfizer preventive with a similar level of neutralising antibodies against the original Wuhan variant.

"Sputnik V shows significantly smaller (2.6 times) reduction of virus neutralising activity against Omicron as compared to reference Wuhan variant," the authors of the study noted. "The vaccine showed an 8.1-fold reduction of virus neutralising activity against Omicron as against 21.4-fold reduction for Pfizer vaccine," they said.

The study demonstrates that Sputnik V neutralises the Omicron variant by inducing robust antibody response associated with high levels of protection. "Among the top quartile of individuals with high receptor-binding domain (RBD)-specific IgG antibodies, 100 per cent of those vaccinated with Sputnik V were able to neutralise Omicron variant in comparison to 83.3 per cent of individuals vaccinated with Pfizer," the researchers said.

RBD is a key part of a virus located on its 'spike' protein that allows it to enter and infect human cells. "Among all samples, 74.2 per cent of Sputnik V-vaccinated sera were able to neutralise Omicron vs 56.9 per cent for Pfizer-vaccinated," they said.

An earlier study by the Gamaleya Institute, also posted on MedRxiv, showed that a booster shot of Sputnik Light vaccine provided a stronger antibody response against Omicron than the two-dose Sputnik V vaccine alone. "Today the necessity of third booster vaccination is obvious. And the most effective approach, already shown in several studies, is the use of heterologous booster vaccination pioneered in COVID-19 vaccines by Sputnik V," the authors of the study added.