An interim analysis of data from a phase III trial of Russia's covid-19 vaccine (Sputnik V) suggests that a two-dose regimen offers 91.6% efficacy against symptomatic disease; protection in the over-60s is very similar, rising to 91.8%.
The preliminary results, published in The Lancet, are based on analysis of data from nearly 20,000 participants, three-quarters of whom received the vaccine and a quarter a placebo.
The trial included 2,144 participants over the age of 60 and the efficacy of the vaccine was 91.8% in this group: the vaccine was well tolerated and safety data from 1,369 of these older adults revealed that the most common adverse effects were flu-like symptoms and local reaction.
These were consistent with most of the adverse effects reported in the other groups, which were also generally mild; serious adverse effects (those requiring hospital admission) were rare in both the placebo group (0.4%) and the vaccine group (0.2%).
None, say the authors, were considered to be associated with vaccination; four deaths were reported in the trial, none of which were considered to be vaccine-related.
In a commentary associated with this research, Ian Jones of the University of Reading and Polly Roy of the London School of Hygiene & Tropical Medicine (who were not involved in the study) say: the development of the Sputnik V vaccine has been criticised for "unseemly haste, shortcuts or lack of transparency", but the result presented here "is clear and demonstrates the scientific principle of vaccination".
"This means that another vaccine can now join the fight to reduce the incidence of covid-19," he says.
Gam-COVID-Vac or Sputnik V is a two-part vaccine that includes two adenovirus vectors - rAd26-S and rAd5-S - that have been modified to express the SARS-CoV-2 S protein.
The adenoviruses are also weakened so that they cannot replicate in human cells and cause disease.
This technique has been used before and its safety has been confirmed in several clinical trials, the journal said in a note.
In this trial, participants received a 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 create a stronger immune response by minimising the risk of the immune system developing resistance to the initial vector, according to the authors.
"Our interim analysis of the randomised, controlled phase III trial has demonstrated high efficacy, immunogenicity and a good tolerability profile in participants aged 18 years and older," says Inna V Dolzhikova of the Gamaleya National Centre for Research in Epidemiology and Microbiology, Russia.
Twenty-one days after receiving the first dose (day of dose 2), 16 cases of symptomatic covid-19 were confirmed in the vaccine group (0.1%) and 62 cases (1.3%) in the placebo group, equivalent to 91.6% efficacy, the study summarises.
The vaccine induced a robust humoral response (antibody response) and a cellular immune response (T-cell response) with data from 342 and 44 participants, respectively.
This efficacy analysis only includes symptomatic cases, so further research is needed to understand the efficacy of the vaccine in asymptomatic cases.
In addition, the median follow-up was 48 days from the first dose, so the study cannot assess the full duration of protection.
As part of their secondary analyses, the authors explored efficacy against moderate or severe covid-19. At 21 days after the first dose, there were no cases of moderate or severe covid-19 in the vaccine group and 20 cases in the placebo group, which equates to 100% efficacy against moderate or severe covid-19.
Although the study was not designed to assess the efficacy of a single-dose regimen, the results point to the early onset of a partially protective effect 16-18 days after single-dose immunisation; the team recently received approval to investigate the efficacy of a single dose.
The authors also point to the need for further research to study the vaccine in adolescents and children and in pregnant women. The trial is ongoing and aims to include a total of 40,000 participants.
In addition to Russia, the Russian vaccine has been licensed in 14 other countries, including Hungary, Algeria, Serbia, Argentina, Bolivia, Venezuela, Paraguay, Turkmenistan, Belarus, Kazakhstan, United Arab Emirates and Iran. The Czech Republic, Mexico and Sri Lanka are also interested.