Two early-phase Russian coronavirus vaccine trials have produced promising results, with participants experiencing no serious adverse effects and evidence of an antibody response.

Controversy greeted the announcement last month that Russia had approved the world’s first Covid-19 vaccine – before it had completed final “phase 3” clinical trials.

Those still have not been done. But the findings of two smaller 42-day trials published in the Lancet medical journal on Friday do suggest the vaccines produce a double protection in the form of a T-cell response within 28 days.

T-cells attack viruses directly by recognising and killing cells that have been taken over by a virus.

The two-part vaccine includes two adenovirus vectors which have been modified to express the SARS-CoV-2 spike protein.

These types of vaccines are based on weakened versions of adenoviruses, which are a group of viruses that typically infect membranes of the eyes, respiratory tract, urinary tract, intestines and nervous system, and include the common cold.

Lead author Dr Denis Logunov, of the N F Gamaleya National Research Centre for Epidemiology and Microbiology in Russia, said: “When adenovirus vaccines enter people’s cells, they deliver the SARS-CoV-2 spike protein genetic code, which causes cells to produce the spike protein.

“This helps teach the immune system to recognise and attack the SARS-CoV-2 virus.

“To form a powerful immune response against SARS-CoV-2, it is important that a booster vaccination is provided.”

To compare post-vaccination immunity with natural immunity formed by Covid infection, the authors obtained convalescent plasma from 4,817 people who had recovered from mild or moderate coronavirus.

The frozen vaccine was tested with a mind for large-scale use through existing global supply chains for vaccines and the freeze-dried version for hard-to-reach regions as it is more stable and can be stored at 2 to 8 degrees centigrade.

The most common adverse side effects were those characteristic of those seen with other vaccines, including pain at injection site, hyperthermia, headaches, lack of energy, and muscle and joint pain.

The trials took place in two hospitals in Russia, with the 76 participants knowing they were receiving the vaccine. They were all healthy adults aged 18 to 60, who self-isolated as soon as they were registered for the trial and remained in hospital for the first 28 days of the trial.

The frozen vaccine (Gam-Covid-Vac) was trialled in a branch of Burdenko Hospital, an agency of the Russian ministry of defence, and involved both civilian and military volunteers. The freeze-dried vaccine (Gam-Covid-Vac-Lyo) trial took place at Sechenov University and all volunteers were civilians.

While generally positive, the authors of the study note it was a small sample, without any placebo or control vaccine. Part of the phase 1 trials only included male volunteers and while efforts were made to recruit volunteers of a broad age range, most were in their 20s and 30s. The authors admit more research is needed to evaluate the vaccine in different populations, including older age groups, those with underlying medical conditions and those in at-risk groups.

Explaining the next steps of their research, Professor Alexander Gintsburg, of the N F Gamaleya National Research Centre for Epidemiology and Microbiology, Russia, said the next phase of the trial was approved on August 26.

He added: “It is planned to include 40,000 volunteers from different age and risk groups, and will be undertaken with constant monitoring of volunteers through an online application.”

Dr Naor Bar-Zeev, of the International Vaccine Access Centre, Johns Hopkins Bloomberg School of Public Health, USA – who was not involved in the study – said the research was encouraging, though small-scale.

And he warned: “To be sure, most past vaccines were designed to target disease and not infection as such, but with Covid-19, the general public could be expecting striking reductions in disease transmission after widespread vaccine introduction.

“Such effects would be very welcome if they occur, but they are far from certain. A vaccine that reduces disease but does not prevent infection might paradoxically make things worse. It could falsely reassure recipients of personal invulnerability, thus reducing transmission-mitigating behaviours.

“In turn, this could lead to increased exposure among older adults in whom efficacy is likely to be lower, or among other higher-risk groups who might have lower vaccine acceptance and uptake.”

Researchers across the world are working hard to develop coronavirus vaccines and treatments.