The COVID-19 vaccine, Sputnik V, approved by Russia last month, has been shown to elicit antibody response with no serious adverse events in small human trials, according to preliminary results published in The Lancet journal on Friday.

Results from early-phase non-randomised vaccine trials in a total of 76 people show that two formulations of the vaccine have a good safety profile detected over 42 days, and induce antibody responses in all participants within 21 days.

Secondary outcomes from the trial suggest the vaccines also produce a T cell response within 28 days, the researchers said.

The findings are based on two small phase trials lasting 42 days -- one studying a frozen formulation of the vaccine, and another involving a lyophilised (freeze-dried) formulation of the vaccine, they said.

The frozen formulation is envisaged for large-scale use in existing global supply chains for vaccines, while the freeze-dried formulation was developed for hard-to-reach regions as it is more stable and can be stored at 2-8 degrees Celsius, the researchers said.

The two-part vaccine includes 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, which usually cause the common cold, are also weakened so that they cannot replicate in human cells and cannot cause disease, according to the researchers.

These vaccines aim to stimulate both arms of the immune system -- antibody and T cell responses -- so they attack the virus when it is circulating in the body, and also attack cells infected by SARS-CoV-2.

"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," said study lead author Denis Logunov, from Gamaleya National Research Centre for Epidemiology and Microbiology, Russia.

"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," Logunov said.

However, Logunov said, booster vaccinations that use the same adenovirus vector might not produce an effective response.

He said this is because the immune system may recognise and attack the vector, adding this would block the vaccine from entering people's cells and teaching the body to recognise and attack SARS-CoV-2.

"For our vaccine, we use two different adenovirus vectors in a bid to avoid the immune system becoming immune to the vector," Logunov added.

The trials took place in two hospitals in Russia, and were open-label and non-randomised, meaning that participants knew that they were receiving the vaccine and were not assigned by chance to different treatment groups.

The trials involved healthy adults aged 18-60 years, who self-isolated as soon as they were registered, and remained in hospital for the first 28 days of the trial from when they were first vaccinated.

In the phase 1 of each trial, participants received one component of the two-part vaccine -- four groups of nine participants were given the frozen or freeze-dried rAd26-S or rAd5-S component.

In the phase 2, which began no earlier than five days after the phase 1 trial began, participants received the full two-part vaccine -- they received a prime vaccination with the rAd26-S component on day 0, followed by a booster vaccination with rAd5-S component on day 21.

There were 20 participants each in the frozen and freeze-dried vaccine groups, the Lancet study noted.

To compare post-vaccination immunity with natural immunity formed by infection with SARS-CoV-2, the authors obtained convalescent plasma from 4,817 people who had recovered from mild or moderate COVID-19, they said.

Both vaccine formulations were safe over the 42-day study period and well tolerated, according to the study.

The most common adverse events were pain at the injection site, hyperthermia, headache, asthenia (weakness or lack of energy), and muscle and joint pain.

The authors note that these adverse effects are also seen with other vaccines, particularly those based on recombinant viral vectors.