Results from two early-phase Russian non-randomised vaccine trials (Sputnik V) released on Friday show that two formulations of a two-part vaccine have a good safety profile with no serious adverse events over 42 days, and induce antibody responses within 21 days.

The two-part 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, medical journal The Lancet reported.

SARS-CoV-2 causes Covid-19.

These types of recombinant adenovirus vectors have been used for a long time, with safety confirmed in many clinical studies. Currently, several candidate Covid-19 vaccines using these vectors and targeting the SARS-CoV-2 spike protein have been tested in clinical trials.

These include an adenovirus type 5 vector-based vaccine (CanSino Biological/Beijing Institute of Biotechnology, China), an adenovirus type 26 vector-based vaccine (Johnson & Johnson, USA), and a vaccine containing a simian adenoviral vector (AstraZeneca/University of Oxford, UK).

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In July, results from the Oxford vaccine trial showed no early safety concerns, inducing strong immune responses in both parts of the immune system.

The journal said secondary outcomes (planned outcome measures that are not as important as the primary outcome measure, but are still of interest in evaluating the effect of an intervention) from the trial suggest the vaccines also produce a T cell (antibody) response within 28 days.

The paper reported the findings from two small phase 1/2 trials lasting 42 days – one studying a frozen formulation of the vaccine, and another involving a lyophilised (freeze-dried) formulation of the vaccine.

The frozen formulation is envisaged for large-scale use using 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 centigrade.

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Explaining why they used two adenovirus vectors, lead author Denis Logunov of the N F Gamaleya National Research Centre for Epidemiology and Microbiology, Russia, says: “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”.

“However, booster vaccinations that use the same adenovirus vector might not produce an effective response, because the immune system may recognise and attack the vector. 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.”

The trials took place in two hospitals in Russia, involving healthy adults aged 18-60 years, who self-isolated as soon as they were registered for the trial and remained in hospital for the first 28 days of the trial (from when they were first vaccinated).

The authors noted that more research is needed to evaluate the vaccine in different populations, including older age groups, individuals with underlying medical conditions, and people in at-risk groups.