The impact of different Covid-19 vaccine dosing regimens on patients’ immune responses and whether mixed regimens have higher or lower immune responses will the focus of a new trial.

Eight National Institute for Health Research (NIHR) supported sites will gather immunological evidence on different intervals between the first and second dose for a mixed-vaccine regimen against control groups when the same vaccine is used for both doses. 

The news comes after authors of the Sputnik V Covid-19 vaccine trial published in The Lancet this week, said that using a different adenovirus vector for the booster vaccination may help create a more powerful immune response (compared with using the same vector twice), as it minimises the risk of the immune system developing resistance to the initial vector.

Initial findings are expected to be released in the summer. The study run by the National Immunisation Schedule Evaluation Consortium (NISEC) has received ethics approval from the Research Ethics Committee, as well as approval from the Medicines and Healthcare products Regulatory Agency (MHRA).

Should the study show promising results, then the government may consider reviewing the vaccine regimen approach if needed, but only if proven to be safe and recommended by the Joint Committee on Vaccination and Immunisation (JCVI). 

Advantages to more flexible immunisation programme

Deputy Chief Medical Officer and Senior Responsible Officer for the study, Professor Jonathan Van-Tam, said: "Given the inevitable challenges of immunising large numbers of the population against Covid-19 and potential global supply constraints, there are definite advantages to having data that could support a more flexible immunisation programme, if needed.

"It is also even possible that by combining vaccines, the immune response could be enhanced giving even higher antibody levels that last longer; unless this is evaluated in a clinical trial we just won’t know. This study will give us greater insight into how we can use vaccines to stay on top of this nasty disease."

The study will initially have eight different arms testing eight different combinations, but more products may be added. The eight arms include:

  • 2 doses of the Oxford/AstraZeneca vaccine at 28 days apart
  • 2 doses of the Oxford/AstraZeneca vaccine at 12 weeks apart – as a control group
  • 2 doses of the Pfizer/BioNTech vaccine at 28 days apart
  • 2 doses of the Pfizer/BioNTech vaccine at 12 weeks apart – as a control group
  • the Oxford/AstraZeneca vaccine for the first dose, followed by the Pfizer/BioNTech vaccine for the second, at 28 days apart
  • the Oxford/AstraZeneca vaccine for the first dose, followed by the Pfizer/BioNTech vaccine for the second, at 12 weeks apart
  • the Pfizer/BioNTech vaccine for the first dose, followed by the Oxford/AstraZeneca vaccine for the second, at 28 days apart
  • the Pfizer/BioNTech vaccine for the first dose, followed by the Oxford/AstraZeneca vaccine for the second, at 12 weeks apart

Over 800 patients are expected to take part in the study, referred to as the COVID-19 Heterologous Prime Boost study or ‘Com-Cov’, across 8 different sites across England – including in London, Birmingham and Liverpool.

Patients will be recruited over the course of February via the NHS Covid-19 Vaccine Research Registry, with vaccinations expected to start towards the middle of the month and initial results to be made available over the summer period.

The study has been classified as an Urgent Public Health study by the NIHR and is being undertaken by NISEC and the Oxford Vaccine Group, with funding of £7 million from the government through the Vaccines Taskforce.

Chief Investigator Matthew Snape, Associate Professor in Paediatrics and Vaccinology at the University of Oxford, said: "This is a tremendously exciting study that will provide information vital to the roll out of vaccines in the UK and globally. We call on those aged 50 years and above who have not yet received a Covid-19 vaccine to visit our website to find out more about the study and see if there is a study site near them.

"If we do show that these vaccines can be used interchangeably in the same schedule this will greatly increase the flexibility of vaccine delivery, and could provide clues as to how to increase the breadth of protection against new virus strains."