Six thousand British volunteers are to be injected with an experimental Covid vaccine modelled on an Ebola jab. It is the third Covid vaccine to enter large-scale clinical trials in the UK. Pursuing multiple candidates is essential to guarantee UK supply and ensure that most effective vaccine is identified, researchers stressed. Already undergoing large clinical trials in British patients are the Oxford Covid vaccine, and one being developed by the US biotech company Novavax. A phase 3 trial of the latest vaccine, developed by global pharmaceutical company Janssen (a subsidiary of Johnson & Johnson), begins on Monday. Phase 3 trials are designed to test the safety and effectiveness of drugs or vaccines in thousands of people. It will initially involve 6,000 volunteers from 17 sites across the UK, including Belfast, Bristol, Cardiff, Dundee, Leicester, London, Manchester, Sheffield and Southampton. A further 24,000 volunteers are to be recruited from other countries. Like the Oxford vaccine, the Janssen candidate uses a weakened common cold virus called an adenovirus to deliver instructions for making the coronavirus’s surface spike protein to our cells. Cells infected begin making the viral protein and expressing it on their surface, triggering an immune response. This includes the development of “memory” immune cells, which should protect against future coronavirus infections. However, whereas the Oxford vaccine is based on an adenovirus from chimpanzees, Janssen’s vaccine uses a human adenovirus, which has been modified so it can no longer multiply or cause disease. The same virus forms the backbone of a recently approved Ebola vaccine, deployed during the 2019 outbreak in the Democratic Republic of the Congo. Janssen’s vaccine has already undergone smaller phase 1 and 2 trials, and interim analysis of a single-dose study suggests that it induces a robust immune response and is generally well-tolerated. Assuming it proves safe and effective in larger trials, 30m doses could be made available to the UK by mid-2021, the business secretary, Alok Sharma, said. Experts stressed the need for large-scale trials of multiple vaccine candidates, despite last week’s announcement of promising interim results from a trial of the Pfizer/BioNTech vaccine. “The recent news is enormously exciting for the whole world, but we must not take our focus off continuing the important research to work out which vaccines work best for different people to provide long-lasting, effective protection against Covid-19,” said Kate Bingham, chair of the UK vaccine taskforce. “We also can’t be certain that vaccine supply will be efficient and effective and secure from any one manufacturer, wherever it’s being made in the world,” said Saul Faust, a professor of paediatric immunology and infectious diseases at the University of Southampton, and principal investigator of the Janssen trial in the UK. So far, about 25,000 Britons have participated in Covid vaccine trials, and more than 310,000 have indicated their willingness to participate by signing up to the NHS vaccine research registry. Bingham said there was a particular need for volunteers from those who are at greatest risk of coronavirus, including frontline health and social care workers, and people from black, Asian and minority ethnic backgrounds. Faust stressed that participating in a vaccine trial would not exclude anyone from routine immunisation against Covid-19 later on. People who suspect they might have had Covid-19, but were never tested, are also eligible to participate in the Janssen trial.