Thousands of volunteers will be injected with a third dose of Covid-19 vaccine as part of a trial to investigate which ones could be used as “booster” doses to protect against new variants and prevent hospitals from being overwhelmed by a potential fresh wave of infections this winter. The Cov-Boost trial – the first in the world to provide vital data on the impact of a third dose on patients’ immune responses – will study the effect of giving an additional dose of one of seven different vaccines to 2,886 UK participants, on top of the two doses of the Pfizer/BioNTech or Oxford/AstraZeneca vaccines they have already received. As well as examining the level of protection afforded by a third dose, the trial will also look at any side-effects triggered by these booster jabs. The results will be used by the Joint Committee on Vaccination and Immunisation (JCVI) to decide whether a booster dose is necessary, and which of the vaccines would be best suited for this use. Results are expected by the end of August, with a final decision taken by the JCVI in September. The UK has already ordered a further 60m doses of the Pfizer/BioNTech Covid vaccine to ensure that booster jabs can be given from this autumn, if necessary. These doses will be of the same formulation as those currently offered, rather than a modified version of the vaccine tweaked to address new circulating variants such as B.1.617.2, which was first detected in India. In addition to the Pfizer jab, the Cov-Boost trial will examine the impact of giving the Oxford/AstraZeneca vaccine or five other vaccines as a booster, on top of the two doses of the Pfizer or AstraZeneca vaccines volunteers will already have received. In some cases, people will receive a half dose of vaccine as the booster jab. The other vaccines which will be tested are the Moderna vaccine; the Janssen and Novavax vaccines, which are undergoing assessment by the Medicines and Healthcare products Regulatory Agency (MHRA); and two vaccines which are in phase 3 clinical trials in the UK – the CureVac vaccine, which uses the same mRNA technology as the Pfizer/BioNTech vaccine, and the Valneva vaccine, which uses killed Sars-CoV-2 virus to trigger an immune response. The UK government has already secured access to doses of all of these vaccines. “The hope of the booster is that we raise antibody levels enough to be able to cover existing and variant strains of coronavirus,” said Prof Saul Faust, the director of the NIHR Southampton Clinical Research Facility, and the trial’s chief investigator. “There are two ways of doing this: one is to have high enough antibody levels from the original vaccine strain, and the other is to change the vaccines. This is not a trial about the changed vaccines, which is a study for the future.” Faust and colleagues will measure the level of immune response triggered by a third vaccine dose and how successful this is against new variants including the India variant, those first detected in Kent and South Africa, and any new variants of concern that emerge, as well as against the original Wuhan strain. The team will also investigate any side-effects triggered by a booster dose of these vaccines in both younger and older adults, compared with a placebo vaccine – the meningococcal vaccine, used to protect against meningitis. The health secretary, Matt Hancock, said: “The UK vaccination programme has been a phenomenal national effort, with seven in 10 UK adults now having had their first Covid-19 jab. “We will do everything we can to future-proof this country from pandemics and other threats to our health security, and the data from this world-first clinical trial will help shape the plans for our booster programme later this year. “I urge everyone who has had both doses of a Covid-19 vaccine, and is eligible, to sign up for this study and play a part in protecting the most vulnerable people in this country and around the world for months and years to come.” The researchers are asking adults aged 30 years or older who received their first vaccine dose in December or January to sign up for the study, with volunteers expected to be invited for screening at one of 18 trial sites across the UK from early June.