To date, the Covid vaccination programme in Britain has involved two doses of one of three vaccines – AstraZeneca, Pfizer or Moderna. But it has stuck rigidly to giving people two doses of the same vaccine. The NHS has not allowed patients to mix vaccines except in a few strict scenarios, such as allowing a second dose of Pfizer when someone developed a blood clot from a first dose of AstraZeneca.

But could we actually improve vaccine efficacy by mixing doses? An Oxford study suggests that we could. The study recruited 830 volunteers who were given one vaccine shot. Some – on a blind, randomised basis – were, four weeks later, given a second shot of the same vaccine and others were given a different vaccine for their second shot.

The volunteers’ blood was then analysed for antibodies and T-cell response. The researchers found that the regime which produced the highest level of antibodies was two shots of Pfizer. However, the highest level of T-cell response was found in those given one shot of AstraZeneca followed by one shot of Pfizer. That order was better than a shot of Pfizer followed by a shot of AstraZeneca. The lowest antibody response was found in those given two shots of AstraZeneca. However, this needs to be put into context, in that the AstraZeneca vaccine is slower to build an immunity response. The Oxford group will shortly publish further results from a group of volunteers given their doses 12 weeks apart.

The findings build on a German study which I wrote about here earlier in the month – which studied immune response in two groups of volunteers: one given two shots of AstraZeneca and one given a shot of AstraZeneca followed by a shot of Pfizer. That study found similar levels of antibodies among both groups, but higher levels of T-cell response among the group given the mixed doses.

The Pfizer and AstraZeneca vaccines work on the following principles: AstraZeneca is a ‘viral vector’ vaccine which uses a harmless virus to deliver DNA coding for the spike protein of Sars-CoV-2 into the body. Pfizer – like the Moderna vaccine – is a messenger RNA vaccine which effectively programmes human cells to produce the spike protein. All three vaccines cause the immune system to target the same thing – the spike protein from the Covid virus – and so should be compatible.

The Oxford study on mixing vaccines has not yet been peer-reviewed and involves a relatively small number of people, but it could be used to introduce more flexibility into the vaccination programme as well as to increase overall immunity. It could also be used to offer an alternative to people with justified fears about receiving a second shot of the AstraZeneca vaccine. Although the potentially fatal side effects of blood clots have been routinely dismissed by the government, NHS and scientific advisers as being very small, the AstraZeneca vaccine has been linked by the Medicines and Healthcare Regulatory Agency (MHRA) to 68 deaths – 47 of them among people aged under 60. The government has already decided that the AstraZeneca vaccine should not be given to the under-40s, but the latest study will fuel demands that people in older age groups at least be given a choice of vaccine.