Should the NHS be mixing vaccines for better effect, or at least offering people who have had one dose of the AstraZeneca vaccine the choice of having the Pfizer vaccine for their second shot?

The question arises because that is exactly the regime which has been followed in Germany since the risk of clotting from the AstraZeneca vaccine became evident. When the German authorities made the decision to administer the Pfizer vaccine in place of a second shot of AstraZeneca there was not much evidence as to whether this would be an effective strategy.

But a study from a group of hospitals in Berlin suggests that a shot of AstraZeneca followed by Pfizer is at least as good as two shots of Pfizer and may even be slightly better at producing immunity.

The study involved 340 healthcare workers at the Charité hospital in Berlin, around half of whom had received AstraZeneca as their first shot of vaccine, and half of whom had received Pfizer.

All were given a second shot of Pfizer. Those who had had Pfizer first time around were given their second shot after three weeks, in accordance with the manufacturer’s recommendation. Those who were given AstraZeneca as their first shot were given their second shot 10 – 12 weeks later – again, as is recommended practice with the AstraZeneca vaccine. The team, led by the Charité hospital, compared the level of antibodies in both groups in the days after the second dose and found them to be comparable in both groups.

Both groups were also measured for T cell responses three weeks after their second dose – and in this case the group which had been given the mixed doses had a slightly better reaction.

There are the usual caveats to be added: this is a preprint of a paper which has not yet been peer-reviewed. It did not involve anyone who was given two AstraZeneca shots, and so we cannot compare the mixed dosage strategy with that regime. It involved a relatively small number of people and did not measure the actual risk of contracting Covid infection – only the levels of antibodies and T-cell responses.

However, the study begs two questions: should we think about mixing vaccines in Britain? At the very least: why aren’t NHS patients who have had one dose of AstraZeneca being offered the choice of having Pfizer for their booster vaccination?

The risk of suffering a blood clot after the AstraZeneca vaccine might be statistically low; nevertheless, according to MHRA figures 61 people have so far died. Worse, many of those who have died have been relatively young – there have been seven deaths among 18 to 29 year olds and nine deaths among 30 to 39 year olds.

The government has already made the decision not to give any further doses of the AstraZeneca vaccine to people under the age of 40, but has declined any choice to older people.