Is mixing Covid vaccines safe? New study shows it could be key to fighting Delta variant

Vaccine: Expert discusses getting second jab before 12 weeks

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The UK has some of the highest numbers of coronavirus vaccines administered in the world, amid the relentless work of the NHS in getting jabs in arms. Currently, more than 76 million doses have been administered, with nearly 50 percent of the British population fully vaccinated – having received two doses – against coronavirus.

Is mixing vaccines safe?

A new study has found that mixing two different brands of the vaccine is more than just safe – it’s actually even more effective.

The Com-Cov trial looked at the efficacy of either two doses of Pfizer, two of AstraZeneca, or one of them followed by the other.

All combinations worked well, priming the immune system to effectively fight the virus.

Two doses are important to give the fullest protection and teach the body to make antibodies and T cells to block and kill Covid.

The Com-Cov study, which looked at giving the doses four weeks apart in 850 volunteers aged 50 and above, found:

  • AZ followed by Pfizer induced higher antibodies and T cell responses than Pfizer followed by AZ
  • Both of these mixes induced higher antibodies than two doses of AZ
  • The highest antibody response was seen after two doses of Pfizer, and the highest T cell response from AZ followed by Pfizer

This knowledge could offer flexibility for the vaccine rollout and solve the issue of the AstraZeneca jab being potentially less effective against the Delta variant.

The UK’s deputy chief medical officer, Prof Jonathan Van-Tam, said there was no reason to change the current successful same dose vaccine schedules in the UK, however, given vaccines were in good supply and saving lives.

But he says it might be something to look at in the future.

He said: “Mixing doses could provide us with even greater flexibility for a booster programme, while also supporting countries who have further to go with their vaccine rollouts, and who may be experiencing supply difficulties.”

Some countries are already using mixed doses – Spain and Germany are offering the Pfizer or Moderna mRNA vaccines as a second dose to younger people who have already received a first dose of the AstraZeneca vaccine.

However, this decision came after concerns about rare but serious blood clots, rather than about efficacy.

Prof Matthew Snape, from the University of Oxford, a lead investigator in the Com-Cov Study, said the findings should not undermine the UK’s current policy.

He said: “We already know that both standard schedules are very effective against severe disease and hospitalisations, including against the Delta variant when given at eight to 12 weeks apart.”

He said the new results showed mixed dose schedules were also effective, even though the interval of four weeks studied was shorter than the eight to 12-week schedule most commonly used in the UK.

He said: “This longer interval is known to result in a better immune response.”

Mixed dose trial results for a 12-week jab interval will be available in the UK next month.

However, there have been reports that mixing vaccines does produce more short-term side effects such as chills, headaches and muscle pain.

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