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June 28, 2021

An Oxford University-led study published Monday found mixing and matching doses of two different Covid-19 vaccines still generates good protection against the virus, marking the latest evidence in support of a more flexible vaccine rollout that some countries have already started to adopt.


The Com-Cov vaccine trial looked at how immune responses differed in the 850 volunteers aged 50 and older when they were given either two doses of the Pfizer-BioNTech vaccine (an mRNA vaccine), two doses of AstraZeneca (a vector vaccine), or one dose of each.

All the different combinations generated strong immunity, the researchers concluded, as both “mixed” schedules induced “high concentrations of antibodies” when administered four weeks apart.

“This means that all possible vaccination schedules involving the Oxford-AstraZeneca and Pfizer-BioNTech vaccines could potentially be used against Covid-19,” the study said.


But while all combinations offered strong protection against the virus, the researchers noted different combinations of the two types of vaccines induced differing levels of immunity. 

Two doses of the AstraZeneca vaccine (which has tested less effective than other vaccines and is recommended to be given eight to 12 weeks apart) proved the least effective combination, while two doses of Pfizer spurred the highest antibody response.

However, a shot of AstraZeneca followed by Pfizer generated the highest T cell response (immune cells that can target and destroy virus-infected cells) and induced more antibodies than getting the jabs the other way around. 

While the study doesn’t undermine the U.K.’s current policy of giving people the same vaccine brand for both shots, the researchers said it suggests countries could be more flexible if needed. “These results are an invaluable guide to the use of mixed dose schedules,” said the trial’s chief investigator Matthew Snape, an associate professor in pediatrics and vaccinology at the University of Oxford. Snape also said the team is looking into how the results vary when volunteers wait the recommended 12 weeks between AstraZeneca jabs, explaining: “This longer interval is known to result in a better immune response.” 

Some countries have already begun allowing—and even recommending—vaccine mixing in their rollouts. Spain and Germany both offer Pfizer and Moderna shots to young people who had the AstraZeneca vaccine as their first dose. Meanwhile, Canada changed its guidelines on vaccine mixing to recommend following AstraZeneca with one of the two mRNA vaccines (Pfizer or Moderna). Previous clinical trials found the efficacy of AstraZeneca against symptomatic Covid-19 infection could reach 82.4% with a longer gap. Pfizer and Moderna have both tested at roughly 95% effective.  

Jonathan Van-Tam, the U.K.’s deputy chief medical officer, emphasized the potential impact of the study’s findings on how governments use booster shots. “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,” he said. 

Source: Forbes
Image Source: Getty Images