Study shows mixing AstraZeneca and Pfizer vaccines generates strong immune response against COVID-19

Mixing doses of the Pfizer and AstraZeneca vaccines can generate a strong immune response against COVID-19, an Oxford University study has found. The study, conducted by Oxford Vaccine Group's Com-Cov vaccine trial, found that those who are given a dose of AstraZeneca followed by a dose of Pfizer four weeks later generated a better immune response.

Oxford developed the AstraZeneca vaccine, which is used in the U.K. In the U.S., three other COVID-19 vaccines are available: Moderna, Johnson & Johnson and Pfizer-BioNTech.

The Oxford study recruited 830 volunteers, 50 years or older, in the U.K. to test four different vaccine combinations.

While taking AstraZeneca followed by Pfizer yielded higher antibodies and T-cell responses, the study found that mixing Pfizer and AstraZeneca – regardless of the order – resulted in higher antibodies than the "standard" two-dose AstraZeneca shot. 

The study also tested the standard vaccine doses and found that the highest antibody response was seen after two doses of Pfizer. 

Professor Matthew Snape, chief investigator on the trial, said the results point to "potentially allowing flexibility in the U.K. and global vaccine roll-out."

"These results are an invaluable guide to the use of mixed dose schedules, however the interval of four weeks studied here is shorter than the eight to 12-week schedule most commonly used for the Oxford-AstraZeneca vaccine," Snape, an associate professor in pediatrics and vaccinology at the University of Oxford, said.

"This longer interval is known to result in a better immune response, and the results for a 12-week interval will be available shortly," he said. 

Deputy Chief Medical Officer Professor Jonathan Van-Tam said the 12-week results will play an instrumental role "in decisions on the future of the U.K.'s vaccination program."

"Our non-mixed (homologous) vaccination program has already saved tens of thousands of lives across the U.K. but we now know mixing doses could provide us with even greater flexibility for a booster program, while also supporting countries who have further to go with their vaccine rollouts and who may be experiencing supply difficulties," Van-Tam said.

In May, the Com-Cov study revealed that mixing vaccines resulted in more frequent mild to moderate reactions, but these reactions were short-lived, according to the press release. 

Researchers are also testing Moderna and Novavax vaccines in a new study. Six new "arms" of the trial recruited approximately 175 candidates each, adding a further 1,070 people to the program. 

The intent of the study is to "demonstrate that mixing is not substantially worse than not mixing – and will compare the immune system responses to the gold-standard responses reported in previous clinical trials of each vaccine," according to the press release.