Is It Safe to Mix COVID-19 Vaccines?

A first-of-its-kind study is launching in the United Kingdom to determine the effects of using a different COVID-19 vaccine for the first and second dose. For example, patients could receive the AstraZeneca vaccine for the first dose, followed by the Pfizer vaccine for the second.

Dr. Patrick Hwu, President and CEO

Moffitt Cancer Center President and CEO Dr. Patrick Hwu says there is no scientific reason not to mix and match vaccines. “Although this hasn’t been tested formally, there are a number of animal models demonstrating that utilizing different kinds of vaccines to boost the effects of a vaccine can work,” he said. “Immunologically, it makes sense to try this.”

The 13-month study will gather immunological evidence on different intervals between the first and second dose for mixed-vaccine regimens compared to using the same vaccine for both doses.

While the Pfizer vaccine uses messenger RNA, the AstraZeneca vaccine uses a weakened version of the common cold virus (adenovirus) that is engineered to attack COVID-19 if it later infects the body. The AstraZeneca vaccine has emergency use approval in the UK.

“The major difference is that the vaccines based on viral vectors may eventually induce neutralizing antibodies when given repeatedly, which may inactivate the vaccines,” said Hwu. “So, if multiple boosting is planned, this should be done with the mRNA-based vaccines in my opinion.”

The study will test eight vaccine combinations:

• Two doses of the Oxford/AstraZeneca vaccine at 28 days apart
• Two doses of the Oxford/AstraZeneca vaccine at 12 weeks apart — as a control group
• Two doses of the Pfizer/BioNTech vaccine at 28 days apart
• Two doses of the Pfizer/BioNTech vaccine at 12 weeks apart — as a control group;
• The Oxford/AstraZeneca vaccine for the first dose, followed by the Pfizer/BioNTech vaccine for the second, at 28 days apart
• The Oxford/AstraZeneca vaccine for the first dose, followed by the Pfizer/BioNTech vaccine for the second, at 12 weeks apart
• The Pfizer/BioNTech vaccine for the first dose, followed by the Oxford/AstraZeneca vaccine for the second, at 28 days apart
• The Pfizer/BioNTech vaccine for the first dose, followed by the Oxford/AstraZeneca vaccine for the second, at 12 weeks apart

Even though the trial is ongoing, the same-dose regimen will continue to be implemented in the UK vaccination program.

More than 800 patients are expected to take part in the study, and results should be released over the summer.