Can you mix Pfizer and AstraZeneca doses?

Let’s hypothetically say you wanted to get one dose of the Pfizer COVID-19 vaccine, and the second dose as AstraZeneca, would you be able to do that in Australia? The answer is no, we’ll explain why below. 

Current advice and justification 

“You need to match, don't mix", Australian Chief Medical Officer, Paul Kelly, said to Australians in July this year on mixing COVID-19 vaccine brands. Australian health authorities are showing no signs of changing that advice. It’s also important to clarify, when referring to ‘mixing’ the vaccine, we don’t mean stirring two brands together in one shot, but rather the first dose being one brand, and the second being the other. 

The justification provided in June this year by Australian Medical Association (AMA) Vice President, Dr Chris Moy, said, “this idea about mixed schedules, having a first AstraZeneca and having a second Pfizer — the studies on that are minimal. I know some countries have switched to that but on minimal scientific advice". 

The evidence from the studies on mixing the vaccine was not enough for the national decision to change, with Dr Moy saying, "I've seen these studies, [based on] 600 people and another based on 25 people. They showed good response, but we don't make scientific decisions on 600 people". 

However, the Guardian reported, some people who have already mixed vaccine brands (for example, those who switched brands for health reasons on the advice of their doctors, or received left over doses of Pfizer) in Australia are finding COVID-19 vaccine certificate system is not recognising mixed doses as fully vaccinated. The health department is working to fix the issue, however, until then, those individuals cannot obtain proof of vaccination. 

What are other countries doing? 

Multiple countries, including Canada, Spain, and South Korea, allow mixing vaccine brands. 

Where did the mixing vaccine discussion come from?  

Some studies (including the one Dr Moy mentioned above) came out earlier this year to suggest that mixing vaccine brands. The Com-COV trial from Oxford University found combining Pfizer and AstraZeneca jabs provided a "robust" immune response, as reported by the ABC. Smaller studies conducted in Spain (600 participants), and a smaller study in Germany, showed effective results when mixing the vaccine. However, at the time of reporting, both those studies are yet to be peer-reviewed. 

Whether Australia will permit using different brands for ‘booster’ shots only, later down the track remains unclear.