National | Novel Coronavirus / COVID-19

Can different Covid-19 vaccines be mixed?

As supply bottlenecks lead to Covid-19 vaccine shortages around the world, several countries like Canada, Spain, Australia and Germany are allowing mix-and-match vaccinations from different manufacturers.

The two main types of vaccines that have been developed to protect people from the virus are the adenovirus-based vaccines like AstraZeneca and Janssen, and mRNA vaccines like Pfizer and Moderna.

Most of these vaccines require two doses.

New Zealand has used the Pfizer vaccine exclusively in its roll-out until recently but the Government recently announced that AstraZeneca will be available to anyone who prefers it, raising the possibility that some people may end up receiving a dose of each. As of mid-December 2021, about 2600 people had opted to get a dose of AstraZeneca as their first, second or third shot.

One other vaccine, Janssen, has also been approved for use here but has not actually been used yet. A fourth vaccine, Novovax, is still going through the Medsafe approval process.

There are also some circumstances where recent arrivals who were administered a different first dose overseas are receiving Pfizer as their second dose now they are back in New Zealand. As of November, the Covid-19 immunisation register shows approximately 550 people in New Zealand had Pfizer for only their second dose, likely because they received a first dose of a different vaccine overseas.

Research is still being gathered about mixing and matching vaccines but, encouragingly, the evidence so far shows mixing vaccines to complete a two-dose regime is safe and effective.

There is even evidence to suggest it provides a stronger immune response.

Data available from German and Swedish studies showed an 11.5-fold increase in antibodies with an initial AstraZeneca vaccine and a second shot of Pfizer.

Two doses of AstraZeneca showed a 2.9 fold increase.

Another ongoing United Kingdom study showed similar outcomes, which suggests mixing vaccines leads to a strong immune system.

The UK research involved 800 participants who volunteered for the study and investigated the efficacy of either two doses of AstraZeneca and Pfizer, or one of each.

Mixing AstraZeneca and Pfizer showed a boost in T-cells and neutralising antibodies - both very important in fighting viral infections.

It also showed a better response to the Alpha, Beta and Gamma variants of the Covid-19 virus.

Countries like Bahrain, Canada, Italy, South Korea, Thailand and the United Kingdom have begun mixing vaccines.

World leaders like German Chancellor Angela Merkel, 66, received a first dose of AstraZeneca, and a second of the Moderna vaccine.

Italian Prime Minister, Mario Draghi, 73, switched to Pfizer for his second dose after getting a first AstraZeneca jab, and Canadian Prime Minister Justin Trudeau was administered Moderna and AstraZeneca.

Nothing suggests that mixing vaccines can lead to serious side effects and the reaction remains the same as, if not better than, a same-dose regime.

Reporting disclosure statement: This post was reviewed by The Whole Truth: Covid-19 Vaccination expert panel members Dr Dianne Sika-Paotonu (an immunologist and senior lecturer in pathology and molecular medicine at the University of Otago), and Dr Rawiri Jansen (GP and clinical director for a primary healthcare organisation).