Australian health officials say there are no plans to approve the mixing and pairing of COVID-19 vaccines here, although the Canadian regulator recently cleared the use of combined doses.

In Canada and several European countries, people who received AstraZeneca for their first dose can now receive the Pfizer or Moderna vaccine for their second dose.

But closer to home, experts predict Australians may not be offered a vaccine combination until the end of the year, if at all.

Burnet Institute epidemiologist Mike Toole said that even if Australia were to approve the mixing and pairing of vaccines, there would not be enough Pfizer to offer it to people for their second dose.

Canada’s move comes after regulators in Australia and abroad concluded that AstraZeneca was the likely cause of a rare side effect called thrombosis with thrombocytopenia syndrome (TTS).

Professor Toole said almost all of the TTS cases reported to date have happened after the first dose of AstraZeneca, which meant Australians could feel safe to receive their second dose.

But what does early science tell us about the safety and effectiveness of mixing vaccines?

And why is Canada in such a different position from Australia?

Trials in Spain, United Kingdom

Spanish researchers recently concluded a study involving 600 people in which participants received either one dose of AstraZeneca or one dose of AstraZeneca followed by a second dose of Pfizer.

“They found a very good antibody response to this [mixed regime] combination, ”said Professor Toole.

“The antibody response was much higher than what you would get with two doses of AstraZeneca.”

In the Spanish study, which has not yet been peer-reviewed, participants in the mix and match cohort reported mild local side effects (such as headaches and chills) comparable to those from receive AstraZeneca.

In the UK, researchers are examining whether the vaccine mix will give longer lasting immunity to SARS-CoV-2, testing different combinations of approved COVID-19 vaccines.

More than 800 people are participating in the research.

The UK study has yet to release efficacy data, although it has published safety data: People in mix and match groups report more vaccine-related side effects (such as fever) than people who have received two doses of the same vaccine.

Why does the vaccine mix work better?

RMIT vaccine expert Kylie Quinn said scientists often experiment with mixing and pairing different vaccines as this could result in a higher level of immunity than using the same one.

AstraZeneca is a viral vector vaccine that uses a chimpanzee adenovirus to deliver genetic material into our cells, eliciting an immune response.

“Think of it as a vehicle and a cargo: so the vehicle is the adenovirus, and the cargo is the instructions on how to peak. [protein] SARS-CoV-2, ”said Dr Quinn.

Dr Quinn said that although the body has developed its strongest immune response to the spike protein, it has also responded to the chimpanzee adenovirus.

She theorized that the mixed diet might work better in this case because the body was not responding to the “vehicle” delivering the “cargo”.

“If you keep the same cargo but find a different way to deliver it, you escape your immune system,” she explained.

Canada vs. Australia

Professor Toole said Canada is in a position to approve vaccine mixing and pairing because it has enough mRNA vaccines to provide people with their second dose.

Canada has agreements for up to 76 million doses of Pfizer and 44 million doses of Moderna.

It also has agreements with AstraZeneca (20 million), Johnson and Johnson, Novavax, Medicago and Sanofi and GlaxoSmithKline.

For comparison, Australia has secured 40 million doses of Pfizer and 25 million doses of Moderna, but the bulk of those will not be available until the end of 2021.

Australia’s biggest vaccine deal is with AstraZeneca (with deals to buy 53.8 million doses).

“We are too dependent on AstraZeneca until the fourth quarter, when we have a lot more Pfizer, Moderna, and maybe Novavax,” Professor Toole said.

Australia has ordered 51 million doses of Novavax, a protein-based vaccine, but it has yet to be approved by regulators.

“The fourth quarter will be an abundance of wealth,” said Professor Toole.

ATAGI recommends a “full course”

A spokeswoman for the Federal Department of Health said the Australian Immunization Technical Advisory Group (ATAGI) will continue to monitor evidence as it arrives.

But for now, their advice has remained unchanged.

“It is important that a person completes the full cycle of a vaccine in order to provide the highest possible level of immunity to COVID-19,” the spokesperson said.

Dr Quinn said it was a sensible move by the regulator, given that science has shown that two doses of the same vaccine (whether two doses of Pfizer or two doses of AstraZeneca) provides a robust protection against COVID-19.

“And what we do know right now is that these vaccines have gone through the gamut of phase 1, phase 2, phase 3 clinical trials, and have worked incredibly well and are very protective.”

Dr Quinn said it was difficult to speculate if, or when, Australians would be offered a mix of vaccines.

“What we are rolling out right now are the first generation vaccines,” she said.

“It’s quite likely that if we need boosters we’ll switch to what they call second-generation vaccines.”

Professor Toole said he could not see Australia offer mixed vaccines until October at the earliest, when the country had more vaccines.

But by then, Professor Toole said, it might be pointless to do so as Australia would have millions of additional doses of mRNA vaccines available.

The two experts agreed that the most important thing for all Australians to do was take the vaccine offered to them and roll up their sleeves as soon as they were eligible.