The evidence is strongest for receiving two doses of the same Covid-19 vaccine, rather than mixing different types, medical authorities have said amid concerns that some people may be cancelling their second AstraZeneca dose in the hope of securing a shot of Pfizer.
As outbreaks of the Delta variant affect Nieu-Suid-Wallis and Victoria, people are being urged by state premiers and health authorities to get vaccinated as soon as possible with whatever vaccine is available, and to show up to their existing first and second-dose appointments.
“The best vaccine … is the one you can get today,” the Victorian premier, Daniel Andrews, said on Tuesday. “Go online right now, there are literally thousands of AstraZeneca appointments available for you.”
Intussen, the NSW health minister, Brad Hazzard, said a lot more Pfizer doses are needed to meet demand. “The mixed messaging from the last six or eight months on the vaccines, particularly AstraZeneca, has not been helpful to the broader community’s understanding of how important it is to get vaccinated from any vaccine you’re eligible to get," hy het gesê.
In non-outbreak settings, the Australian Technical Advisory Group on Immunisation (Atagi) advice remains that the Pfizer vaccine is the preferred Covid-19 vaccine for people aged under 60, due to the risk of a rare clotting disorder known as TTS associated with the AstraZeneca vaccine in this age group.
But people under 60 have been signing up for the AstraZeneca vaccine in accordance with the Atagi advice that in outbreak settings where the supply of Pfizer is constrained, adults younger than 60 should re-assess the benefits from being vaccinated with AstraZeneca.
The prime minister, Scott Morrison, announced on Tuesday that half a million Pfizer doses would arrive this week from Singapore, which already has 80% Covid-19 vaccination rates. Earlier in August, vaccine rollout coordinator Lt Gen John Frewen said Australia was working to bring forward 32m Pfizer doses so that 1m would arrive every week.
The announcements have led to hope from some of those in Victoria and New South Wales awaiting second-dose AstraZeneca appointments that they might be able to receive Pfizer instead. Egter, vaccination hubs and GPs in Australia do not yet allow this, aside from in exceptional circumstances.
The head of the Australian Medical Association (AMA) Dr Omar Khorshid said “there’s definitely people vaccinated with AstraZeneca interested in getting Pfizer as a second dose, but that really doesn’t make any sense”.
“The majority of the small risk of TTS is associated with the first dose, and in addition vaccination with AstraZeneca may also provide better longer term protection than Pfizer," hy het gesê.
“For those who have had their first dose of AstraZeneca it’s a no-brainer to have the second. It’s a fantastic vaccine with excellent efficacy and safety. The studies on mixing vaccines are much smaller.”
The head of the Royal Australasian College of General Practitioners (RACGP) Dr Karen Price said GPs were having a lot of discussions with patients about mixed-dosing.
“Our advice is mixed dosing is not approved in Australia and we are proceeding as per Atagi guidelines," sy het gese. “I am not quite sure what people are concerned about given recent studies on AstraZeneca showing good results, and the second dose is linked with hardly any TTS.”
Some people also prefer Pfizer because they can be vaccinated sooner. A gap of at least 21 days between doses is recommended. This is compared to a gap of between four and 12 weeks between AstraZeneca doses, met 12 weeks recommended for maximum efficacy.
“If people are really worried about the gap they can talk with their doctor about bringing their second AstraZeneca dose forward to earlier than 12 weke, particularly in areas where there is an outbreak,” Price said.
Canada was among the first countries to allow mixed vaccine dosing, with the National Advisory Committee on Immunization in June issuing guidance permitting the AstraZeneca, Pfizer and Moderna vaccines to be used interchangeably.
Studies on administering a first dose of AstraZeneca followed by Pfizer are also promising, with a preliminary, pre-print study from the University of Oxford involving 830 participants finding the combination provoked a powerful immune response against Covid, stronger than other vaccine combinations.
But Atagi co-chair Prof Allen Cheng, who is also an epidemiologist and infectious diseases physician in Melbourne, said by comparison, “the clinical trials for two doses of the same vaccine have tens of thousands of people, and many millions of people have received two doses of the same vaccine, whereas the experience with mixed schedules is much more limited”.
“We have provided some early advice on the use of different vaccines in clinical situations where this is necessary," hy het gesê. “For example, people who have had a serious reaction to the first dose of one or the other vaccine, or people who have received a vaccine overseas that is not available in Australia. In terms of making mixed schedules available routinely, we note that while supplies of one vaccine are still constrained it probably isn’t possible to offer a choice to consumers.”
He said the evidence available so far did suggest receiving different vaccines generates a good immune response, is effective, and seems to be safe. But much more work was needed to understand the best timeframe between dosing when mixing vaccines, and which vaccines mixes would be considered valid in other countries. Some countries are also yet to recognise vaccination passports from those who have received a mixed-dose.
“There is ongoing work in defining what is a ‘valid’ vaccine – it’s a complex area involving Atagi, the TGA [Therapeutic Goods Administration] and government, noting proving vaccination status is only currently required for a few workforces and we don’t yet need vaccine passports,” Cheng said.