RESPONSES TO A MIXED APPROACH
Studies are being undertaken in six overseas countries into immune responses to Covid-19 when AstraZeneca and Pfizer vaccines are given as first and second shots, writes Jane Hansen
COMBINING different types of Covid-19 vaccines may offer a better immune response and is currently being trialled in six countries around the world. Spain was the first country to study those who had one AstraZeneca shot, followed up with the Pfizer vaccine eight to 12 weeks after. It found the approach produced a higher antibody response. Subsequent German studies reported that the mix and match regimen elicited an immune response as good as or better than two shots of Pfizer. The United Kingdom Com-COV study has just released its findings into the combination regime. Mixed schedules of Pfizer, followed by AstraZeneca and AstraZeneca, followed by Pfizer both induced high doses of antibodies when given four weeks apart.
“Evidence from the German nonrandomised studies suggest that the immune response is the same or better than having the same type of vaccine for both first and second doses,” Professor Terry Nolan, head of the Vaccine and Immunisation Research Group at the Doherty Institute, said.
While the World Health Organisation is cautious on the idea of mixing and matching, our experts assess the possibilities:
WHICH COMBINATION IS BEST?
Which vaccine works best before or after was also analysed in the ComCOV study.
“The UK Government-funded study using a proper randomised controlled clinical trial confirmed the German results but shows a very strong immune response if the AstraZeneca vaccine is given first then followed by the Pfizer vaccine four weeks later, and not nearly as strong if given in the reverse order, that is Pfizer first, followed by AstraZeneca,” Prof Nolan said.
Side effects of the mixed regime were slightly higher but of the moderate kind.
“They showed a moderate increase in rates of minor events like injection site soreness and feeling off colour and very transient tiredness with a mixed schedule.
A further stage of the UK study is looking not
just at Pfizer and AstraZeneca vaccines, but also at Moderna and Novavax Covid vaccines, both of which are being purchased by the Australian Government,” Prof Nolan said.
“Theoretically, we could expect both improved protection, longer lasting immunity, and possibly better protection against variant strains because the immune responses were so strong. “But while very likely, this is not certain, and the new data following use of mixed schedules in the population will answer only some of these questions (e.g. actual protection against Delta and other variant strains, and any possible longer lasting immunity will take time to assess).”
Professor Matthew Snape, Asso
ciate Professor in Paediatrics and Vaccinology at the University of Oxford, and Chief Investigator on the UK Com-COV study, said mix and match allowed for flexibility in vaccine rollouts.
“The results show that when given at a fourweek interval both mixed schedules induce an immune response that is above the threshold set by the standard schedule of the AstraZeneca vaccine,”
Deputy Chief Health Officer
Prof
Professor Allen Cheng.
Snape said in The Lancet medical journal.
The results for a 12-week interval between different shots are yet to be released.
CAN I GET THE COMBINATION IN AUSTRALIA?
Professor of infectious diseases epidemiology at Monash University and Chair of the Advisory Committee for Vaccines Allen Cheng said the combination approach was not routinely offered in Australia.
“There are several studies being done into getting different types of vaccines, but at the moment these are not routinely recommended.
The main exception is if someone has a serious complication after the first dose (e.g. anaphylaxis, or severe allergy), then they can’t get the same vaccine again.
The other exception is if someone has received a vaccine overseas that yet isn’t available in Australia (e.g. Moderna or Sinovac). But these situations are both uncommon, and you’ll need to discuss this with a specialist vaccine provider,” Prof Cheng said.
“The other concern that people have is about the clotting complication (TTS) and whether they should
have Pfizer instead of AstraZeneca as their second dose.
“This isn’t currently recommended, as the risk of clotting after the second dose is much lower than after the first dose (in the UK, there have been 36 cases reported after second doses, with an estimated risk of less than two cases per million second doses).”
IS IT SAFE?
The current studies into combination regimes have also been much smaller than original safety studies.
“It should be noted that the clinical trials that used two doses of the same vaccine (Pfizer, AstraZeneca or others) have generally tested their vaccines on tens of thousands of participants; these studies looking at mixed schedules have only been done on a few hundred participants.
Thus, we don’t yet know if mixed schedules are more effective in preventing infection, or if there are any rare but serious side effects associated with mixed schedules,” Prof Cheng said. For this reason the World Health Organisation has issued a caution on the practice.