Mixing doses of different Covid-19 vaccines could help tackle supply shortages, but more trials are needed to find the best combinations and dose intervals, experts say.
The latest study, led by the University of Oxford, suggested that a vaccine cocktail of AstraZeneca’s shot followed by Pfizer’s four weeks later increased side-effects such as fatigue and headaches.
But the study, of 830 people aged over 50, has yet to show how well the combination defends against the coronavirus.
The study did not identify any safety issues and the stronger side-effects vanished after a few days, according to Matthew Snape, an Oxford paediatrics and vaccinology professor who is leading the trial.
“Whether or not this will relate to an improved immune response, we don’t know yet; we’ll be finding out those results in a few weeks’ time,” Bloomberg quoted Snape as saying.
The researchers are exploring a wider interval of 12 weeks between jabs and further studies with vaccines by Moderna and Novavax are continuing, they said in a paper published in medical journal The Lancet on Wednesday.
“It’s not really surprising that each dose from a different vaccine comes with its own first-dose adverse effects – but most self-limiting and mild,” said Julian Tang, a clinical virologist at Britain’s University of Leicester, who is not involved in the trial.
He said similar results could be expected for vaccine combinations of the same types as those tried so far. The Oxford-AstraZeneca vaccine is adenovirus-vectored and Pfizer-BioNTech’s is mRNA-based.
“[Combining vaccines] is a practical solution to cover the variable supply, delivery and vaccine efficacy, mixed adverse effect profiles and hesitancy globally,” he said. “The ability to mix vaccines will also be useful where different variants are concerned.”
For example, a future updated vaccine targeting a variant could be used as a third-dose booster, regardless of manufacturer.
In April, China started a trial of 120 people mixing two Chinese vaccines using different technologies to assess safety and whether it could boost immunity. They used a first dose of the adenovirus-vectored vaccine made by CanSino Biologics, followed by a protein subunit vaccine produced by Anhui Zhifei Longcom Biopharmaceutical.
The University of Hong Kong has also started a study of 150 adults who would receive a dose of Pfizer-BioNTech’s vaccine, followed by Sinovac’s. Its medicine faculty said the combination “might improve the vaccine immunogenicity and safety when compared to vaccination with two doses of the same type of vaccine”.
For now, mainland Chinese health authorities do not recommend mixing vaccines that use different technologies, but have said people can be given different brands that use the same technology if a second dose of the same vaccine is not available.
China has approved five domestically developed vaccines, three of which are inactivated vaccines that use dead material from the virus to trigger an immune response.
The country had administered nearly 355 million doses of vaccines as of Wednesday, although its statistics do not specify how many of the 1.4 billion population have had two doses of a two-dose vaccine.