In the face of changing eligibility for the AstraZeneca vaccine, new variants of the coronavirus and supply constraints, many people are wondering whether they can mix and match COVID-19 vaccines. This means, for example, having the AstraZeneca vaccine as the first dose, followed by a different vaccine such as Pfizer as the second dose, and boosters with other vaccines later on.
While many studies are ongoing, data has recently been released from mix and match trials in Spain and the United Kingdom. This data is very promising, and suggests mix and match schedules may give higher antibody levels than two doses of a single vaccine. While Australia's drug regulator, the Therapeutic Goods Administration (TGA), hasn't yet approved a mix and match COVID-19 vaccination schedule, some countries are already doing this.
So how does this work and why might it be a good idea?
If the COVID-19 vaccine rollout can mix and match vaccines, this will greatly increase flexibility. Having a flexible immunisation programme allows us to be nimble in the face of global supply constraints. If there's a shortage of one vaccine, instead of halting the entire programme to wait for supply, the programme can continue with a different vaccine, regardless of which one has been given as a first dose.
If one vaccine is less effective than another against a certain variant, mix and match schedules could ensure people who've already received one dose of a vaccine with lower effectiveness could get a booster with a vaccine that's more effective against the variant.
Some countries are already using mix and match vaccine schedules following changing recommendations regarding the AstraZeneca vaccine because of a very rare side effect of a blood clotting/bleeding condition. Several countries in Europe are now advising younger people previously given this vaccine as a first dose should receive an alternative vaccine as their second dose, most commonly mRNA vaccines such as Pfizer's. Germany, France, Sweden, Norway and Denmark are among those advising mixed vaccination schedules due to this reason.
In a UK mix and match study published in the Lancet in May, 830 adults over 50 were randomised to get either the Pfizer or AstraZeneca vaccines first, then the other vaccine later. It found people who received mixed doses were more likely to develop mild to moderate symptoms from the second dose of the vaccine including chills, fatigue, fever, headache, joint pain, malaise, muscle ache and pain at the injection site, compared to those on the standard non-mixed schedule.
However, these reactions were short-lived and there were no other safety concerns. The researchers have now adapted this study to see whether early and regular use of paracetamol reduces the frequency of these reactions.
Is it effective?
The Spanish study found people had a vastly higher antibody response 14 days after receiving the Pfizer booster, following an initial dose of AstraZeneca. These antibodies were able to recognise and inactivate the coronavirus in lab tests.
There's no data yet on how effective mix and match schedules are in preventing COVID-19. But they're likely to work well as the immune response is similar, or even better, compared with studies using the same vaccine as the first and second dose. This indicates they will work well in preventing disease.