A study into COVID-19 vaccines found that people have higher levels of immunity when receiving the first dose of AstraZeneca or Pfizer followed by a Moderna or Novavax shot nine weeks after, compared to two shots of the same vaccine. While this is a relatively small study, it seems to suggest that the mix-and-match approach works when it comes to COVID-19 vaccines — at least in some combinations.

Researchers at the University of Oxford tested vaccine combinations on a group of over 1,000 volunteers over 50. The Moderna and Novavax vaccines increased immunity after the AstraZeneca vaccine, compared to a second AstraZeneca shot, while only Moderna increased antibodies after Pfizer, compared to a full vaccination with Pfizer. 

Mix and match

The COVID-19 pandemic has already caused more than five million deaths to date — and with new variants constantly emerging, there’s no way out of the pandemic other than vaccination.

Over three billion people were vaccinated with at least one dose, but that figure is only 2% to 8% in low-income countries. Around the world, 24 Covid-19 vaccines have been already approved, but manufacturing and distribution remain major challenges, especially in the less-developed parts of the world. For countries like this, where availability is a problem, mixing between different vaccines could be a game-changer, the researchers suggest.

Results varied based on the particular type of vaccine mix. The researchers found levels of antibodies 17 times higher in those individuals who got the first shot of AstraZeneca followed by a shot of Moderna, and four times higher when followed by Novavax. Meanwhile, for those who got the first jab of Pfizer, antibodies were 1.3 times higher when getting a second shot of Moderna and 20% lower with Novavax. 

The team at Oxford also explored the impact of vaccine combinations against novel COVID-19 variants, specifically the Delta and the Beta ones. In both cases, they registered a reduction in the levels of antibodies and a very little drop in T-cell responses. The Omicron variant, which was just recently discovered, wasn’t included in the study. 

Of the mixed schedules studied, perhaps the most relevant to low-income countries is the AstraZeneca/Novavax, the researchers argued, as neither require ultra-low temperature storage and also given the low cost of the AstraZeneca. The WHO is expected to soon authorize the Novavax to be delivered through the Covax initiative.   

Validating a strategy

While previous studies demonstrated the short-term safety of vaccine combinations, this is the first one to publish data from randomized controlled trials examining the immunity levels and safety of using different vaccines over a longer period of time. 

“Multiple vaccines are appropriate to complete primary immunisation following priming with BNT (Pfizer) or ChAd (AstraZeneca), facilitating rapid vaccine deployment globally and supporting recognition of such schedules for vaccine certification,” the researchers, members of the Oxford Vaccine’s Group Com-Cov, wrote in The Lancet. 

Several countries had already been using vaccine combinations for a while now, especially as they were faced with low vaccine supplies and slow vaccination campaigns. This was the case of Germany, for example, which offered booster shots of Pfizer and Moderna to vulnerable individuals, regardless of their previous vaccine. But not everyone supports this strategy.

Back in July, the World Health Organization advised people against mixing and matching COVID-19 vaccines, and it remains to be seen whether this recommendation will change in light of the new information. Mixing vaccines could offer new hope to lower-income countries that have not completed their primary vaccination campaigns and could now start using different vaccine brands. 

The study was published in the journal The Lancet.