A: No, but a small trial of this approach has just begun in the UK.
What is a mix and match strategy? This refers using different types of vaccines for the first and second dose.
While some incorrect headlines a few weeks back suggested the UK was adopting this strategy, in fact their guidelines merely state that a mixed strategy can be used in rare cases when the type of first dose is unknown or unavailable.
If deemed equally effective compared to normal dosing, mixed dosing would allow much more flexibility in the logistics of vaccine deployment. What’s more, it’s possible that a mixed strategy could actually enhance immune response, a phenomenon referred to as a “heterologous prime-boost regimen.”
Different vaccines stimulate the immune response in slightly different ways, for example mRNA vaccines elicit good antibody response but the viral-vector vaccines have produced better T-cell responses. Animal studies suggest that a combination of an RNA coronavirus vaccine and the AstraZeneca vaccine stimulated better CD8+ T cells responses in mice than either vaccine alone.
The Russian vaccine candidate Sputnik V uses a heterologous prime-boost approach, using a slightly different adenovirus vector for the first and second doses. Adenovirus vector vaccines use harmless non-replicating viruses to enter cells and release the instructions for making the famous SARS-CoV-2 spike protein. The catch is that since adenoviruses are viruses, the immune system will also mount a response to the vaccine itself. Immunity against the vector could prevent the virus from expressing the spike protein after the second dose, lessening the effectiveness of the booster. While Sputnik uses different vectors for the two shots to minimize this risk, Oxford/AstraZeneca’s vaccine uses only one, making testing the heterologous prime-boost studies especially relevant.
The new study will test one dose of the mRNA Pfizer vaccine and one dose of the Oxford/Astrazenca vaccine (in both orders). Results are expected by June, after which time they might inform continued vaccine strategy. Watch this space!
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