Yes, I've been quoted in an interview in the newspapers that I think mixing and matching has several advantages. One is for the viral vector vaccines like the AstraZeneca Oxford vaccine. The second time you come in with the second shot, the host will have already, perhaps, mounted an immuno-response against the vector itself, and so you'll have diminished effectiveness of the vector or the vaccine the second time around, whereas, if you only give it once and then come in, for example, with the RNA vaccine, you're combining the best of both worlds. That's one reason.
The other reason is that there's evidence that the RNA vaccines are particularly good at mounting one arm of our immune system, making antibodies, whereas the viral vector vaccines are particularly good at activating another arm of our immune system, which is the so-called cellular arm of our immune system. By combining the two, you get, again, the best of both worlds.
The third reason, of course, is that, in terms of vaccine availability, if we find that we have a lot of one and not the other, that's another argument for doing both.
I think the bottom line is that we won't know until we do a trial to really measure the effectiveness of that mix-and-match strategy. That trial's begun in the U.K. Here in Canada, my recommendation is that we should also consider doing such a trial as well, perhaps in partnership with the British.