Some assorted thoughts:
In the big picture, the efficacious of a vaccine doesn’t matter per se what matters is getting to herd immunity. If you have a less efficacious vaccine you need to vaccinate more people but herd immunity is herd immunity, i.e. vaccines mostly protect people not because they are efficacious but because we reach herd immunity. I’ve never had measles mostly because I have probably never been challenged with measles not because I have been challenged but due to a vaccine I fought it off. The AZ vaccine at 70% efficacious will work just fine. (One potential issue, as Josh Gans notes, we don’t yet have data on transmission reduction which could vary by vaccine.)
As I mentioned in The Vaccine Works Fast, the first shot of the Pfizer vaccine seems to work well enough so that one *might* consider delaying the second dose a few weeks to get the first dose out more widely. In fact, the accidental low-dose, standard-dose regime for the AZ vaccine had people getting the second dose 7 to 8 weeks after the first dose and that was the 90% efficacious regime. We don’t have full-information but the exact timing of the second-dose does not seem critical, although everyone should get a second dose.
A related point is that we could mix and match vaccines. The UK will run a trial on this question. Mix and matching has two potentially good properties. First, mix and matching could make the immune system response stronger than either vaccine alone because different vaccines stimulate the immune system in different ways. Second, it could help with distribution. It’s going to be easier to scale up the AZ vaccine than the mRNA vaccines, so if we can use both widely we can get more bang for our shot. (As Tyler has noted the British have really stepped up on rational trial design.)
The mRNA vaccines are getting the press but for the world as a whole the AZ, Chinese, Russian and similar more traditional vaccines are going to be the big players because facilities exist for scaling them up around the world.
Addendum: Countries in the world that now have a vaccine: the UK, Canada, Bahrain, China, Russia. One country without a vaccine: the United States. The US FDA advisory committee is meeting today. You can watch here.