Should you take a less efficacious vaccine or wait for a more efficacious vaccine? The individual and social incentives are in conflict. For society as a whole it’s typically much better if everyone takes the less efficacious vaccine sooner. We show one example of this in the supplementary material to the Science Paper with details under different scenarios in a forthcoming paper but the intuition is clear. Herd immunity is herd immunity. In the final analysis what you care about is not your chances of overcoming the disease if challenged (the vaccine efficacy) but your chances of overcoming the disease if challenged times the probability of being challenged. Herd immunity means pushing the latter number close to zero which is more important than modest differences in efficacy rates.
What about at the individual level? If you have a choice, it’s clearly better to get the more efficacious vaccine, especially since both vaccines are free (the price system has its advantages in clearing markets). But if you have to wait for the more efficacious vaccine, the choice isn’t obvious. Many people in Europe aren’t taking the AstraZeneca vaccine in the hopes of getting an mRNA vaccine later but I think that is a mistake. Don’t fail to wear your seatbelt today because your next car may have airbags. I’d be happy to take the AstraZeneca vaccine today, if only the government would let me.
Moreover, there is little reason to believe that you can’t follow-up the J&J or AstraZeneca vaccine with a mRNA vaccine at a later date.* If we will be taking multiple SARS-COV-2 vaccines over the next 10 years, as seems likely, it really doesn’t matter much which one we get first.
Do yourself and your society a favor by getting whatever vaccine is available.
Everything in this post applies even more strongly to a country making decisions. Buy the vaccine with the earliest delivery date! And don’t forget to consider the Gamaleya, Sinovac and Sinopharm vaccines.
* Multiple shots of adenoviral vector vaccines such as AZ may become less efficacious overtime as people’s bodies recognize the vector.