From mathematician Gary Cornell:
For people 65 to 74, while the average number (92.9%) looks great, the confidence interval is not. It says that what we can say is that there is a 19/20 chance that the efficacy is between 53.2 to 99.8. This kind of confidence interval says that didn’t have enough cases in this group to really say much at all and so the confidence range is too large to be really useful.
And when we get to people over 75, what they describe isn’t a confidence interval, it’s a joke. A confidence interval of -12.1 to 100 is a lot like saying they threw a bunch of darts at a dart board at random and did everything from hit bystanders (i.e. the vaccine made things worse) to perfect protection. They simply didn’t have enough cases to say anything meaningful and so what they say is just totally useless.
But I don’t want to end on a depressing note! My friends who think about these questions feel pretty strongly that while the vaccine will likely be less effective in people over 65 than it is in younger people, the dropoff won’t be great enough to make a big difference. For example, if it is 20-25% less effective in these age groups (which they think is the worst case scenario), you still get a vaccine that is roughly between 70% and 75% effective – which is still pretty darn good.
Still I wish they had enrolled enough people >65 to have a better signal!
There is much more useful information at the link.