That's an interesting and relevant multiplyer effect imo. It will be very hard to calculate the impact of it, though. For precision, your point should be accounted for as both the mortality and the percentage vax/unvax should refer to the same month for good comparison. My guess is that for the elder groups this effect will have more impact than for the younger ones for reasons that I mentioned above in my reply to Mark (in the older age groups small changes in the percentage will have more effect on the resulting ratio).