I can't really say whether or not blood donors are representative (most likely that has little impact on the results of the study), but the sample size is much smaller: an initial cohort of 142,758 from July 2021 was reduced down to a final sample size of N=72,748 because of the availability of robust records of immunization and disease history in the context of the study. This represents roughly 0.02%, which may be representative if the sampling was done well, (it likely was, the CDC tends to do good work in that regard), but isn't as robust as the study by the PHAC.
This final cohort was studied based on records generated and collected in four three-month periods between April 2021 and September 2022 which is a good timeline.
This study seems to focus on the efficacy of older (≥65 years) people sticking to a strict vaccination schedule, which is reflected in the numbers; in that age group, vaccination-linked immunity is highest. The counterpoint to that is that younger groups aren't sticking to as strict a vaccination schedule, though best as I can tell, the study stops short of spelling that out, so that's my own conclusion.
This final cohort was studied based on records generated and collected in four three-month periods between April 2021 and September 2022 which is a good timeline.
This study seems to focus on the efficacy of older (≥65 years) people sticking to a strict vaccination schedule, which is reflected in the numbers; in that age group, vaccination-linked immunity is highest. The counterpoint to that is that younger groups aren't sticking to as strict a vaccination schedule, though best as I can tell, the study stops short of spelling that out, so that's my own conclusion.