It was less expensive to study in older populations because they get shingles at higher rates, so it was only studied in older populations. That's all. Early

It was less expensive to study in older populations because they get shingles at higher rates, so it was only studied in older populations. That's all. Early on there were also arguments that supply was limited, so the age minimum was to restrict supply to those who could benefit the most (frail elderly). But that ship has long sailed.