Before people got the shingles vaccine, those just barely eligible for it were just like those just barely ineligible — same health habits, same chronic conditions — so researchers could trust that any differences later were likely due to the vaccine, not pre-existing differences.

Researchers found that people just above and just below the age cutoff for getting the shingles vaccine were pretty similar in their health before getting the shot—so any differences in outcomes later are likely due to the vaccine, not pre-existing differences.

Scientists compared people who were just barely eligible for the shingles vaccine with those who were just barely ineligible—and found they were equally healthy in other ways, so any differences in health later on can likely be blamed on the vaccine, not other factors.

When the government decided who could get a shingles shot based on birth date, people born just after the cutoff date were way more likely to get vaccinated—13% more—because the rule suddenly let them in, showing the policy worked as intended.

People born just after November 2, 1936, got a free shingles shot because of a government rule, and way more of them got vaccinated than people born just before that date—proving the rule worked to get more people protected.

Getting the shingles vaccine didn’t make people go to the doctor more for other things or change how often they were diagnosed with common long-term illnesses like diabetes or high blood pressure—so if people who got the vaccine had less dementia, it’s probably because the vaccine itself helped, not because they got better overall care.