cohort

In everyday use a cohort is just a group. In research it carries more than that. The US National Cancer Institute defines it as a group of individuals who share a common trait, such as birth year, and adds that in medicine a cohort is a group that is part of a clinical trial or study and is observed over a period of time.

The second half is the part that does the work. A cohort is not simply a category of people, it is a group tracked forward through time, which is what allows a study to see what happens to them rather than asking them to remember what already did. It also means a cohort has an age. A group defined by the year it was born or vaccinated moves through life together, and reaches any given age at a predictable moment.

That property is why some public health results can only arrive when they arrive. If a vaccine is given to girls at twelve and the disease it prevents normally appears in middle age, the first honest answer about whether it worked has to wait for that cohort to reach middle age. Reporting before then is not wrong, but it is measuring something earlier in the chain, such as infections or precancerous changes, rather than the outcome that matters most.