You don't often see trials of vaccine efficacy these days but here is an interesting paper on HPV vaccine in 25 - 44 year olds. The outcome is HPV infection, not cervical cancer.
Given that (1) the aim is to prevent infection with a sexually transmitted virus, and (2) the median age at first sexual intercourse for females is around 17, at least in western societies, vaccinating women after the 25th birthday seems to be leaving it too late. But the argument is that nowadays many women may start a new sexual relationship in their thirties, perhaps after a failed first marriage.
I note the use of a new phrase creeping into the literature - 'per protocol' analysis. This is opposed to 'intention to treat' analysis. 'Per protocol' seems to mean 'actually got the vaccine'.
'Intention to treat' seems a strict standard - you would judge the vaccine ineffective even if the problem was that most women never got it. But 'Intention to treat' tells you what happens in the real world - 'I intended to treat all of these patients (but all sorts of things prevented this from happening as intended)'; this is sometimes called effectiveness. 'Per protocol' tells you how much effect the intervention has considered pure and on its own; this is sometimes called efficacy.
But 'per protocol' introduces all sorts of bias because the women who did not get vaccinated after being assigned to the intervention group will differ systematically from the women who did receive the vaccine. For example the sort of woman who attends to receive vaccine may, by comparison to the non-attender, be more health conscious and so more careful about her sexual relationships, use condoms more etc. and so put herself less at risk of HPV infection regardless of whether the vaccine works or not. Be vary careful about what 'per protocol' analysis actually tells you.
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