Enrolling continuous series of cytological specimens enriched with pathological samples from women attending cervical cancer screening provides, after weighing for sampling fractions, a powerful and efficient design to survey the early impact of HPV vaccination.

The Belgian SEHIB study demonstrated lower rates of HPV16 and HPV18 as well as cervical precancerous lesions associated with these types, similar to effects observed in randomised trials in women younger than 30.

Vaccine effectiveness decreased with increasing age.

Lower effectiveness observed in women aged 25–29 is explained by a tendency of women with high-risk behaviour (probably already infected with HPV vaccine types prior to vaccination) to get the vaccine.