Vaccination against human papillomavirus (HPV) types 16 and 18 appears to maintain effectiveness for a minimum of 6.4 years, a study shows.

Findings were presented to the Society of Gynaecologic Oncologists 2008 annual meeting on women's cancer, DGDispatch reports.

HPV vaccine has previously been demonstrated to be 100 per cent effective against HPV 16/18 persistent infections as well as cervical intraepithelial neoplasia (CIN) 1 and CIN2, Diane Harper, Associate Professor of Community and Family Medicine and Associate Professor of Obstetrics and Gynaecology, noted.

Research showed seven months after vaccination, seropositivity against HPV type 16 reached 100 per cent, with seropositivity dropping to 99 per cent briefly, but climbing to 100 per cent after 25 months. It stayed between these levels until a 76-month follow-up when it again reached 100 per cent.

Seropositivity against HPV type 18 was maintained from months seven through to month 39, dropping to 98 per cent but then rising to 100 per cent after 76 months.

The vaccine also appeared to provide a 78 per cent reduction in infection risk in type 45 and a 60 per cent reduction in type 31.

CIN2 cases caused by HPV infection in participants occurred in five out of 505 women receiving the active vaccine, and 12 out of 497 women who received placebo injections.

After 6.4 years there were no cases of CIN1-positive infection caused by types 16 or 18 in women who were given the active vaccine, compared with 15 in the placebo group.

In the same period there were no cases of CIN2-positive infection caused by HPV types 16 or 18 among vaccinated women, compared with nine cases in women who received placebo.

"Every cost-effectiveness analysis that we have seen indicates that the most important factor is the duration of effectiveness of the vaccine," Dr Harper said.

"We have seen high and sustained seropositivity for both [HPV] 16 and [HPV] 18 at levels equal to or greater than 98 per cent for both types," she added.

Participants have been asked to continue follow-up for 9.5 years.

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