This is the longest duration of protection reported so far for the vaccine, which protects against HPV 16 and 18.
The study, presented last week at an American Society of Gynecologic Oncologists conference in Florida, involved 776 women who were tested every six months.
A separate study, presented at the same conference, showed that Gardasil, a vaccine against HPV types 6, 11, 16 and 18, appears to significantly reduce abnormal smear test results.
The government announced last October that it planned to introduce routine HPV immunisation for girls aged 12–13 years. It has yet to decide which vaccine should be used.