Two doses of HPV vaccine may suffice for genital warts prevention
Just two of the recommended three doses of human papillomavirus virus (HPV) vaccine may be enough to reduce the risk of condyloma (genital warts) infections and potentially the risk of cervical cancer, a Swedish study has shown.
Completing the three-dose HPV vaccine series still conferred the most protection, but an examination of data from national Swedish population-based health data registers showed that the difference in risk reduction between the second and third doses was small, especially among girls who received their first dose before age 17. [JAMA 2014;311:597-603]
“The number of condyloma cases prevented by three doses versus two doses was 59 cases per 100,000 person years, which is a small difference,” said researchers from the Karolinska Institutet in Stockholm, Sweden.
The researchers identified 20,383 new cases of condyloma among a population of 1,045,165 females in Sweden aged 10 to 24 years, followed up between 2006 and 2010. Of these new cases, 322 occurred after at least one dose of HPV vaccine.
Risk reduction was highest among females who completed their vaccine course. However, two doses of vaccine also conferred significant protection.
For example, among girls aged 10-16, the incidence rate ratio (IRR) for condyloma was 0.18 for those who completed the vaccine course, 0.29 for those who received two doses, and 0.31 for those who had only one dose (p<0.001 for all), compared with those who did not receive the vaccine.
These corresponded to an incidence rate difference (IRD) of 459 cases of condyloma per 100,000 person years for three doses, 400 cases per 100,000 person years for two doses, and 384 cases per 100,000 person years for a single dose (p<0.001 for all) compared with no vaccine.
The IRR and IRD was consistently the least different between two and three doses among girls of any age, suggesting significant, if not the most, risk reduction.
Accounting for the impact of varying vaccine dose levels is important because “actual vaccination programs include substantial numbers of women who do not complete the full vaccination schedules,” the researchers said.
HPV serotypes 6 and 11 cause about 90 percent of condylomas, which are the first measurable endpoint for HPV infection and have an incubation period between 1 and 6 months. The females included in the study received the quadrivalent HPV vaccine, which also protects against serotypes 16 and 18, which are related to cancer outcomes, including cervical cancer.
The researchers said further investigations need to be done to determine if there is any reduced risk of cervical cancer with fewer than three doses of HPV vaccine. The current data may have also underestimated the number of condyloma cases since some patients can’t or won’t seek medical care, nor did it account for disease outcomes other than condyloma.