New US research has found that if girls miss the recommended HPV vaccine at age 11 to 12, the catch-up is still effective until they turn 20, if they receive all three doses.

Introduced in 2006, the quadrivalent human papillomavirus (HPV) vaccine is recommended in the US for girls aged 11 to 12. However, for those who did not receive the vaccine at this age a catch-up vaccination — approved as a three-dose series — is recommended, for girls and women aged 13 to 26. For girls aged 9 to 14, the US Centers for Disease Control and Prevention also allows a two-dose series.

To look at the effectiveness of the three-dose catch-up vaccine, researchers at Kaiser Permanente, Oakland, analyzed cases of CIN2+ or CIN3+, or cervical intraepithelial neoplasia, which is abnormal growth of cells on the surface of the cervix that could lead to cervical cancer.

Participants included 4,357 women and girls who were aged 26 or younger with CIN2+ or CIN3+, as well as 21,773 women without CIN2+ or CIN3+ who acted as controls.

The results showed that for women aged 14 to 20 years, the catch-up HPV vaccination is effective against the risk of important cervical precancers, if they receive all three doses.

No significant protection was found in women who received their first dose at 21 years or older, or who received fewer than the full three doses. The team said that more data is needed on women aged 21-26 years.

The researchers noted that the study looked at the effectiveness of the quadrivalent HPV vaccine, and not of the more recently introduced nonavalent HPV vaccine, which is expected to prevent more CIN2+ cases than the quadrivalent HPV vaccine. As new vaccines become more widely used, further research will be needed.

They added that as the study was carried out in a single health-care setting and on insured women in the area, the findings may not be generalizable to other health care settings and populations.

However, lead author Michael J. Silverberg concluded, “In comparison to other countries, HPV vaccine uptake in the US has been relatively low. Our findings show that girls and women who did not receive the full vaccine series at age 11 to 12 can still benefit from significant protection if they receive the full three doses of vaccine by the age of 20.”

“The evidence suggests that protection is strongest the earlier the vaccine is initiated, and after the age of 21, the evidence of effectiveness is unclear. Further research in other settings, and using the recently introduced nonavalent vaccine, will now be needed to assess the effectiveness of vaccinating women aged 21 to 26 years.”

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