The Human papillomavirus 16/18 (HPV16/18) is known to cause cervical cancer. In Japan, HPV vaccinations became available to girls aged 13-16 years with public aid from 2010 and subsequently became routine for those aged 12-16 years from April, 2013. However, after several news media reports of adverse events to the vaccination, the Ministry of Health, Labour, and Welfare has suspended its recommendation of the HPV vaccination. This has led to great differences in vaccination rates among different birth year groups. A research team led by Yusuke Tanaka, medical staff member and Yutaka Ueda, assistant professor, of the Department of Obstetrics and Gynecology, Graduate School of Medicine, Osaka University, has now calculated the HPV16/18 infection risk for Japanese females born between 1993 and 2008. The results show that risk of HPV16/18 infection rises significantly with each year that vaccination encouragement is suspended.
For their risk calculation, the researchers created graphs with the positive rate of HPV16/18 infection for women born in 1993 at the time they were 20 years old set at 1. They then conducted their calculations based on the following assumptions: (1) HPV infection risk correlates to the rate of experiencing sexual intercourse without HPV vaccination. (2) The rates of experiencing sexual intercourse at the ages of 20, 19, 18, 17, 16, 15, 14, 13 and 12 are assumed to be 65%, 55%, 42%, 25%, 15%, 5%, 2%, 1% and 0%, respectively. (3) During the encouragement period (except in 2013), female youths were vaccinated at the youngest target age. (4) In 2010, the vaccination rate among 13-16-year old females was at 70%. (5) In 2013, the vaccination rate among 12 and 13 year-olds was at 1% and 4%, respectively. (6) The cumulative vaccination rate after resuming vaccination encouragement is at 70%. (7) There is no correlation between rates of sexual intercourse experience and vaccination rate.
This study delivers important findings that support the resumption of HPV16/18 vaccination encouragement during this year in order to minimize the differences in future infection risks of different age groups. Should encouragement resume, the Ministry of Health, Labour, and Welfare should include those young women who were 12-16 years old during the encouragement suspension period as target vaccination subjects.
This research was featured in the electronic version of the British medical journal The Lancet Oncology on June 29, 2016.