The overall impact of vaccinating a population, i.e., overall effectiveness
always comprises direct effectiveness (vaccine efficacy) and indirect effectiveness
(herd effect) of vaccination. In the case of human papillomavirus (HPV) vaccination
the role of herd effect is especially strong due to the assortative nature of sexual risktaking behaviour and transmission of sexually transmitted micro-organisms, including
genital HPV types. At the time of HPV vaccine licensure we launched a communityrandomized trial in Finland to provide real-life evidence on the impact of different
vaccination strategies one of which was to be implemented into national Finnish
vaccination program. In collaboration with the Finnish National Institute for Health and
Welfare we invited in Autumn 2007 all 80,272 boys and girls from 1992-1995 birth
cohorts, who attended one of the 250 junior high-schools in overall 33 Finnish towns
outside the Helsinki Metropolitan regions. The study sites represented 33 of 34 Finnish
communities with >35,000 inhabitants and were randomized 1:1:1 into 11 genderneutral HPV-vaccination communities, 11 girls-only HPV-vaccination communities,
and 11 control (hepatitis B-virus) vaccination communities. Furthermore, with both
parental and their own informed consent 20,513 girls and 11,662 boys participated in
the school years 2007-2009. Between 2010 and 2014 11,396 cervical samples for HPV
typing were obtained from 18.5 year-old females. We identified superb herd effect in
the gender-neutral HPV vaccination communities against transient HPV18/31/33/35
infection as defined by PCR positivity and against persistent HPV type 16/18/31/35
positivity as defined by serology. Statistically significant rapid elimination of HPV
types 18/31/33 by birth cohort was found only in the gender-neutral HPV vaccination
communities. This study was possible only in the HPV vaccination naïve population,
and the findings supported the implementation of gender-neutral HPV vaccination two
years after their publication.
Keywords: Core group, Cumulative incidence, Effectiveness, Herd effect, Prevalence, Sero-prevalence, Vaccination coverage, Vaccine efficacy.