HPV vaccines are highly efficacious, and significant reduction of high-risk HPV type prevalence is within reach. By model predictions, herd effects are stronger for HPV18 and other faster clearing types than for slow clearing HPV16. An ancillary study was conducted independently of the initial community-randomized HPV-040 trial (NCT 00534638, sponsored by GSK Biologicals S.A.) to verify model-predicted herd effect gained from gender-neutral vaccination of early adolescents.
Objectives: To compare methods for analysing community-specific changes of HPV prevalence in non-HPV vaccinated population due to the herd effect following different HPV vaccination strategies.
In 2007-9, 80.272 1992-95 born adolescents were invited in 33 communities and 20.153 girls and 11.662 boys participated (1). In 11 Arm A communities 90% of participants, and in 11 Arm B communities 90% of girls, received HPV-16/18 vaccine. In 11 Arm C communities all received HBV-vaccine. HPV prevalence were determined by using a MGP primer system PCR from 5.473 non-HPV-16/18 vaccinated 18.5 year-old girls in 2010-2014. By model predictions, herd effects appear with a delay so that they are stronger for 1994-95 than for 1992-93 birth cohorts. Thus, the herd effect into non-HPV vaccinated girls were determined from the (probability density function of) community-specific changes in the HPV prevalence from 1992-93 to 1994-95 birth cohorts by study arms, both in frequentist and Bayesian ways. These estimates are conservative as the 1992-93 cohorts were also vaccinated.
The herd effect against HPV16 was weak/absent: In Arm A RR 1994-95 vs 1992-93 was 0.94 (with 95% CI 0.64-1.30) and 1.03 (0.71-1.41) in Arm B. As predicted, the herd effects were stronger against HPV18 [A: 0.64 (0.43-0.92); B: 0.74 (0.49-1.05)], and against combined HPV16/18/31/33/45 [A: 0.77 (0.59-0.95); B: 0.90 (0.69-1.11)]. In Arm C the corresponding RR ranged from 0.98 to 1.07.
Herd effects were evident in the community specific analysis with the gender-neutral vaccination strategy.
(1) Lehtinen M, et al. Vaccine 33:1284-1290, 2015