The Efficacy and Duration of Vaccine Protection Against Human Papillomavirus

Abstract

Every year, 4600 women in Germany develop cervical cancer (1). The raw incidence rate for 2014 has been estimated at 11.2 cases per 100 000 individuals (1). Persistent infection with a high-risk human papillomavirus (HPV) type is a necessary prerequisite for the development of dysplasia and neoplasia of the cervix (2); incident infections, in contrast, are not a risk factor. HPV types 16 and 18 are among the most common high-risk types in Germany (3).

Dysplasia, or cervical intraepithelial neoplasia (CIN), is classified by severity (grades 1 to 3). The cervical cancer risk increases with the severity of CIN (2). For CIN 2 lesions the risk of developing cervical cancer within 5 to 10 years is 20 to 30%, while CIN 3 lesions that persist for more than two years are associated with a 50% risk (2, 4).

A vaccine for HPV types 6, 11, 16, and 18 has been available since 2006; since 2007 a further vaccine for types 16 and 18 has been available. The addition of HPV vaccination to the vaccination recommendations of the German Standing Committee on Vaccination (Ständige Impfkommission, STIKO) in 2007 led to heated discussions of the benefits of vaccination (5). Since then, further data has been generated that can be seen as confirming the benefit of HPV vaccination identified in its authorization trials, which found that it could prevent persistent HPV infection in HPV-naive girls and young women (6, 7). As of 2012, HPV vaccination recommendations for girls were part of national vaccination plans in 21 of 29 EU countries (8).

The duration of vaccine protection was a central element in the discussion from the very beginning, in both national and international public and specialist debate (5). When this study was conducted, systematic reviews on the efficacy of vaccination were already available (9– 14), but none of these addressed the duration of vaccine protection. In addition, these systematic reviews have limitations in terms of the included study types, and some of them analyzed vaccines (sometimes monovalent) not authorized in Germany and/or have methodological shortcomings in their search for studies and data analysis.

In this context, we performed a systematic search of the literature and a meta-analysis of the existing studies that addressed the duration of protection following HPV vaccination. In particular, we aimed to clarify whether, in individuals vaccinated in childhood, there is still sufficient protection against HPV several years later, when sexual activity and therefore the risk of infection begin. A rapid drop in vaccine protection would raise the question of the need for booster vaccination, or affect the preferred age of vaccination.

Methods

A systematic review was performed to address the following primary questions:

• In long-term follow-up (≥5 years following initial immunization) after HPV vaccination, is vaccination less effective than in short-term follow-up (<5 years following initial immunization) in terms of preventing high-risk HPV infection or the development of CIN 2 or CIN 3 lesions?
• What is the quality (according to the GRADE guidelines) of the evidence on the efficacy of vaccination in long-term follow-up in comparison to evidence obtained during short-term follow-up?

In order to draw conclusions for the target group of STIKO‘s vaccination recommendation (15), these primary questions must be answered independently of which vaccine is used in girls and young women not previously infected with HPV.

Results

The search strategy shown in eBox 1 identified 908 potentially relevant publications. After their titles, abstract, and full text were examined, 38 publications remained in the study pool, of which 23 had to be excluded because they were interim or subgroup analyses of identical study populations. The final analysis therefore included 15 primary studies (19– 33). Of these, 10 were randomized controlled trials (RCTs) (19– 28), and the remaining five were observational studies (29– 33) (see Figure 1 and eTable for details of the study selection process; further details available from the authors).

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Figure 1

Flow diagram showing procedure used to search literature

Study findings

RCTs—short-term follow-up: Figure 2 shows the relative risks and pooled estimates with 95% confidence intervals (95% CIs) from meta-analysis of the RCTs for short-term follow-up. With a median follow-up duration of 25.5 months, incident HPV 16 and HPV 18 infections were prevented with 83% efficacy (95% CI: 70 to 90%). For persistent infections (lasting six months or longer), pooled efficacy was estimated at 90% (95% CI: 79 to 95%) with a median follow-up duration of 27 months, although there was moderate, statistically significant heterogeneity. CIN 2+ cervical lesions were prevented with 84% efficacy (95% CI: 50 to 95%) after a median of 36 months‘ follow-up, also with moderate, statistically significant heterogeneity. Subgroup analysis showed that this heterogeneity was caused by outcome definitions: in the PATRICIA trial (26) and the trial by Konno et al. (24), CIN 2+ lesions were analyzed as an outcome irrespective of the type of HPV found in the lesion; in contrast, the FUTURE I (21) and FUTURE II (22) studies reported CIN 2+ lesions as HPV 16- or HPV 18-positive lesions for participants with no previous infection, which are the participants relevant to this review article (per-protocol susceptible population). None of these two latter studies reported any lesions independent of HPV type for this group of participants. Subgroup analysis for the first two studies mentioned (24, 26) showed pooled vaccination efficacy against CIN 2+ lesions, regardless of HPV type, of 70% (95% CI: 56 to 79%), while efficacy against HPV 16- or HPV 18-positive CIN 2+ lesions specifically was 98% (95% CI: 86 to 100%) according to the FUTURE studies (21, 22). CIN 3+ lesions were prevented with 94% efficacy (95% CI: 83 to 98%) with a median follow-up duration of 36 months.

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Figure 2

Forest plots for follow-up <5 years (RCTs)

RCTs—long-term follow-up: Figure 3 shows the data from meta-analysis of the RCTs on long-term follow-up. The efficacy of prevention of incident infections was 94% (95% CI: 80 to 98%); this data was obtained from a single trial with a follow-up duration of seven years (20). Persistent infections were prevented with a pooled efficacy of 95% (95% CI: 84 to 99%) over a period of six years (median) (20, 28). For CIN 2+ lesions as well, data was obtained from only one study (20). It showed 86% vaccination efficacy (95% CI: –166 to 99%) for HPV 16- or HPV 18-positive lesions after seven years‘ follow-up, which is a non-significant effect with a very wide confidence interval. The same study reported vaccination efficacy of 40.6% (95% CI: –106 to 84.7%) for CIN 2+ lesions regardless of HPV type. No data was available on CIN 3+ lesions.

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Figure 3

Forest plots for follow-up ≥5 years (RCTs)

Observational studies: All five observational studies that were identified reported results from short-term follow-up. Two were cohort studies (30, 33), while the other three were geographical association studies (29, 31, 32). The latter provided indirect evidence for population-based efficacy of HPV vaccination, as they compared data from before and after the introduction of vaccination for specific geographical areas (England, UK [31,], Victoria, Australia [29], and Connecticut, USA [32]) and found a decrease in the incidence of HPV infections (31) or CIN 2+ lesions (29, 32) (population-based before-and-after studies). The cohort studies identified relationships between HPV vaccination status and CIN 2+ lesions (33) or HPV infections (30). No estimates (relative risks) of vaccination efficacy could be obtained from any of the observational studies, so ultimately these studies could not be included in the analysis and evaluation of evidence. Instead, they were taken only as supporting evidence for the efficacy of vaccination in terms of specific outcomes (details available from the authors).

One further observational study was published after the search of the literature had been completed. This analyzed data from the Australian vaccination program for the state of Victoria (34). It was not included in the analysis, as it did not meet all the inclusion criteria (no data on participants‘ infection status).

Evaluation of quality of evidence according to GRADE guidelines

For short-term follow-up, evidence quality was rated as “high” for the outcomes incident infections, persistent infections, and CIN 3+ lesions, as there were no shortcomings in terms of risk of bias or the other areas covered by the GRADE guidelines (inconsistency, indirectness, imprecision, publication bias). For the outcome CIN 2+ lesions, evidence quality was assessed as “moderate,” as the wide confidence interval indicated imprecision (details available from the authors).

As the trial by De Carvalho et al. (20) had a very high risk of bias, evidence quality was downgraded for all outcomes of long-term follow-up. Further shortcomings concerned the definition of the outcome persistent infections (indirectness) and the wide confidence interval for the outcome CIN 2+ lesions. Overall, evidence quality for long-term follow-up was therefore low to very low (details available from the authors).

Conclusion

This systematic review shows that there is no evidence from long-term follow-up that vaccine protection following vaccination for HPV types 16 and 18 decreases. While persistent infections (those lasting six months or longer) were prevented with a pooled efficacy of 90% with a median follow-up duration of 27 months, the pooled efficacy for a period of six years (median) was 95%. For the clinical outcome HPV 16- or HPV 18-positive CIN 2+ lesions, 84% vaccination efficacy was calculated after a median of 36 months, and 86% after seven years‘ follow-up. Data on long-term follow-up was taken from only one study (20), which had a considerably smaller number of participants; this explains why the effect was insignificant and the confidence interval very wide. Because only a few RCTs were conducted for five years or longer and these had considerably fewer participants than studies with shorter follow-up, the quality of evidence for long-term protection is lower than that for short-term protection. However, the premise of stable long-term protection is supported by data that shows induction of a robust immune memory following HPV booster vaccination (35).

Our work focused on study participants in whom incident HPV infection with the types of HPV contained in the vaccine was ruled out when they were enrolled in the studies. The highest vaccination efficacy was achieved when girls and young women were vaccinated before their first possible HPV infection. For example, in the FUTURE II study the efficacy of vaccination against HPV 16- and HPV 18-associated medium-grade dysplasia of the cervix (CIN 2+) in HPV-negative women was almost 100%, while in participants in the same study for whom HPV status was not an inclusion criterion it fell to approximately 50% (22). The main route of transmission of HPV infections of the cervix is sexual contact, and the probability of HPV infection rises substantially when an individual becomes sexually active (36). HPV vaccination should therefore be completed before the beginning of sexual activity.

The observational studies we identified provided no information that went beyond the data obtained from the RCTs. This was partly because all the existing observational studies yielded data only on short-term follow-up, for which there is already relatively good evidence from RCTs. It was also partly because the designs of some observational studies were not suitable for generating data on vaccination efficacy, and others did not provide any data on girls or young women without HPV infection, which was the group focused on here. Nevertheless, these studies are an additional source of evidence showing an effect of HPV vaccination on various outcomes following widespread use in the target group.

This systematic review focuses on studies investigating efficacy of vaccination in short- and long-term follow-up; data on adverse drug reactions (ADRs) was not included in the evaluation according to the study protocol. Two recent systematic reviews have analyzed data on ADRs following HPV vaccination; both concluded that the studies included did not show any significant differences between vaccinated and non-vaccinated participants in terms of relevant outcomes, and that the safety profile of vaccination was acceptable (12, 13). A recently published systematic review also demonstrates the efficacy and safety of HPV vaccination when coadministered with other vaccines (37).

The limitations of this article concern the focus on girls and women with no HPV infection. The results are extrapolated to the actual target group for vaccination, girls and young women who are not yet sexually active. It must also be assumed that the data for other target groups such as older women or young men is different in terms of both efficacy data and evidence quality.

The particular strength of this article is that this is the first time a comprehensive systematic review has provided a conclusion on the long-term efficacy of HPV vaccine protection for the most important vaccination target group on the basis of meta-analysis. In addition, the standardized, international GRADE guidelines have been used to provide a conclusion on the quality of the evidence, differentiated by length of follow-up. This supports a critical assessment of this data.

Acknowledgments

Footnotes

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