In general, the higher anti-HPV-16 and -18 immune responses induced by the HPV-16/18 vaccine when compared with the HPV-6/11/16/18 vaccine at Month 7 in Study HPV-010 30
were maintained up to Month 24 after first vaccination. GMTs of anti-HPV-16 and -18 serum neutralizing antibodies measured by PBNA remained significantly higher up to Month 24 in women aged 18–26 y who received the HPV-16/18 vaccine than in women who received the HPV-6/11/16/18 vaccine. The HPV-16/18 vaccine also induced significantly higher anti-HPV-16 and -18 neutralizing antibody titers in women aged 27–35 and 36–45 y at all timepoints through to Month 24. For both neutralizing and IgG anti-HPV-16 and -18 ELISA antibody titers, the magnitudes of the differences between the two vaccines (GMRs) at Month 24 were similar to those observed at previous timepoints. In the HPV-naive subset of the ATP cohort for immunogenicity (i.e., seronegative and DNA-negative for the antigen under analysis prior to vaccination), which approximates the target population for HPV vaccination programs, GMRs of anti-HPV-16 and -18 neutralizing antibody titers between the vaccine groups were greater in the younger age groups studied (18–26 y and 27–35 y) compared with the older age group (36–45 y) at all timepoints from Month 7 to 24. As expected from observations in other studies,10,13–15,17,18
for both vaccines, levels of vaccine-induced anti-HPV-16 and -18 antibodies peaked at Month 7 30
and subsequently declined toward plateau by Month 18.31,32
Through to Month 24, both vaccines induced higher anti-HPV-16 neutralizing antibody titers than those observed in women who had cleared natural infection (as measured by PBNA in the TVC);30
vaccine-induced anti-HPV-18 neutralizing antibody titers remained higher than those observed after natural infection in women who received the HPV-16/18 vaccine and reached levels similar to those associated with natural infection with the HPV-6/11/16/18 vaccine. In a previous study, for HPV-16, analysis by antibody titer quartile showed a significantly reduced risk of new HPV-16 infection with an increasing naturally-acquired antibody titer (measured by ELISA);33
an epidemiological study reported similar findings with women in the highest HPV-18 antibody tertile having a significantly reduced risk of subsequent infection with HPV-18.34
In our current study, across all age groups, at Month 24 the ELISA antibody titers are higher in the ATP cohort for immunogenicity with the HPV-16/18 vaccine compared with the HPV-6/11/16/18 vaccine for HPV-16 (2.3–4.2-fold) and HPV-18 (5.2–9.7-fold). Although an immunological correlate of protection has not been defined, these observed differences in the magnitude of immune response between both vaccines might therefore represent determinants of duration of protection.
In the ATP cohort for immunogenicity (seronegative and DNA-negative prior to vaccination), seropositivity rates for anti-HPV-16 neutralizing antibodies remained high (≥97.5%) in both vaccine groups at all timepoints up to Month 24. In the HPV-16/18 vaccine group, seropositivity rates for anti-HPV-18 neutralizing antibodies were ≥97.1% at all timepoints. In the HPV-6/11/16/18 vaccine group, seropositivity rates for anti-HPV-16 and −18 IgG antibodies remained high (≥96.5%) through Month 24 in both vaccine groups.
High serum antibody titers have been shown to induce enhanced concentrations of antibodies in CVS, thereby providing the first line of defense against HPV infection and subsequent disease.35,36
At Month 7, a greater proportion of women who received the HPV-16/18 vaccine compared with those who received the HPV-6/11/16/18 vaccine had detectable HPV type-specific neutralizing antibodies in CVS, correlating with the higher serological immune response observed with the HPV-16/18 vaccine.30
CVS antibody levels in women with detectable antibodies were similar between groups at most timepoints through Month 24. For both HPV-16 and HPV-18, the HPV-16/18 vaccine induced higher GM ratios of HPV-specific IgG antibodies to total IgG antibody content in CVS at Month 24, as previously observed at Month 7.30
This suggests that, up to 18 mo after the third dose, vaccination with the HPV-16/18 vaccine resulted in the transudation of more HPV-specific antibodies from serum to the site of potential infection, when compared with the HPV-6/11/16/18 vaccine. Interpretation of CVS data are, however, limited by the small number of samples analyzed, the technical difficulty and the limited sensitivity of the analyses, which we believe may be due to the presence of inhibitors such as blood and sample dilution resulting from heterogeneity of the sampling and from the IgG extraction process.
The proportion of HPV-18-specific circulating memory B-cell responses was statistically higher at all timepoints up to Month 24 in the HPV-16/18 vaccine group compared with the HPV-6/11/16/18 vaccine group, as was the mean frequency of circulating antigen-specific cells in responders at all timepoints except Month 12. In terms of HPV-16-specific circulating memory B-cell responses, at Month 7, although the proportion of responders was similar in both groups, the geometric mean of HPV-16-specific circulating memory B cells was significantly higher in the HPV-16/18 vaccine group compared with the HPV-6/11/16/18 vaccine group.30
After Month 7, no significant differences were observed between the vaccine groups in the proportion of responders and geometric means of HPV-16-specific circulating memory B-cells, except for at Month 18 when a significant difference in the proportion of responders was observed for HPV-16-specific circulating memory B-cells. Within each vaccine group, HPV-16- and HPV-18-specific circulating memory B-cell frequencies generally remained stable between Months 12–24. It should be noted that B-cells may migrate to central lymphatic organs (e.g., spleen) at later timepoints.
For CD4+ T-cells, the proportion of responders and mean frequency of circulating antigen-specific cells calculated on all subjects (i.e., responders and non-responders) were significantly higher with the HPV-16/18 vaccine compared with the HPV-6/11/16/18 vaccine at Month 24, for both HPV-16 and HPV-18. For the HPV-16/18 vaccine, higher values observed at later timepoints might in part be due to variability in the batch testing utilized, as samples from different timepoints were evaluated in different testing runs. For instance, assay variability may be a result of the batch of reagents used at that timepoint or the assay set up, which, even if bridged, may have evolved. The most consistent comparisons are with the group responses at each timepoint. In the HPV-6/11/16/18 vaccine group, HPV-16- and HPV-18-specific circulating CD4+ T-cell frequencies plateaued between Months 12–24.
Both vaccines were generally well tolerated and exhibited similar safety profiles up to Month 24; incidences of MSCs, NOCDs, SAEs and pregnancy outcomes were similar between the groups and consistent with previous studies of each vaccine. The incidence of autoimmune diseases was similar in women vaccinated with the HPV-16/18 vaccine [1.1% (95% CI: 0.4, 2.3)] and women vaccinated with the HPV-6/11/16/18 vaccine [1.8% (0.9, 3.3)]; these values are comparable with those from a large integrated safety database of participants in studies of the HPV-16/18 vaccine [N = 19,723; 0.49% (0.39, 0.59)] vs. control [N = 19,437; 0.54% (0.44, 0.65)].37
Additionally, incidence rates of autoimmune disease in vaccinated women were comparable with rates of other neuroinflammatory events within the general population.
The differences in anti-HPV-16/18 immunogenicity observed between the HPV-16/18 vaccine and the HPV-6/11/16/18 vaccine may reflect the different adjuvant systems used in each. The HPV-16/18 vaccine employs AS04, which contains aluminum hydroxide and MPL, a detoxified derivative of the immunomodulatory cell wall lipopolysaccharide (LPS) molecule of the Gram-negative Salmonella minnesota
MPL is an agonist of Toll-like receptor 4 (TLR4) 39
and TLR4 stimulation can contribute to activation of the innate immune response.40
In vitro and in vivo data suggest that the addition of MPL to aluminum hydroxide enhances vaccine-induced immune response by rapidly triggering a local cytokine response leading to optimal activation of antigen-presenting cells.39
A separate study demonstrated enhanced antibody and memory B-cell responses when HPV-16/18 L1 VLP vaccine was formulated with AS04 compared with aluminum hydroxide alone.6
The HPV-6/11/16/18 vaccine contains a proprietary AAHS adjuvant. When formulated with HPV-16 vaccine, AAHS has demonstrated greater inherent capacity for adsorption of HPV-16 L1 VLPs and greater anti-HPV-16 L1 VLP antibody responses compared with aluminum hydroxide.8
However, no study has directly compared the relative contributions of AS04 and AAHS (using identically expressed antigens) to vaccine-induced anti-HPV-16/18 immune responses.
Strengths of our study include the use of a PBNA, which measures a range of neutralizing antibodies, and identical methodology for assessment of both vaccine groups. The cell line used in the PBNA is not used in the production of either vaccine and the pseudovirions closely resemble the natural viral particles, making the PBNA unbiased to either vaccine. As discussed previously, our results are also unlikely to be biased by the ELISA and B-cell ELISPOT assays used to measure HPV type-specific immune responses.30
Although these assays are based on the HPV-16/18 vaccine constructs, data are not expected to be significantly impacted by the use of these truncated proteins, given their overall similarity of 93% with the full-length L1 protein sequences. In a sub-analysis of sera from women included in Study HPV-010, a good correlation was observed between GMTs (HPV-16/18 vaccine over HPV-6/11/16/18 vaccine) generated using GSK's ELISA and Merck's competitive Luminex immunoassay (cLIA); the HPV-16/18 vaccine induced higher GMTs of anti-HPV-16 and -18 serum antibodies compared with the HPV-6/11/16/18 vaccine irrespective of the assay used.41
The assay used to evaluate CD4+
T-cell responses is also unlikely to favor either vaccine, as the HPV peptide pools used for in vitro stimulation were designed from the HPV-16 and HPV-18 L1 VLP sequences used in the HPV-16/18 vaccine but included the portions truncated from the HPV-16/18 vaccine but present in the HPV-6/11/16/18 vaccine.
Study limitations include the exclusion of pre-teen/young adolescent girls (as discussed previously in ref. 30
) and reliance on subjects for accurate reporting of history (e.g., sexual activity, number of partners, previous abnormal Pap results and HPV positivity). This study primarily evaluated vaccine-induced immune responses by measuring GMTs of serum neutralizing antibodies (by PBNA) and, in the absence of a defined correlate of vaccine-induced protection, our data may or may not reflect clinical outcomes. Given the very high efficacy observed with both vaccines, conducting a trial capable of detecting a difference between the two would require a very large study population and a prolonged follow-up period. Due to the methodological challenges of assessing CVS samples by PBNA (e.g., presence of inhibitors such as blood, timing relative to menstrual cycle, sample dilution), antibody titers in CVS were measured by ELISA. A previous study of women vaccinated with the HPV-16/18 vaccine observed a high correlation when anti-HPV-16 and -18 antibody responses were measured by direct ELISA (which is based on multiple epitopes) and PBNA, suggesting that the direct ELISA is a surrogate for neutralizing activity.42
Both the HPV-16/18 vaccine and the HPV-6/11/16/18 vaccine have demonstrated high efficacy against disease and virological endpoints in clinical trials and are expected to substantially reduce HPV-related disease burden.43
Both vaccines were licensed based on their excellent efficacy profiles in adolescents and young adults, with high immunogenicity demonstrated across all licensed age groups. Currently, the longest published immunogenicity follow-up for either vaccine is 7.3 y.44
Higher immunogenicity can reasonably be expected to contribute to a longer duration of vaccine-induced protection.
In the absence of a surrogate marker for a correlate of protection against HPV, the number of breakthrough cases in vaccinees (fortunately rare) for each type will become particularly important to establish normalized serocurves. If a correlate of protection against HPV is established, a standardized assay would be important to ensure that this is appropriately and universally defined. PBNA, developed independently without using antigens from specific vaccines, could be used for such comparison.
In this follow-up analysis of Study HPV-010, the HPV-16/18 vaccine demonstrated generally higher immunogenicity and a similar safety profile to the HPV-6/11/16/18 vaccine through 24 mo after first vaccination in healthy women aged 18–45 y. This observation may be of relevance for healthcare providers and other stakeholders in cervical cancer prevention. Any clinical efficacy differences between the two vaccines in terms of prevention of HPV-16/18-associated cervical disease, should they exist, are not currently evident and will only become apparent through long-term assessment.