We have developed and implemented a robust, scalable and sensitive assay for the evaluation of IgG antibody responses to vaccination with HPV − 6, -11, -16 and − 18 VLPs. The seroprevalence of HPV 16 and HPV 18 anti-HPV IgG antibodies was significantly higher among vaccinated than unvaccinated girls, as were the mean anti-HPV antibody levels. The median antibody titers did not differ significantly between vaccinated and unvaccinated girls.
Neutralizing antibodies are perceived to be the single most important determinant of prophylactic HPV vaccine performance [16]. As such, their long-term persistence in human serum is consistently a matter of significance in population-based vaccine studies [10]. A number of assays such as ELISA, competitive Luminex immunoassays (cLIA) and in-vitro neutralization assays are available for evaluating the viability of the immune response in clinical trials and in post-vaccination surveillance [8]. Among these, ELISAs stand out for being versatile enough to be automated on third-party laboratory systems to reduce cost, rapidly scalable, and can be optimized to quantitatively measure neutralizing antibodies as a marker of vaccine durability [10].
The titer of antibody activity calculated from a dilution curve of each sample is the most accurate quantitative method for measuring antibody responses. Antibodies of the immunoglobulin G class were demonstrated to be within detectable limits within serum drawn from the GAVI/EPI immunization pilot 36 months prior. There was indication that this antibody activity was more pronounced in vaccinated girls compared to non-vaccinated HPV-naïve girls or in natural infection. Since the GAVI/EPI-sponsored pilot vaccination of pre-adolescent girls in primary 4 and 5 some years ago, the young girls will now be in their mid-late adolescence with likely initiation of sexual activity and heightened risk of HPV infection [5, 17].
The rates of seroconversion to HPV-18 and 16 at month 36 observed here are consistent with other studies [5, 17, 18]. Current seropositivity assays utilizing spectroscopy suffer from significant background noise especially at low concentration. This may cause loss of sensitivity at antibody levels typical of long-term vaccinees and inability to distinguish between low antibody titres from non-immune sera [18]. More sensitive antibody assays should make it possible to prove this hypothesis. Qualitative assays like this help to circumvent the problem of false negatives associated with qualitative assays by estimating an antibody titer rather than reporting a categorical outcome. In the meantime, the evidence from studies conducting repeat testing suggests that the prevalence of vaccine-type HPV infection among immunized girls is very low [19].
The antibody activity/titer detected in this work would suggest that the strength of the initial immune response may have waned according to the expected kinetics of anti-HPV antibody responses [16, 20]. Previous studies in HIV-negative girls and young women from sub-Saharan Africa indicate that the time-curve for antibody activity following vaccination with HPV VLPs follows a bell-shape with a peak antibody concentration expected approximately 4 months following the third dose of vaccine with sub-peak antibody titers detected 12 months after the initial dose [5, 21–24]. Some modeling results have predicted that the protection induced by currently available vaccines should last for at least 21 years irrespective of the vaccination schedule [16, 25]. Our results provide empirical evidence that the initial antibody response is sustained even at 36 months post vaccination and around the average age of sexual debut for African girls [26]. These findings should reinvigorate the discussion to introduce prophylactic HPV vaccination of girls as a national strategy for preventing HPV infection and its sequelae among women in the country.The persistence of these antibodies will be expected to confer much-needed protection against HPV genotypes covered by the vaccine [27]. Cross-protection to closely-related genotypes may also be expected [28, 29].
All the girls who received the vaccine were enrolled in school at the time of vaccination making it easy for follow up. However, it is unlikely that this may have significantly affected the outcome of this study. There is evidence to suggest that a similar outcome could be expected for girls who receive the vaccine in other settings provided they are able to receive all three doses [7, 20].
In most cases except with HPV-11-targeted antibodies, vaccinated girls registered a significantly increased antibody index/threshold and sero-conversion rate relative to unvaccinated girls. This could be as a result of a number of reasons. It is conceivable that the initial response had waned to natural levels or the small sample size of this pilot was inadequate to detect such significant differences. Ultimately, larger studies may be required to clarify this observation.
Limits of the Assay
The assay detects and quantifies IgG antibodies directed to the L1 protein. Patients may have HPV infection without producing antibodies specific to L1. Anti-HPV antibody levels of an infected patient may be below detection threshold related to the time course of the infection, e.g., too early for positive titer development. Samples from non-patients may be elevated due to prior exposure to the human papillomavirus.