Immunogenicity and Safety of Extended-Interval 2-Dose Regimens of 9vHPV Vaccine

The overall design for this study has been described in detail previously.¹⁵  Briefly, with this study, we are evaluating the safety and immunogenicity of 2 doses of 9vHPV vaccine administered 1, 2, 3, or 5 years apart in girls and boys 9 to 14 years of age (Cohorts 1, 2, 3, and 4, respectively) compared with a standard 3-dose regimen (administered at day 1 and months 2 and 6) in women 16 to 26 years of age (Cohort 5). This study was designed to reveal that extended-interval 2-dose regimens of 9vHPV vaccine in boys and girls induce noninferior antibody responses to each of the 9vHPV vaccine types compared with young women receiving a 3-dose regimen. The study is ongoing, and data from Cohorts 1 to 5 will be reported separately.

This study also included an early assessment cohort of participants who previously received a single dose of 9vHPV vaccine (dose 1) but who did not complete the series (Cohort 0). For this cohort, eligible participants were generally healthy, sexually naïve adolescents 10 to 15 years of age who had received 1 9vHPV vaccine dose at least 12 months before enrollment. These participants received 1 dose of 9vHPV vaccine on day 1 of the study (dose 2). Because the first dose of 9vHPV vaccine was received outside the context of the study, the HPV status of Cohort 0 before the administration of dose 1 of the 9vHPV vaccine was unknown.

The primary immunogenicity objective for Cohort 0 was to describe HPV 6/11/16/18/31/33/45/52/58 antibody geometric mean titers (GMTs) at 1 month after the second dose of 9vHPV vaccine was administered as part of the study. As prespecified, we assessed the immunogenicity data from Cohort 0 when it was available.

We enrolled all participants in Cohort 0 at 7 clinics within Kaiser Permanente Northern California (KPNC), a large integrated health care system in the United States with ∼4.5 million members. KPNC has a stable population and comprehensive long-term electronic health records that maintain data on all health care encounters, including all vaccinations; KPNC therefore had reliable vaccination records for all study participants regarding the date when the first HPV vaccine dose was received.

Parents or legal guardians provided written informed consent, and participants provided assent. KPNC’s institutional review board approved the study, and it was conducted in accordance with the principles of Good Clinical Practice.

We collected blood samples on day 1 (pre-dose 2) and at 1 month post-dose 2 (month 1) to evaluate immunogenicity. We used the HPV-9 competitive Luminex^® immunoassay (cLIA) to measure serum HPV 6, 11, 16, 18, 31, 33, 45, 52, and 58 antibody levels.^16,17  Serostatus cutoffs were defined as the antibody level above the assay’s lower limit of quantification that reliably distinguished seropositive samples. This assessment was based on serum samples that were classified by clinical likelihood of HPV infection and positive or negative status and by alignment to serostatus, which was determined by using previous versions of the cLIA and monitoring of assay performance. Based on the recent performance of the assay, the serostatus cutoffs used in this analysis were 65, 37, 79, 85, 46, 26, 21, 30, and 31 mMU/mL for HPV types 6, 11, 16, 18, 31, 33, 45, 52, and 58, respectively.

We collected safety information from day 1 (day of vaccination) through day 5 post-vaccination, with participant-recorded oral body temperatures (starting on the evening after vaccination) and solicited injection-site adverse events (AEs; redness or erythema, swelling, and tenderness or pain) in their electronic vaccination report card. We considered a documented oral temperature of ≥37.8°C as a fever. From day 1 through day 15 post-vaccination, participants recorded any other injection-site or systemic AEs in their electronic vaccination report card. We collected all serious AEs through 12 months after the last dose.

This was a descriptive study to assess the immunogenicity of extended-interval dose regimens. There were no hypotheses tested for Cohort 0. The sample size for this cohort was capped at 200 with a limited enrollment period to ensure early availability of data.

We analyzed immunogenicity in the per-protocol immunogenicity (PPI) population. We defined PPI as Cohort 0 participants who received the correct dose of study 9vHPV vaccine (dose 2) on day 1, had evaluable serology results based on a serum sample collected within 21 to 49 days after study vaccination, and had no protocol deviations that could interfere with the evaluation of participant’s immune response to 9vHPV vaccination. Because HPV status before dose 1 was unknown, HPV seropositivity before study vaccination was not a PPI population exclusion criterion.

As described previously,¹⁵  we evaluated anti-HPV GMTs at 1 month post-dose 2 using simple linear regression modeling, accounting for the time interval between administration of the first and second 9vHPV vaccine doses. A regression model for antibody response to each HPV type was fitted on the natural logarithm (log) of the anti-HPV titer at 1 month post-dose 2 (ie, the dependent variable) with the time interval between administration of the first and second doses as a predictor (ie, the independent variable). The mean of the log anti-HPV titers at 1 month post-dose 2 could be written as β0 + β1.log (time interval between administration of the first and second doses of the 9vHPV vaccine), where the β0 and β1 are the regression coefficients. For every 1 unit increase in time between doses, the anti-HPV GMT at 1 month post-dose 2 was expected to change by 100% × (exp[β1] − 1). We estimated the effect of the time interval between doses on the expected GMT as a change (increase or decrease) in the unadjusted GMT for every unit (month) increase in time between doses.

We included all Cohort 0 participants who received the single dose of study vaccine in the safety analysis and reported injection site, systemic and serious AEs in the all-participants-as-treated population using descriptive statistics.

Participants and their parents or legal guardians who returned for study visit 2 were eligible to participate in an additional prospective observational cohort study. With this study, we aimed to survey reasons for participants’ noncompliance to the 9vHPV vaccination schedule within 12 months and required a separate consent. The questionnaire included information on caregivers’ demographic characteristics (age, sex), level of education, household income, and reasons for their child’s noncompliance with the 9vHPV vaccine schedule using a choice of 9 prespecified reasons (full questionnaires available in Supplemental Information). Respondents could select multiple reasons on the questionnaire and enter free-text responses.

We enrolled a total of 146 adolescents (boys [n = 85] and girls [n = 61]) aged 10 to 15 years (median: 13 years) between March 15, 2021 and October 28, 2021, which included a racially and ethnically diverse study population (Table 1). All participants received the study vaccine on day 1, and 141 (96.6%) completed the study (Fig 1). Five participants were lost to follow-up or withdrew after day 1.

The median time interval between the first and second 9vHPV vaccine dose on day 1 was 25 months (range 12 to 53 months; Fig 2). All participants received their first dose of the non-study 9vHPV vaccine at ≥9 years of age.

Anti-HPV GMTs and seropositivity rates on day 1 varied, reflecting the participants’ varying time intervals post-dose 1 (ie, 12 to 53 months). For all 9 of the vaccine-targeted HPV types, anti-HPV GMTs increased from day 1 (pre-dose 2) to 1 month post-dose 2 (Table 2). Analyses adjusting for the time interval between dose 1 and dose 2 (ie, 12 to 53 months) did not detect any differences in the anti-HPV GMTs 1 month post-dose 2 for any of the 9 HPV types (Table 3). We estimated that the fold-change in unadjusted GMTs for every 6-month increase in the time interval between dose 1 and dose 2 for each vaccine-targeted HPV type was 1.0.

Across the 9 vaccine-targeted HPV types, the day 1 seropositivity rate ranged from 27.3% to 59.7% (Table 4). The proportions who were seropositive for each HPV vaccine type on day 1 were generally similar across dose interval subgroups (Supplemental Table 7). At month 1, all participants were seropositive to each of the vaccine-targeted HPV types (Table 4).

A total of 83 (56.8%) participants reported any AE. Of the 65 (44.5%) who reported solicited injection site AEs, the most common was injection-site pain (n = 63/65), and all were reported as mild or moderate in intensity (Table 5). Of the 20 (13.7%) who reported vaccine-related systemic AEs, the most common was headache (n = 13/20; 8.9% of total; Table 5). No fever was reported from day 1 to day 5 post-dose 2. There were no serious AEs and no deaths (Table 5).

Of the 141 eligible participants, 99 (70.2%) parents and legal guardians agreed to participate in the survey. The baseline demographic characteristics for survey participants were generally similar to those of the entire cohort (Supplemental Table 8).

The most common reasons for noncompletion of the 2-dose 9vHPV vaccine regimen were not being aware of a scheduled second dose (44.4%) and being unable to receive the dose because of COVID-19 (36.4%; Table 6). Only 3 respondents (3.0%) reported delaying the second dose over fear of side effects, whereas few believed the vaccine was not needed because their child was not sexually active (n = 2) or were concerned the vaccine would make their child more sexually active or not practice safe sex (n = 1).

This study reveals that among adolescents aged 10 to 15 years, administering the second dose of 9vHPV vaccine 12 to 53 months after the first generated robust antibody responses. After the second dose of 9vHPV vaccine, regardless of the time interval between the first 2 doses, antibody responses increased similarly across the range of 12 months to 53 months. All participants (100%) were seropositive to each vaccine-targeted HPV type at 1 month post-dose 2. The second dose was also generally well tolerated, with the most common AEs being mild to moderate injection-site reactions. These results suggest that extending the interval between the first and second doses of the 9vHPV vaccine should be considered a feasible dosing regimen.

The findings from this study are consistent with 2 previous descriptive studies, in which the authors evaluated individuals who only received a single HPV vaccine dose followed by a booster dose of a different HPV vaccine 6 to 8 years later and observed a robust response after the second dose.^18,19  The authors of these studies provided encouraging initial evidence of strong immune responses with longer dosing intervals, but were limited by relatively small sample sizes and the use of different vaccines for the prime and booster doses. In the first, Toh et al¹⁸  assessed 40 girls aged 15 to 19 years who were previously vaccinated with 1 dose of the qHPV vaccine, followed by a single dose of the bivalent HPV vaccine 6 years later, whereas Gilca et al¹⁹  evaluated 31 girls aged 9 to 14 years who received 1 dose of the qHPV vaccine followed by 1 dose of the 9vHPV vaccine 3 to 8 years later. These studies were limited by relatively small sample sizes, the use of different vaccines for the prime and booster doses, the use of different immunoassays, and the fact that antibody responses against the 9 HPV types targeted by the 9vHPV vaccine were not assessed,¹⁵  but the results offered encouraging initial evidence that longer dosing intervals still yielded strong immune responses. The current study revealing that extending the dosing interval up to 53 months did not negatively affect 9vHPV vaccine immunogenicity further supports the idea that 2 doses of the 9vHPV vaccine with an extended interval may provide appropriate protection in adolescents.

Although this study was not designed to assess immunogenicity after a single dose, anti-HPV seropositivity was <100% by both cLIA (Table 4) and Immunoglobulin G Luminex assay^17,20  (data not shown) across all 9 vaccine HPV types when measured at 1 to 4.5 years after the administration of 1 dose of 9vHPV vaccine. When assessed at time intervals from 2 to 8 years after 1 dose of an HPV vaccine, the authors of some studies have reported nearly 100% seropositivity,^19,21–23  whereas the authors of other studies have noted lower seropositivity rates when evaluated at 2 to 7 years after 1 dose.^5,18,24,25  These results suggest variability in seropositivity after vaccination with 1 dose, depending on the study, the population, the time interval between dose 1 and the immunogenicity assessment, and other factors, such as the serologic assay. More investigations on this topic are needed.

In our study, the most common reasons participants reported for not completing the 2-dose 9vHPV regimen per recommendation were being unaware of the need for the second dose (44.4%) and the inability to receive the dose because of the COVID-19 pandemic (36.4%). COVID-19 as a reason for delay was provided as a free-text response by survey respondents, and we are unable to discern in what way(s) the pandemic resulted in nonreceipt of a second dose (eg, participant or caregiver COVID-19 infections or health system service disruptions). These results indicate that patient education surrounding HPV vaccination needs to include information about the 2-dose schedule and the importance of completing the series. The authors of previous studies have also highlighted that provider recommendations and education around the completion of the full HPV vaccine series can result in improved rates of completion.^26–28  The responses reported in our study further highlight the degree to which the COVID-19 pandemic and other large-scale interruptions may affect vaccine series completion.

This study provides early results that may help inform public health decision-makers. The Advisory Committee on Immunization Practices states that HPV vaccine schedules can be completed if interrupted without needing to be restarted.⁹  This recommendation was based on limited data under the assumption that such situations would be the exception. However, studies have subsequently revealed that many individuals do not complete HPV vaccination regimens,²⁹  suggesting that administering the second dose after an extended delay (ie, >1 year) may be beneficial for several reasons. These include facilitating catch-up vaccinations to increase vaccination completion rates in the setting of COVID-19 pandemic disruptions,^12,13,30  increased time to complete the series,^31,32  and expanding HPV vaccination programs in low- and middle-income regions in which many individuals may not complete vaccine regimens.^33,34 

The strengths of this study include a racially and ethnically diverse population, which included boys and girls across a wide range of ages and time intervals between vaccine doses. Participants were enrolled from a single health care delivery system with robust vaccination records. Relative to previous studies with a similar design,^18,19  this cohort included a large number of participants. The immunogenicity evaluation in this study was the same as that used throughout the 9vHPV vaccine clinical development program; therefore, results can be interpreted in the context of previous clinical trials of the 9vHPV vaccine.

Finally, the use of the same vaccine for both the first and second doses enabled the evaluation of antibody responses to each of the 9 vaccine-targeted HPV types.

This study had limitations. The small sample size could have resulted in random variation in antibody responses. However, a previous study revealed that GMT and seropositivity at 12 months after 1 dose did not differ by age, consistent with the data reported in this current study.⁵  There were likely to be additional unmeasured confounders associated with the time interval between doses, which we were unable to account for given the observational study design. We were also unable to differentiate between individuals who were HPV exposed or unexposed before dose 1 of the 9vHPV vaccine in the immunogenicity analysis by HPV type. However, given the age when they received their first dose, it is unlikely that participants had previous exposure to HPV. In addition, we only evaluated intervals of up to 53 months between the first 2 doses, limiting inferences regarding even longer time intervals. Although the sample size was small, the safety profile we describe with extended interval dosing was consistent with previous studies.^5,6  Finally, the survey participants only included those who consented to enroll and therefore may not be fully representative of all individuals who do not receive the second 9vHPV vaccine dose per recommendations.

In summary, this study provides early results supporting a regimen that extends the interval between doses of the 9vHPV vaccine. Ongoing cohorts from this study will provide rigorous demonstration of the noninferior immunogenicity of 2-dose regimens in girls and boys, compared with 3-dose regimens in young women.

The authors thank all study participants, investigators, and contributors. Medical writing assistance, under the direction of the authors, was provided by CMC AFFINITY, a division of IPG Health Medical Communications, in accordance with Good Publication Practice (GPP 2022) guidelines.

9vHPV

9-valent human papillomavirus

AE

adverse event

cLIA

competitive Luminex^® immunoassay

COVID-19

coronavirus disease 2019

GMTs

geometric mean titers

HPV

human papillomavirus

IgG LIA

Immunoglobulin G Luminex assay

KPNC

Kaiser Permanente Northern California

log

logarithm

PPI

per-protocol immunogenicity

qHPV

quadrivalent HPV