Introduction
The 9-valent human papillomavirus vaccine (9vHPV; Gardasil 9) targets seven high-risk oncogenic HPV types—16, 18, 31, 33, 45, 52, and 58—and two low-risk types, 6 and 11, which are primarily associated with genital warts; together, these types account for approximately 90% of cervical cancers and a substantial proportion of other HPV-attributable malignancies and anogenital diseases. Since licensure, an expansive body of global cohort evidence has characterized the vaccine's immunogenicity, tolerability, and preventive efficacy across pediatric, adolescent, and adult populations. With more than 200 million Gardasil 9 doses distributed worldwide as of the FDA’s 2020–2024 postmarketing review period, and more than 300 million cumulative Gardasil 9 doses administered globally by the end of 2024 according to Merck China, post-licensure surveillance continues to refine the vaccine's risk-benefit profile 126. This review synthesizes current evidence for clinicians and researchers, drawing on Phase III trials, long-term follow-up cohorts, post-marketing surveillance databases, and emerging single-dose regimen data.
Immunogenicity Across Age Groups
Seroconversion and Geometric Mean Titers
One of the most consistent findings across global cohort studies is near-universal seroconversion following 9vHPV vaccination. Across five pooled Phase III trials enrolling 11,304 participants aged 9–26 years, seroconversion rates at one month after the third dose ranged from 99.6% to 100% for all nine vaccine HPV types, regardless of sex or geographic region 9. These findings were replicated in a large bridging immunogenicity study of 3,066 girls, boys, and young women aged 9–26 years, in which greater than 99% of all participants seroconverted for each vaccine type 6, and in male cohorts, where over 99.5% of both heterosexual men (HM) and men who have sex with men (MSM) aged 16–26 years achieved seropositivity 7.
A critical age-dependent immunogenicity gradient has been consistently observed. In the pooled Phase III analysis, anti-HPV-16 geometric mean titers (GMTs) were 7,159.9 milli-Merck units per milliliter (mMU/mL) in girls aged 9–15 years and 8,444.9 mMU/mL in boys of the same age, compared with only 3,159.0 mMU/mL in women aged 16–26 years—representing a two- to threefold immunogenicity advantage in younger vaccinees across all nine HPV types 9. This pattern has been demonstrated in Asian subgroup analyses spanning India, Japan, South Korea, Taiwan, Thailand, and Hong Kong, where seroconversion rates in girls and boys aged 9–15 years reached ≥98.8%, exceeding rates in adult women (≥97.9%), and GMTs were consistently higher in the younger cohort 11. In a dedicated Japanese cohort of 100 girls aged 9–15 years, 100% seroconversion was achieved for each HPV type at month 7, with antibody persistence documented through two years after the third dose 14.
Two-Dose and Single-Dose Immunogenicity
A noninferiority immunogenicity trial of 1,518 participants demonstrated that 2-dose regimens (administered 6 or 12 months apart) in girls and boys aged 9–14 years elicited antibody responses noninferior to the standard 3-dose schedule in young women aged 16–26 years for all nine HPV types 8. The DoRIS randomized controlled trial (RCT) long-term extension, evaluating 930 Tanzanian girls aged 9–14 years, provided the first randomized evidence that a single dose of 9vHPV sustains stable antibody responses through 60 months post-vaccination, with HPV16 seropositivity exceeding 99% and HPV18 seropositivity at 93% at month 60 2. Importantly, single-dose GMCs for both HPV16 and HPV18 remained stable from month 12 to month 60, whereas two-dose GMCs declined over the same interval—suggesting an immunological plateau effect following single-dose vaccination in young adolescents. Antibody avidity indices did not differ significantly between one-dose and two-dose recipients, indicating comparable antibody quality regardless of dosing schedule 2.
An inverse age-immunogenicity relationship also applies within the adult range: post hoc analysis of five Phase III studies revealed that GMTs after three doses were generally higher in participants aged 16–21 years than in those aged 22–26 years, and that MSM demonstrated lower GMTs than HM across all nine types 4.
| Population | Seroconversion at Month 7 | GMT Pattern |
|---|---|---|
| Girls/Boys 9–15 years | ≥99% | Highest (2–3x women) |
| Women 16–26 years | ≥97.9–99% | Moderate |
| Heterosexual Men 16–26 years | >99.5% | Noninferior to women |
| MSM 16–26 years | >99.5% | Numerically lower than HM |
| Single-dose, Girls 9–14 years | HPV16 seropositivity >99% and HPV18 seropositivity approximately 93% at month 60 | Stable antibody concentrations over 5 years |
Safety Profile
Local and Systemic Adverse Events
The safety profile of 9vHPV is well characterized across age groups and geographic regions. Injection-site adverse events (AEs)—predominantly pain, swelling, and erythema—are the most frequently reported reactions. In the large bridging immunogenicity study, injection-site AEs were reported in 81.9% of girls, 72.8% of boys, and 85.4% of young women, with the majority classified as mild to moderate 6. In the Asian subgroup (Study 002), injection-site AEs in children and adolescents ranged from 62.4%–85.7% across countries, with systemic AEs more frequent in Thailand (26.4%) but predominantly mild with a median duration of one day 11. Discontinuation rates due to AEs were rare across all cohorts—for instance, only 2 of 1,003 Asian 9vHPV recipients discontinued due to AEs 11.
Concomitant administration with routine adolescent vaccines—meningococcal conjugate vaccine (MCV4) and tetanus-diphtheria-acellular pertussis (Tdap)—did not compromise immunogenicity to any component and was not associated with serious vaccine-related AEs, though injection-site swelling was more frequent in the concomitant group 13.
Serious Adverse Events and Postmarketing Surveillance
The FDA's comprehensive postmarketing safety review covering June 2020 to September 2024 analyzed data from 42.5 million US doses and 202.5 million worldwide doses. Among pediatric patients under 18 years, 333 serious non-fatal US reports and 6 deaths were identified. Eight unique pediatric deaths were subject to detailed case review; etiologies were diverse and unrelated to vaccination (including acute demyelinating encephalomyelitis, anti-MOG antibody-associated disease, and metastatic cervical carcinoma), and no clustering or causal patterns were identified 1. Postural orthostatic tachycardia syndrome (POTS) was reported in 43 pediatric cases; however, no discernible causal pattern was detected, consistent with the European Medicines Agency’s 2015 assessment 1. The only regulatory labeling update during this period was a March 2025 supplement adding "injection-site nodule" to the Postmarketing Experience section 1. The multi-country long-term follow-up (LTFU) extension involving 1,272 vaccinees across 13 countries reported no vaccine-related serious AEs or deaths through a median 7.6 years of follow-up, further reinforcing long-term tolerability 5.
Preventive Efficacy and Durability of Protection
Efficacy Against Vaccine-Type Infection and Precancerous Lesions
Preventive effectiveness data from the per-protocol effectiveness population of the LTFU multi-country cohort are compelling: among girls vaccinated at ages 9–15 years, no cases of vaccine-type high-grade cervical, vulvar, or vaginal intraepithelial neoplasia were observed through a median 7.6 years post-vaccination 5. In young women (16–26 years), the 9vHPV vaccine prevented HPV-31/33/45/52/58–related high-grade cervical, vulvar, and vaginal dysplasia with 92.3% efficacy (95% CI, 54.4%–99.6%) in Latin American participants 10, and with 95.8% efficacy (95% CI, 87.8%–98.9%) against 6-month persistent infection with these types in the Asian subgroup 11. A systematic review of RCT evidence confirmed direct efficacy of 96.7% (95% CI, 80.9%–99.8%) against high-grade cervical, vulvar, or vaginal disease in women aged 16–26 years 15.
In HPV-positive women who had not previously been vaccinated, administration of the 9vHPV 3-dose regimen was associated with significantly higher HPV clearance rates: 72.4% achieved complete viral clearance versus 45.7% in unvaccinated controls (p < 0.001), with even higher clearance for vaccine-type genotypes (85.2% vs. 60.0%, p < 0.001). However, these findings should be interpreted cautiously because HPV vaccination is not indicated as treatment for existing HPV infection or HPV-related disease, and a therapeutic effect has not been established. 3.
Long-Term Durability
Seropositivity remained above 90% by competitive Luminex immunoassay (cLIA) for all nine vaccine HPV types at month 90 (approximately 7.5 years post-vaccination) in the global LTFU cohort 5. Single-dose immunogenicity data from the DoRIS trial document stable antibody concentrations through 60 months, supporting the broader evidence base behind WHO’s off-label alternative single-dose HPV vaccination schedule recommendations first issued in 2022 and reflected in later product-specific updates; implementation and eligibility should follow local guidance 2. Population-level evidence from countries with mature vaccination programs confirms substantial reductions in high-grade cervical lesions and HPV type prevalence, validating laboratory immunogenicity findings with clinical outcomes 2425.
Evidence Quality, Limitations, and Clinical Implications
The global cohort evidence base for 9vHPV is methodologically robust, anchored by multiple Phase III RCTs, pooled analyses exceeding 10,000 participants, and a multinational LTFU cohort spanning 13 countries. Nevertheless, several limitations warrant acknowledgment. Observational studies evaluating viral clearance in HPV-positive adults are susceptible to selection bias, given that 78.1% of eligible patients self-selected vaccination 3. Published immunogenicity data specifically from mainland Chinese populations remain limited, despite 33 million doses administered domestically; systematic population-level surveillance data have not been extensively published 19. Single-dose efficacy evidence relies substantially on immunobridging from multi-dose efficacy trials and observational cohorts rather than randomized efficacy endpoints, and the DoRIS trial's 9-year follow-up remains ongoing 2. Furthermore, MSM-specific efficacy data remain inferential, as GMTs—though high—are numerically lower in this population than in HM 74.
Collectively, these findings support a clear clinical strategy: vaccination should be initiated as early as age 9 to capitalize on superior immunogenicity in younger recipients. Two-dose schedules are recommended or approved for most immunocompetent individuals who initiate HPV vaccination at ages 9–14 years, while emerging single-dose data may support simplified regimens in settings where such schedules are endorsed by public health authorities. Catch-up vaccination remains recommended for older adolescents and adults through age 26 who were not adequately vaccinated earlier; in HPV-positive adults, vaccination may still help prevent infection with HPV types not yet acquired. Gender-neutral programs and concomitant administration with routine adolescent vaccines are safe and programmatically feasible, supporting integration into existing immunization schedules globally.