Gardasil 9 — Single-Dose Evidence & Dosing Schedule

The Vaccine

Gardasil 9 (Merck) is the only HPV vaccine currently available in the US (since 2014, FDA-approved). It is a nine-valent L1 VLP vaccine targeting:

HPV Types	Disease Association
6, 11	Genital warts
16, 18	~70–80% of cervical cancers
31, 33, 45, 52, 58	Additional ~10–15% of cervical cancers

Combined, the vaccine offers up to ~90% protection against HPV-associated squamous cell cancers worldwide. (Harrison's, p. 5651)

Current Dosing Schedule (CDC / Harrison's)

Age at Series Start	Doses	Timing
< 15 years	2 doses	0 and 6–12 months
≥ 15 years	3 doses	0, 1–2, and 6 months
Immunocompromised (e.g., HIV/AIDS)	3 doses	0, 1–2, and 6 months

The rationale for 2 doses in younger adolescents is the more robust immunologic response in that age group — geometric mean concentrations (GMCs) after 2 doses are non-inferior to those after 3 doses. (Harrison's, p. 5653)

Single-Dose Schedule — Current Status

The official current recommendation remains 2 doses (age <15) or 3 doses (age ≥15), not a single dose. However, single-dose evidence has grown significantly:

Key Evidence

The KEN SHE trial (Kenya, NEJM 2022) showed a single dose of Gardasil 9 or bivalent HPV vaccine provided 97.5% vaccine efficacy against persistent HPV 16/18 infection over 18 months in women aged 15–20.
The IARC/WHO ESCUDDO trial (Costa Rica) and Costa Rica Vaccine Trial (CVT) long-term data showed durable protection from a single dose over 11+ years.
A WHO Strategic Advisory Group of Experts (SAGE) review (2022) concluded that a single dose delivers "comparable efficacy and effectiveness" to multi-dose schedules and recommended that a single-dose schedule can be used for girls aged 9–14 and women up to age 20 as an alternative.

WHO Position (2022 Update)

WHO now endorses a 1-dose or 2-dose schedule for girls/women 9–20 years as equivalent in efficacy, with the goal of improving global coverage in resource-limited settings. For individuals ≥21 years and immunocompromised persons, 2–3 doses remain recommended.

US (CDC/ACIP) Position

As of 2024, the US CDC/ACIP has not yet formally adopted a routine single-dose recommendation — the US schedule remains 2 doses (age 9–14) or 3 doses (age 15–26+). ACIP continues to review accumulating single-dose data.

Key Considerations

Best time to vaccinate: Before sexual debut; most effective at ages 9–12
Catch-up vaccination: Recommended up to age 26 for all; shared clinical decision-making for ages 27–45
Immunocompromised patients: Always require 3 doses regardless of age
Route: Intramuscular (deltoid or anterolateral thigh)
Contraindication: Severe allergic reaction to a prior dose or vaccine component (e.g., yeast allergy)

Bottom line: A single dose of Gardasil 9 appears highly effective (WHO-endorsed for girls 9–20 years), but in the US, the standard of care remains 2 doses for those starting before age 15 and 3 doses for those starting at age 15 or older. Single-dose protocols are most relevant in global public health contexts aiming to maximize coverage.

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