Only One Dose of HPV Vaccine Needed To Be Effective, Researchers Find

Study comes ahead of a key CDC advisory committee hearing, which will review whether current multi-dose recommendations should be updated.

Although Merck recommends multiple doses of their controversial Gardasil vaccine, new research suggests that the drug is more than 90% effective at preventing the human papillomavirus (HPV) after only one dose, raising questions about whether the HPV vaccine is overused among teens and young adults.

Gardasil was introduced in 2006, and is currently the only HPV vaccine available in the United States. It is recommended for girls ages 9 to 26 and boys ages 9 to 15, to help prevent HPV infections, which can lead to the later development of cervical cancer. Although another vaccine, Cervarix, has been developed, it is no longer marketed in the U.S.

Currently, two doses of the Gardasil vaccine are recommended for full effectiveness. However, amid growing safety concerns about the potential side effects of Gardasil vaccines, researchers and healthcare professionals have begun to question whether a single dose could offer similar efficacy with fewer potential health risks.

Although Merck has promoted the vaccine as safe and effective, with few serious side effects, the manufacturer now faces hundreds of Gardasil lawsuits brought by individuals throughout the U.S., each raising similar allegations that the drug maker failed to disclose the risk of orthostatic tachycardia syndrome (POTS), premature ovarian insufficiency (POI) and other autoimmune disorders.

In this new study, a group of researchers from the National Cancer Institute conducted a clinical trial involving 20,000 girls between the ages of 12 and 16 to determine the effectiveness of a single dose of both Gardasil and Cervarix. Their findings were introduced at the American Association for Cancer Research annual meeting, which was held from April 25 through 30 in Chicago.

The study involved the teen girls randomly being assigned one of the two HPV vaccines, and randomly being chosen to receive one or two doses. According to the findings, a single dose of either vaccine exceeded 97% effectiveness, suggesting only one dose is actually necessary.

The findings have not yet been submitted for publication and peer review, but come ahead of a meeting by the CDC’s Advisory Committee on Immunization Practices (ACIP), which plans to vote on whether to recommend single-dose HPV vaccinations.

Gardasil Litigation Status

Merck & Co currently faces more than 200 product liability lawsuits, each raising similar allegations that the drug maker failed to adequately warn about the potential long-term health risks associated with the Gardasil HPV vaccine.

Since August 2022, these cases have been centralized as part of a federal Gardasil multidistrict litigation (MDL), which is assigned to U.S. District Judge Kenneth D. Bell, Sr. in the Western District of North Carolina for coordinated discovery and pretrial proceedings.

The litigation primarily involves claims that Gardasil caused serious autoimmune complications, such as postural orthostatic tachycardia syndrome (POTS) and premature ovarian insufficiency (POI). However, in March 2025, Judge Bell granted summary judgment in favor of Merck on many of these failure-to-warn claims, ruling that federal law preempts certain state-based legal theories related to vaccine warnings. This decision is currently being appealed to the U.S. Court of Appeals for the Fourth Circuit.

While the appeal is pending, plaintiffs’ attorneys have called for other claims involving different injuries to be paused, or stayed, until the appellate court provides guidance on how Judge Bell’s ruling should be interpreted more broadly. These additional lawsuits involve nearly 100 plaintiffs who allege that Gardasil caused a wide range of other injuries unrelated to POTS and POI.

Despite the ongoing litigation, Gardasil lawyers continue to investigate new injury claims involving side effects that may have developed shortly after receiving the vaccine, including: