Study: Two Doses of HPV Vaccine as Effective as Three

Cervical cancer, while no longer the leading cause of death by cancer for women in the U.S. according to the Centers for Disease Control and Prevention (CDC), is still a huge problem in areas across the globe without access to quality vaccinations and care.

Forty years ago cervical cancer—a disease in which cancerous cells spread by the human papilloma virus (HPV) grow in the tissue of the cervix—was the number-one cause of cancer death in women in the U.S. Due to Pap smears and proper care, the incidence rate has declined, according to the CDC. Today, girls and young women are often given an HPV vaccine to protect them against cervical cancer. In other parts of the world, however, they aren’t so lucky. 

Led by Dr. Simon Dobson, researchers from the University of British Columbia are looking for a way to reduce the cost of HPV vaccination treatment, simply by cutting back on the number of vaccines needed for protection. Currently, three vaccinations are normally administered. In a study published in the Journal of the American Medical AssociationDobson and his team studied the efficacy of just two doses and found that results could be just as protective.

“We have established that the immunogenicity of a 2-dose schedule at 0 and 6 months in girls 9 through 13 years of age is statistically noninferior for HPV-16 and HPV-18 to the immunogenicity in women receiving 3 doses, assessed 1 month after the final dose,” the study states. 

Who wouldn’t says yes to one less trip to the doctor and one less shot?

Studying the Effectiveness of Two-dose HPV Vaccinations

Over the course of the study period, 830 participants were randomly divided into groups that received either two or three doses of the HPV vaccine at zero and two months, or zero, two, and six months, respectively. One month after the last dose, researchers checked levels of the immune system's response to the vaccine and found that the two doses provide a comparable and just as protective response as the three doses.

The study took place at three different centers between 2007 and 2008, and was approved by Health Canada and an external monitoring board. Participants were otherwise healthy girls between the ages of 9 and 13 years or young women between the ages of 16 and 26 years and were limited to four or less total lifetime sexual partners. Participants were excluded if they were pregnant at the time of enrollment or vaccination, had a history of sexually transmitted diseases such as genital warts, or had previously received an HPV vaccine. The vaccinations were commercially available options, and were administered to the participants as they would have been in a non-study setting.

In the following two and a half years, the majority of participants in both groups retained antibodies for the HPV vaccinations, hinting at the longevity and effectiveness of a two-dose vaccination schedule, although the immunogenicity levels were found to be inferior to those on a three-dose schedule at 36 months. 

Timing, however, may be just as important as number of vaccinations. When compared to a general population of women, the efficacy and importance of vaccination for girls and women earlier rather than later becomes clear. “Both girls groups continued to maintain higher plateau levels of antibody at 36 months than women,” the study states. HPV vaccination for adult women may be too little, too late.

However, “the vaccine is thought to provide protection through the production of serum neutralizing anti-HPV IgG antibodies…and only small amounts of antibody need to be present,” the study authors wrote. “The clinically meaningful different between the 2- and 3-dose schedules for girls cannot yet be determined.”

So while further study is needed to confirm the results before it becomes standard practice, it’s possible that for adolescent groups, a two-dose schedule could be enough. 

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