But no data shows that Gardasil offers direct protection against cancer. Instead, the vaccine has the potential to be protective based on its ability to prevent the persistent HPV infections that can be precursors to cancer.
"We can't demonstrate it directly yet but we believe it has the same cancer-protective effect in men and boys [as girls]," said Dr. Joel Palefsky, associate dean for Clinical and Translational Research at the University of California at San Francisco and co-lead investigator for Merck's study on Gardasil in males. "It's a public health intervention to offer to boys as well as girls."
Beyond its ability to reduce HPV transmission between males and females or prevent cancer, further unanswered questions about Gardasil may give experts pause.
For example, Gardasil has been proven effective in males for up to three years and in females up to five years, but not beyond.
"Just because we have data up to a point doesn't mean that the vaccine will stop working after that," said Anna Giuliano, chair of the department of Cancer Epidemiology and Genetics at the H. Lee Moffitt Cancer Center in Tampa, Fla. and co-leader of the Merck study on Gardasil. "That is true of every new vaccine."
But without such data, clinicians may question the optimal age at which to administer Gardasil, and if booster shots will be necessary.
And experts said the potential reaction from parents will be mixed.
"Boys may be interested as they go into high school and college," said Dr. Diane Harper, director of the Gynecologic Cancer Prevention Research Group at University of Missouri. "But I don't think parents are going to be at all interested."
On the other hand, Dr. Lauren Streicher, an obstetrician-gynecologist at Northwestern Medical School, who supports use of the vaccine, said she vaccinates boys off-label because some parents ask for Gardasil for their sons as well as their daughters.
"Parents on board with getting their daughters vaccinated will be equally on board with getting their sons vaccinated," said Streicher, adding that the opposite would be true as well, that parents with no interest in having their daughters receive Gardasil would not be likely to vaccinate their sons.
Recommendations from the FDA and organizations such as the CDC's Advisory Committee on Immunization Practices, which will discuss Gardasil at the next meeting in October, as well as the American Academy of Pediatrics and the American Academy of Family Physicians will be crucial as clinicians, public health officials and families decide whether to use Gardasil to vaccinate boys against HPV.
Zanga noted that further research from Merck will help move the vaccine in the appropriate direction.
"I'd like to see the manufacturer present at the outset what the limits of protection are -- whether this is a boostable vaccine, whether it will require more doses, or whether having gotten it once, will it never be protective again," Zanga said. "Before we push this [vaccine] and make it a routine part of preadolescent and adolescent immunizations for boys, we ought to know the answers to these questions."