GlaxoSmithKline’s FDA approval for Cervarix, received last week as rival Merck expanded its Gardasil label, heralds a market-share battle that may hinge on science.

Cervarix is approved to protect women and girls 10-25 from cervical cancer and precancerous lesions due to HPV types 16 and 18, but the vaccine also guards against HPV types not included in the vaccine. “The vaccine’s impact against the overall burden [of] cervical disease…will be critical,” said a spokesperson.

The drugmaker is looking for an edge against Gardasil, which has been on the US market for the same indication since 2006 and was just approved to prevent genital warts among men and boys. “We are not pursuing that,” the spokesperson said with regard to an indication for treating males.  “GSK is committed to providing a vaccine specifically designed to protect girls and young women.”

Whether Cervarix’s collective efficacy in females matters to patients or doctors remains an open question. One analysis showed the vaccine to be 89% effective against HPV type 31, the third most common cancer-causing virus type in North America. Gardasil has not been demonstrated to protect against diseases due to HPV types not contained in the vaccine, Merck’s website states.

The next step for both vaccines will be a late-October meeting of the Centers for Disease Control and Prevention Advisory Committee on Immunization Practices, to review Cervarix for girls and Gardasil for boys. The committee, which sets national guidelines, could have an effect on sales of both vaccines.