Yesterday, Income Investor pick GlaxoSmithKline (NYSE:GSK) announced that the FDA had issued a complete response letter for its human papillomavirus (HPV) vaccine Cervarix -- a sign that the agency has completed its review, but still has questions.

Because of its large-cap pharma size, Glaxo provided few details about any additional information the FDA might want on Cervarix. Most importantly, the company's brief press release said nothing about whether the FDA simply wanted some final labeling changes, or a whole other clinical study with the drug.

If the delay owes to something as minor as labeling changes -- a good possibility, given the FDA's recently acknowledgment of its major staffing issues -- Cervarix's U.S. approval could only be a few months away. But extra trials or manufacturing concerns could spell a much longer delay.

HPV is the leading cause of cervical cancer in women. Cervarix has been found effective at preventing the most common types of HPV, strains that can cause cervical cancer in as many as 100% of some subgroups of women. In September, the vaccine was approved for sale in the European Union. Considering Cervarix's strong efficacy and immunogenicity, and the European Union's lack of problems with Glaxo's manufacturing practices, I'm guessing the holdup here in the U.S. is probably a more minor factor.

I could be wrong, though. More than one drug has gotten past the European Medicines Agency -- like Sanofi-Aventis' (NYSE:SNY) Zimulti or Novartis' (NYSE:NVS) Galvus, to name a few -- only to be denied by the FDA. With Merck's (NYSE:MRK) competing HPV vaccine Gardasil already approved in the U.S., the agency faces less urgency to rush Cervarix to the market.

This round of the HPV vaccine wars clearly goes to Merck. But both drugmakers have more up their sleeves, as Merck tries to expand the patient population for Gardasil, and Glaxo tests Cervarix in head-to-head studies against the rival vaccine. But while Glaxo taps its feet waiting on the FDA, Merck will be adding to the $925 million in sales that Gardasil has generated in the U.S. this year.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. A quick shot of the Fool's disclosure policy is good for your financial health.