Last Thursday, British pharmaceutical powerhouse GlaxoSmithKline (NYSE:GSK) filed for United States FDA marketing approval of its human papillomavirus (HPV) vaccine candidate, Cervarix.

HPV is an especially insidious virus because some of its strains are the main cause of cervical cancer. Other strains are the cause of genital warts. If Cervarix gains regulatory approval in the U.S., it would be the second HPV vaccine on the market here, after Merck's (NYSE:MRK) Gardasil.

The sales opportunity for HPV vaccines is huge. Gardasil has been on the market in the U.S. since June of last year and in the European Union since September. After less than half a year of sales and with regulatory approval in 54 countries, Merck brought in Gardasil sales of $235 million in 2006. This  level of sales uptake is phenomenal for a vaccine product that costs less than $400 for a full course of therapy and that has a fairly restrictive marketing label at the moment.

Even with the delay in getting Cervarix to the market, GSK does not lose out on Gardasil sales -- it receives royalties on sales of the drug due to a settlement agreement over patent rights with Merck back in 2005. However, Cervarix should be on the market soon enough; GSK filed for EU marketing approval in March of last year, so Cervarix could receive EU regulatory approval in the first half of this year. U.S. approval is possible in the second half of the year if the vaccine gets the same priority review status at the FDA that Gardasil did.

In January, GSK initiated a clinical study comparing Cervarix to Gardasil to see which vaccine confers more protection against different strains of the virus, including the main two types of HPV that cause the majority of cervical cancer cases. If the trial is successful, GSK may be able to claim to have the vaccine that provides protection for the longest period of time.

Merck isn't resting on its laurels, though. Gardasil would still have the advantage of protecting against some of the strains of HPV that account for the majority of genital warts cases -- which would be a major selling point for the use of Gardasil in men -- which Cervarix does not do. In addition, Merck plans on filing an application to expand the Gardasil label to cover women older than 26 later this year and in men in 2008. It it looks like the HPV vaccine wars are just about to heat up.

GlaxoSmithKline is an Income Investor recommendation, and Merck is a former selection. Learn why with a free 30-day trial.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has a disclosure policy.