Nothing like an after-hours plunge in a stock. Dendreon shares fell as much as 23 percent this evening after the Centers for Medicare & Medicaid Services announced it is reviewing the prostate cancer vaccine to determine whether national coverage is “reasonable and necessary,” but a final decision won’t be made for an entire year. The stock later regained some of its losses to close at $26.69, but remains well below its 52-week high of $57.67 on May 3.

The agency will take public comments through July 30 “on the evidence regarding the effects of this treatment on health outcomes in patients with prostate cancer,” according to a statement, adding that it is “particularly interested in clinical studies and other scientific information relevant to the subject under review.” A meeting of the Medicare Evidence Development and Coverage Advisory Committee will be held later this year.

Obviously, if CMS covers the vaccine, sales will increase because the number of eligible patients would presumably increase, prompting private insurers to also provide coverage. Aetna, for instance, recently agreed to provide some coverage. By the same token, though, a CMS denial would likely dampen revenue, which Wall Street is already estimating could reach several billion dollars a year. The vaccine is priced at $93,000 a year per patient, well above initial estimates.

In a statement this evening, Dendreon writes: “The CMS National Coverage Analysis “is not a change in Medicare coverage policy. NCAs do not impact existing coverage decisions, nor do they restrict local Medicare contractors from covering Provenge. Therefore, Medicare beneficiaries are still able to access Provenge and private payers can also still cover Provenge.”