In terms of determining a drug's competitive profile and success in the marketplace, the most important traits are its efficacy and safety, but a close third is often its convenience in dosing. Early last week, MedImmune (NASDAQ:MEDI) announced that the U.S. Food and Drug Administration approved for marketing an improved formulation of its flu vaccine, FluMist.

FluMist is only one of a small number of flu vaccines on the market, with GlaxoSmithKline (NYSE:GSK), Novartis (NYSE:NVS), and Sanofi-Aventis (NYSE:SNY) as the other major players. Unlike the other vaccines being sold, it's a nasal spray rather than an injectable shot, which obviously appeals to patients with needlephobia. Earlier this week, the FDA gave the go-ahead for MedImmune to sell a version of the drug that's much more convenient to store and can be kept in refrigerators, rather than freezers. Lest this sound like a minor development, convenience in dosing is important not just from the patient's perspective, but also for doctors. Hard-to-preserve or -administer drugs often have limited markets, if similar competing products are available, and if time and money can be saved in prescribing the more convenient option.

Despite its benefit of not having to be injected, FluMist hasn't been a big hit financially so far, with sales of just $21 million in 2005 and only $16 million in the most recent third quarter that heralds the start of flu season (and a large portion of sales).

Regardless of the disappointing FluMist sales so far, MedImmune has been proactive in taking steps to expand the market for the drug. Besides finally getting approval to sell the much easier-to-store formulation of FluMist, MedImmune is waiting on possible approval to market the drug to children as young as one year of age, after having filed for approval to sell FluMist to this population back in July. FluMist is currently approved for use in individuals between the ages of 5 and 49, limits which hold back its sales potential.

FluMist is already approved for use in children as young as age 5, but MedImmune hopes getting the drug approved for younger kids will be a boon to sales, since young children are the group of patients most likely to favor FluMist over an injection with a needle. Getting FluMist approved in this population could finally allow FluMist to bring in a much more meaningful amount of sales to MedImmune, so investors should be mindful of the FDA's actions on the drug in the coming months.

GlaxoSmithKline is an Income Investor selection.

Fool contributor Brian Lawler does not own shares of any company mentioned in this article.