We're quickly transitioning from a battle to see which company will get an all-oral hepatitis C treatment approved to one in which the length of treatment will likely dictate which company wins the war.

Gilead Sciences (NASDAQ:GILD) fired off a solid warning shot over AbbVie's (NYSE:ABBV) bow on Thursday. The biotech released phase 2 data today, demonstrating that a combination of sofosbuvir and ledipasvir works extremely well, perhaps even better, when a third generic drug ribovarin is added.

Eight weeks after treatment, all 21 patients taking the three-drug regimen for just eight weeks had undetectable virus levels in their system. When just sofosbuvir and ledipasvir were given for eight weeks, 19 of 20 patients appear to be cured. The results look even better -- 19 out of 19 -- when the two-drug combination was given for 12 weeks, although the results measured the presence of virus just four weeks after treatment, so they could change if patients relapse.

The trial also tested hepatitis C patients who had failed a previous treatment. Both the two-drug and three-drug combinations taken for 12 weeks produced 95% interim cure rate, again measured just four weeks after treatment.

Current treatments, Vertex Pharmaceuticals' Incivek, which Johnson & Johnson sells overseas as Incivo, and Merck's Victrelis, require at least 24 weeks of treatment -- Vicrelis requires at least 28 -- and need to be taken with peginterferon, which must be injected. If patients don't respond initially, both drug regimens recommend continuing the treatments for a total of 48 weeks. That's nearly a year.

AbbVie has solid data testing a 12-week course of a four-drug combination -- ABT-450/r, ABT-267, ABT-333, and ribovarin -- where 99% of patients showed no detectable virus 12 weeks after treatment. But when that combination was tested for eight weeks, just 88% of patients had undetectable virus levels after 12 weeks.

The Gilead and AbbVie data isn't directly comparable, because they didn't measure the response at the same time after treatment; Gilead's number could go down a little if patients relapse. But given what we know now, it appears that Gilead's drug combination works better than AbbVie's when used for just eight weeks.

It's more than just a convenience factor for patients to take drugs for the shortest amount of time possible. Viruses mutate as they replicate, so knocking them down, and keeping them down, is really important for the likelihood of a cure. The longer a patient is taking a medication, the more likely he or she will stop, or miss a couple of doses, allowing the virus to mutate in a way that it becomes resistant to the medication.

Safety first
Treatment time will be important, but safety will still trump it. Gilead's data included around 20 patients per treatment group. AbbVie's was around 80. Neither is enough to say much about the long-term safety of the drugs.

We'll have to wait until the companies complete phase 3 trials of the drug combinations to declare a clear winner of this war. AbbVie has already begun phase 3 trials testing its combination for 12 weeks, and after seeing this new data, Gilead just announced it will test the two-drug and three-drug combination for eight weeks in a phase 3 trial.

Fool contributor Brian Orelli has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences, Johnson & Johnson, and Vertex Pharmaceuticals. The Motley Fool owns shares of Johnson & Johnson. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.