GlaxoSmithKline’s ViiV Healthcare already boasts an approval for Dovato, a two-drug HIV regimen, in newly diagnosed patients. And it just added some new data that aims to at least draw level with market leader Gilead in already-treated patients.

Six months after patients switched to Dovato from a triple-drug regimen that contains Gilead’s Vemlidy (TAF), the ViiV drug was as effective at suppressing HIV as the triple-drug regimens were for who stayed on their original drugs and no Dovato patients developed treatment resistance, the company said Wednesday.

The top-line data from the Tango study showed that virally suppressed people can switch from TAF-containing triplets to Dovato and therefore reduce the number of medicines they’re exposed to, ViiV’s head of global research and medical strategy, Kimberly Smith, M.D., said in a statement.

However, the concern among industry watchers has been around the two-drug approach’s long-term efficacy since it uses only one nucleoside analog, or nuke, that stops HIV from replicating, rather than the traditional two. ViiV is determined to prove its case to that end. The Tango study will continue to follow patients and compare the two groups’ viral loads through week 144.

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Still, data from a controlled clinical trial might not be enough to change the treatment paradigm. Many clinicians worry that more patients could miss doses in the real world, and when that happens, patients on dual regimens like Dovato might develop resistance more easily than those on triple combos.

In a recent survey, Credit Suisse conducted in 30 U.S.-based doctors, Gilead’s TAF-based three-drug regimen Biktarvy turned out to be the most preferred therapy for anticipated use over the next two years in both treatment-naïve and switch patients.

“While Dovato (dolutegravir + lamivudine) has comparable efficacy and safety in a clinical trial setting, physicians generally preferred a three-drug single-tablet regimen like Biktarvy, as respondents demonstrated concern over disease resistance in two-drug regimens,” CS analyst Evan Seigerman said in a Wednesday note. Because of “a clear preference for Biktarvy” and a “modest” enthusiasm for ViiV’s doublets, “Gilead is well-positioned to maintain market share in HIV,” he concluded.

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New IQVIA U.S. prescription data also provides a glimpse into the hard time ViiV is having with its dual-drug regimens, including Dovato and Juluca, which is approved for previously treated patients. Gilead’s and ViiV’s HIV new-to-brand market shares were “very stable” in the second quarter, Credit Suisse analysts said in a recent investor note. Currently, Gilead holds a majority 73% new-to-brand share, while ViiV only has 16%. Both Juluca and Dovato—both containing Tivicay (dolutagravir)—are gradually growing, but that growth isn't coming at Gilead's expense.

RBC Capital analyst Brian Abrahams’ team, also tracking IQVIA numbers, noted that Dovato scripts are running at less than one-sixth the rate that Biktarvy enjoyed in its ninth full week of launch, “reaffirming our view that GSK’s dual regimen is unlikely to be a significant competitor to Gilead’s HIV franchise,” he said in a recent report.

In the first quarter, ahead of Dovato’s U.S. go-ahead, Juluca generated sales of £70 million ($87.5 million), only £8 million more than its Q4 total. In comparison, Biktarvy chipped in $739 million to Gilead’s Q1 U.S. sales after just a year on the market.