Short Report: Frontier Group short positioning continues gaining altitude

Welcome to this week’s installment of “The Short Interest Report” – The Fly’s weekly recap of short interest trends among some of the most widely followed high-short-float stocks. Using the data from our partner Ortex.com, which utilizes the latest information from stock lenders to estimate short interest changes for thousands of publicly traded companies, this report will screen for some of biggest changes in short interest as a percentage of free float and days-to-cover ratios while also considering the short interest data on some of the more volatile and heavier-traded names of the week. Based on the availability of data from Ortex, the report tracks the trading period that covers prior Friday through Thursday of this week, excluding holidays. As a basis of comparison for stocks discussed below, the S&P 500 index was flat, the Nasdaq Composite was flat, the Russell 2000 index was flat, the Russell 2000 Growth ETF (IWO) was down 0.4%, and the Russell 2000 Value ETF (IWN) was up 0.6% in the four-day trading session range through September 4.

SHORT INTEREST GAINERS

• Ortex-reported short interest in Frontier Group (ULCC) has climbed gradually from the 22% lows seen all the way back in late February, though this week, the rise in bearish expression has been particularly pronounced. Shorts as a percentage of free float on the stock rose from 32.5% all the way to 35.9%, the highest level since early 2024, while days-to-cover on the name rose 100 basis points to 7.1 – a near-match to 2025 highs of 7.2 seen in mid-August. Shares of the discount airline are up 54% from August 6 and were also up 10% over the past four sessions, though year-to-date, the stock is still down 27%.
• Ortex-reported short interest on Grindr (GRND) had declined from a record high of 111% over the prior two weeks, though this week, bears renewed their assault on the stock, boosting shorts as a percentage of free float from 99.7 to 109%, with days to cover also going up from 2.7 to 3.4 amid relatively subdued trading volume. Grindr, which is also at the top of our screen in terms of short interest percentage, has been a popular short since the start of June, when the short interest was at a far more benign level of 18%. Since then however, shares have fallen about 40%, supporting the bearish conviction. This week’s 4-session span saw Grindr lose about 4% to reach a four-week low.
• The short squeeze in shares of Sionna Therapeutics (SION) discussed here last week that was spurred by 45% post-earnings rally appears to have taken a breather as the sharp rise in the stock price has slowed. This week, shorts as a percentage of free float on the biopharmaceutical name rose from 21.5% to 23.7% while the stock price slipped about 10%. Investors are also exercising greater caution following a pair of conflicting sell-side research notes regarding the potential of the company’s SION-451 drug trial in cystic fibrosis – Raymond James, which launched the stock at Strong Buy this week, said Sionna has the best chance in the industry at breaking Vertex’s (VRTX) strong technological monopoly in the indication, whereas RBC’s neutral initiation cautioned that the existing treatments create a high bar and there are questions as to how meaningfully de-risking Sionna’s next set of clinical data in mid-2026 will be.

SHORT INTEREST DECLINERS

• Ortex-reported short interest in Wheels Up (UP) had peaked as high as 63% in early June but has since plummeted from those elevated levels, punctuated by this week’s sharp decline from 33% to about 20%. The stock has also seen a spike in trading activity since mid-August, driving down its days-to-cover to 2.4 from 3.0 in the four-day period covered and from the highs of 36.0 in the first week of May. Despite the reduced exposure by the bears, the on-demand private aviation company has continued to trade as a penny stock and saw some extreme volatility last month, more than doubling from mid-August but giving most of those gains back. This week, shares of Wheels Up were down 25%, though year-to-date, the stock is still up 21%.
• Ortex-reported short interest on TeraWulf (WULF) was down for the third consecutive week this week, slipping from 32.4% to as low as 29.4% – a two-month low – before ending the week at 30.1%. Bears are bailing as their expectations of mean reversion following a 60% spike in the stock price on August 14, when the company announced a 200+ MW, 10-year AI hosting agreement with Fluidstack along with a long-term ground lease at the Cayuga site, did not materialize. Shares are still up 63% relative to the pre-announcement levels, though this week, the stock slipped about 6% in the four-day period covered.