IBB Shorts Hold Firm as Biotech ETF Rebounds

IBB enters the final day of October with an unusual split: the ETF has gained nearly 5% on the week to $217.61, yet short sellers have barely flinched, keeping their positions close to a one-month high.

The persistence of that short positioning is the week's central tension. Short interest has crept up 0.9% over the past seven days to 23.2% of free float, and is now 5.4% above where it stood a month ago. These are not marginal moves. For context, the FINRA fortnightly figure put settled short interest at 11.57 million shares as of October 15, with days-to-cover running at 5.5, meaning it would take more than a week of average volume to unwind the entire short book. Despite a 5% rally in the underlying, that book has not meaningfully shrunk.

The borrow market adds a layer of interest. Cost to borrow has jumped 30% over the past week to 2.45%, reaching its highest point since early October. Availability has loosened somewhat, running at 273% relative to current short interest, meaning there are roughly 2.7 shares available for every one already borrowed. That is well off the tightest level seen in the past 52 weeks, when availability compressed to just 1.3%, but the cost move suggests demand for fresh borrows is picking up even as the ETF climbs. Short sellers are paying more to maintain positions they are not abandoning.

Options positioning reinforces the cautious mood. The put/call ratio has drifted up to 1.51, about one standard deviation above its 20-day average of 1.36. That is not yet an extreme reading relative to the 52-week range of 0.77 to 3.54, but it has been grinding higher through October: the PCR was below 1.10 at the start of the month and has risen steadily as the ETF rallied. That pattern, rising put demand alongside rising prices, points to investors buying downside protection into strength rather than chasing the move.

The ORTEX short score, a composite measure of short-side pressure, is holding in the high-60s range this week, peaking at 70.8 on October 16 before easing slightly to 69.2. The score has been largely flat over the ten-day window, suggesting the short-side conviction is stable rather than building into a crescendo. On the holder side, the most notable institutional change as of June 30 (the latest available filing) was Two Sigma adding over a million shares, while Citigroup trimmed by roughly 362,000. Analyst data for IBB is too stale to be actionable, the most recent price target on record dates to 2008.

What to watch next: whether cost to borrow continues rising into November even as the ETF holds its gains, and whether the put/call ratio breaks above its recent range toward the 2.0 level that would mark a more decisive shift in options sentiment.