Short Interest, Days to Cover, and Why Short Sellers Are the Third Expert Signal in Biotech

In our breakdown of Verdad Capital's 2026 white paper, we covered two of the three "expert" signals that predict biotech returns: specialist fund ownership (Part 1) and insider trades (also Part 1). But Verdad identifies a third signal that's equally powerful: short interest. Their data shows the most-shorted biotech quintile returned -19% annualized, while the least-shorted returned +16% — a 35-percentage-point spread. Short sellers, Verdad writes, are "equally valuable to biotech specialist funds and company insiders" as guides through the sector. This article unpacks the academic research behind short interest as a signal, explains the key metrics (short interest ratio, days to cover, short % of float), and shows how BiotechEdge now incorporates this data alongside fund holdings and insider trades.

Short interest is the total number of shares that have been sold short but not yet covered (bought back). When an investor shorts a stock, they're borrowing shares and selling them, betting the price will fall so they can buy them back cheaper. Short interest matters because short sellers are typically sophisticated — they do deep research, and they're putting capital at risk with unlimited downside. Unlike a long investor who can simply hold and wait, a short seller pays borrowing costs every day and faces margin calls if the stock rises. This creates strong incentive to be right. Three metrics capture different dimensions of short interest: Short Interest (raw shares) — The absolute number of shares sold short. Useful for tracking changes over time, but meaningless without context about the stock's float and volume. Short % of Float — Short shares divided by the total tradeable shares (float). A stock with 20% of its float sold short has meaningful bearish positioning. Above 30% is extreme. Days to Cover — Short shares divided by average daily volume. This measures how many days it would take all short sellers to buy back their shares at normal trading volume. This is the metric that matters most — and the one with the strongest academic backing.

The relationship between short interest and future returns has been studied extensively since the 1990s. The key papers: Desai et al. (2002) studied short interest on Nasdaq stocks from 1988-1994 and found that heavily shorted stocks exhibited persistent negative abnormal returns for up to 12 months. The effect was strongest for stocks with the highest short interest ratios, even after controlling for size, book-to-market, and momentum. Asquith, Pathak, and Ritter (2005) confirmed these findings with a broader dataset and showed that the predictive power of short interest is strongest when combined with institutional ownership data — exactly the combination BiotechEdge now provides. Hong, Li, and Ni (2015) published the definitive paper on days to cover specifically, titled "Days to Cover and Stock Returns." Their key finding: days to cover is a better predictor of poor stock returns than the short interest ratio alone. A long-short strategy based on days to cover generated approximately 1.2% monthly returns. They argue that days to cover captures both the intensity of bearish conviction (short shares) and the liquidity constraint (how hard it is to exit the short), making it a more complete signal. Dechow, Hutton, Meulbroek, and Sloan (2001) added another layer: short sellers tend to target stocks with poor fundamentals — low earnings quality, inflated valuations, and deteriorating business metrics. In other words, shorts aren't just betting on price direction; they're doing fundamental analysis and reaching bearish conclusions that the market hasn't yet reflected.

The general academic findings on short interest are amplified in biotech for several reasons: Binary outcomes make shorts more deliberate. Shorting a biotech ahead of a Phase 3 readout or PDUFA date is an explicit bet that the catalyst will fail. Unlike shorting an overvalued tech stock where the thesis might take years to play out, biotech shorts often have defined timelines — making the signal more interpretable. High borrow costs filter for conviction. Biotech stocks, especially small caps, are often expensive to borrow. A short seller paying 10-30% annualized borrow fees is expressing extreme conviction. The cost alone filters out casual or speculative shorts. Biotech has more zeros. In most sectors, a stock going to zero is rare. In biotech, it's common — a failed Phase 3 trial can destroy 80%+ of market cap overnight. Short sellers are specifically trying to identify which companies are heading toward those zeros, and their track record (per Verdad's data) suggests they're good at it. The combination with specialist fund data is unique. When short sellers are heavily short a stock that no specialist biotech funds own, the signal is reinforced from both sides. Conversely, when specialist funds are buying and shorts are covering, you're seeing two independent expert groups converge on a bullish view.

BiotechEdge now displays short interest data on every company page, updated twice monthly from FINRA settlement data. Here's how to interpret each metric: Short Shares — The raw count of shares sold short. Watch for large changes between periods (the "Change %" metric) more than the absolute number. Change % — How short interest changed from the prior reporting period. A 20%+ increase means shorts are piling in. A 20%+ decrease means shorts are covering — potentially a bullish signal, especially if specialists are simultaneously adding positions. Days to Cover — The key metric per the academic research. Our color coding: - <5 days (default) — Normal. Not much to read into. - 5-10 days (amber) — Elevated. Worth noting alongside other signals. - >10 days (red) — High short interest relative to volume. This is the zone where academic research shows the strongest predictive power for negative returns. Composite badges combine short interest with specialist fund data: - "High short interest · No specialist buying" — Bearish alignment. Both expert groups are negative. - "Shorts covering · Specialists adding" — Bullish convergence. Shorts are retreating while smart money enters. - "Specialists adding · Low short pressure" — Clean bullish signal. Smart money is buying without meaningful short opposition.

With short interest, BiotechEdge now tracks all three expert signals identified in Verdad's research: 1. Specialist fund holdings (13F filings) — Who's buying? Which funds have conviction? Where are multiple funds converging? 2. Insider trades (Form 4 filings) — Are the people closest to the science buying or selling? Is it routine or conviction-driven? 3. Short interest (FINRA data) — What do the bears think? How intense is the short positioning? Are shorts covering or adding? Each signal is informative on its own. Combined, they form a composite quality measure that — per Verdad's research — spreads biotech returns by 35+ percentage points annually. The runway screener page now includes a "Days to Cover" column, and the weekly email digest highlights notable short interest changes. The data updates twice monthly with each FINRA settlement cycle.