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Prothena Corporation

Nasdaq • PRTA • IE00B91XRN20

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About this data: PRTA short interest, cost to borrow and days to cover

This page tracks how heavily Prothena Corporation (Nasdaq:PRTA) is being shorted: ORTEX's daily short interest estimate, the cost to borrow the shares, utilization of the lendable supply and days to cover, next to the official exchange-reported figure.

Short interest
The number of shares that have been sold short and not yet bought back, usually shown as a percentage of free float (the shares available to trade). ORTEX estimates it every day from securities-lending data, instead of waiting for exchange reports that are published only twice a month and about two weeks late.
Cost to borrow
The annualised fee a short seller pays to borrow the shares, as a percentage of the position's value. A rising cost to borrow means lendable shares are getting scarce and holding the short is getting more expensive.
Utilization
The share of the lendable supply of a stock that is currently on loan. Close to 100% means almost no shares are left to borrow.
Days to cover
Short interest divided by average daily trading volume: roughly how many days of normal trading it would take for every short seller to buy back their shares.
Exchange-reported short interest
The official figure published by the exchange or regulator (in the US, FINRA twice a month). It is accurate but delayed, which is the gap the ORTEX daily and intraday estimates fill.

Source: ORTEX. Some figures are limited on the free view; live data and full history are part of ORTEX Advanced.

PRTA key data

Prothena Corporation (Nasdaq: PRTA) has a market capitalization of $435.85M. Last close $8.51 (as of 2026-10-21); 52-week range $7.73 to $11.80. ORTEX data as of 2026-10-19: short interest is 23.58% of free float, 14.5 days to cover and the average cost to borrow is 0.59%.