What is PRTA's short interest?

8,544,664 shares, as of the 15 September 2026 settlement date. That is the figure FINRA published, unadjusted.

What percentage of the float is short?

We do not publish that, because FINRA does not publish float. The regulator reports the number of shares held short; turning it into a percentage needs a share count from somewhere else, and different sources disagree about what counts as float. We would rather give you the number that was actually filed than a percentage built on an assumption.

It is 21 days old, and that is as fresh as short interest gets. FINRA measures it twice a month and publishes it seven business days after the settlement date. Any site showing you a "live" short interest figure is showing you something else.

What does days to cover mean?

It is the short position divided by average daily trading volume — here, 26.53. It describes how many days of ordinary trading the position is equivalent to. It is arithmetic from the filing, not a prediction about what will happen.

Where does this come from?

FINRA's consolidated short interest file, which is public. We read it directly and store it unchanged, except that we blank the days-to-cover placeholder FINRA writes when a stock has almost no volume.

Ticker

PRTA

Prothena Corporation plc

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PRTA in the last 90 days

Prothena Corporation plc (PRTA) had no US House transactions and 7 insider purchases worth $4,157,395 by 1 insider in filings from the last 90 days.

FINRA’s latest report (settlement 15 SEPT 2026) shows 8,544,664 shares held short, 26.5 days to cover, up 0.6% from the report before.

Insiders: open-market purchases only (Form 4 code P); a purchase filed by related owners counts once.
Delay: each filing below shows how many days passed between the trade and its publication. None of this is real-time.

Latest filing on file: 30 SEPT 2026 · FINRA settlement 15 SEPT 2026 · public filings, reported as filed

What the filings show

What the filings show · last 90 days · Quiet Money indicator

2 of 4 sources

+13Balanced

Leaning outBalancedLeaning in

The disclosed money is split, on what has actually been filed. This reads on two of the four sources we track.

Short interest: Short interest rose 4.2% — 8,196,964 to 8,544,664 shares between 15 JUL 2026 and 15 SEPT 2026. See the filings →; -11
Insider purchases: 1 insider disclosed buying $4,157,395 across 6 filings.; +46

This summarises filings that have already been published — it is not a forecast, a rating of the company, or advice. Every figure above is listed in full further down this page, with the document it came from.

FINRA short interest

Settlement 15 SEPT 2026 · shares held short

8,544,664shares held short

Days to cover

26.53

Avg daily volume

322,027

Since last report

+0.58%

As of 15 September 2026, 8,544,664 Prothena Corporation plc shares were held short — the total across every reporting firm, not one fund's position. That is 48,868 shares more than the previous report, a 0.58% increase. At the average daily volume of 322,027 shares, that position represents 26.53 days of trading — what the filing calls days to cover.

This is the most recent figure that exists, and it is 21 days old. FINRA collects short interest twice a month, on a settlement date, and publishes it seven business days later. So nobody has a live short-interest number for PRTA — not us, and not anyone quoting one. We print the settlement date on the figure so you always know which day you are looking at.

Across the 12 reports we hold, the position has risen from 7,016,080 shares on 31 March 2026 to 8,544,664 today, a change of 21.8%. The highest reading in that window was 8,545,127 shares on 14 August 2026.

Settlement	Shares short	Days to cover
15 SEPT 2026	8,544,664	26.53
31 AUG 2026	8,495,796	24.85
14 AUG 2026	8,545,127	19.56
31 JUL 2026	8,172,438	13.96
15 JUL 2026	8,196,964	16.40
30 JUN 2026	8,369,568	11.02
15 JUN 2026	7,866,857	15.98
29 MAY 2026	7,737,902	13.85
15 MAY 2026	7,097,854	12.96
30 APR 2026	6,871,465	18.33
15 APR 2026	7,076,832	15.33
31 MAR 2026	7,016,080	12.32

What is PRTA's short interest?: 8,544,664 shares, as of the 15 September 2026 settlement date. That is the figure FINRA published, unadjusted.
What percentage of the float is short?: We do not publish that, because FINRA does not publish float. The regulator reports the number of shares held short; turning it into a percentage needs a share count from somewhere else, and different sources disagree about what counts as float. We would rather give you the number that was actually filed than a percentage built on an assumption.
How current is this?: It is 21 days old, and that is as fresh as short interest gets. FINRA measures it twice a month and publishes it seven business days after the settlement date. Any site showing you a "live" short interest figure is showing you something else.
What does days to cover mean?: It is the short position divided by average daily trading volume — here, 26.53. It describes how many days of ordinary trading the position is equivalent to. It is arithmetic from the filing, not a prediction about what will happen.
Where does this come from?: FINRA's consolidated short interest file, which is public. We read it directly and store it unchanged, except that we blank the days-to-cover placeholder FINRA writes when a stock has almost no volume.

Disclosed insider buying on file

$4,157,395across 7 purchases

Open-market purchases only — the amount is a fact, not a verdict.

On file here

Short positions0

Insider purchases7

US House transactions0

Latest change30 SEPT 2026

SHORT POSITIONS · 0INSIDER BUYS · 7US HOUSE · 0

Insider purchases on file

SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$1,405,674
162,000 sh @ $8.677
29 SEPT 2026 → filed 30 SEPT 2026
Filed 1 day after the tradeSEC EDGAR · 0001045463-26-000018 →
SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$889,872
103,500 sh @ $8.598
28 SEPT 2026 → filed 29 SEPT 2026
Filed 1 day after the tradeSEC EDGAR · 0001045463-26-000017 →
SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$843,460
100,000 sh @ $8.435
18 SEPT 2026 → filed 21 SEPT 2026
Filed 3 days after the tradeSEC EDGAR · 0001045463-26-000016 →
SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$255,739
27,000 sh @ $9.472
02 SEPT 2026 → filed 03 SEPT 2026
Filed 1 day after the tradeSEC EDGAR · 0001045463-26-000015 →
SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$439,430
50,000 sh @ $8.789
20 AUG 2026 → filed 21 AUG 2026
Filed 1 day after the tradeSEC EDGAR · 0001045463-26-000014 →
SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$30,167
3,500 sh @ $8.619
07 AUG 2026 → filed 10 AUG 2026
Filed 3 days after the tradeSEC EDGAR · 0001045463-26-000010 →
SEC — Form 4 insider purchase
SCULLY WILLIAM P10% owner
BOUGHTOrdinary Shares, par value $0.01 per share
$293,053
34,000 sh @ $8.619
07 AUG 2026 → filed 10 AUG 2026
Filed 3 days after the tradeSEC EDGAR · 0001045463-26-000010 →

What this page will never showNo buy or sell ratingsNo price targetsNo forecastsIndicators are rules-based summaries of the filingsDocuments, dates, and how long they took to become public.