The short interest in biotech stocks is mixed, judging by the short interest changes between the September 14 and September 28 settlement dates. There were notable swings in shares sold short from Biogen Idec, Celgene and Gilead Sciences. Short interest had largely risen in the previous period.

We have tracked the key short interest changes in the following biotech and emerging pharma companies: Alexion Pharmaceuticals Inc. (NASDAQ: ALXN), Amgen Inc. (NASDAQ: AMGN), Arena Pharmaceuticals Inc. (NASDAQ: ARNA), Biogen Idec Inc. (NASDAQ: BIIB), Celgene Corp. (NASDAQ: CELG), Dendreon Corp. (NASDAQ: DNDN), Gilead Sciences Inc. (NASDAQ: GILD), Illumina Inc. (NASDAQ: ILMN), Onyx Pharmaceuticals Inc. (NASDAQ: ONXX), Pharmacyclics Inc. (NASDAQ: PCYC), Regeneron Pharmaceuticals Inc. (NASDAQ: REGN), Vertex Pharmaceuticals Inc. (NASDAQ: VRTX) and VIVUS Inc. (NASDAQ: VVUS). We also tracked the iShares Nasdaq Biotechnology (NASDAQ: IBB) against the group.

Alexion Pharmaceuticals Inc. (NASDAQ: ALXN) short interest fell 6.3% between September 14 and September 28 to 5.03 million shares. Days to cover dropped to less than five.

Amgen Inc. (NASDAQ: AMGN) saw short interest tick up 0.6% to 12.69 million shares. But the average daily volume was the highest it has been since July.

Arena Pharmaceuticals Inc. (NASDAQ: ARNA) short interest rose 4.4% to 47.69 million shares. That was the fourth consecutive increase. Short interest was 22.1% of the float.

Biogen Idec Inc. (NASDAQ: BIIB) shares sold short plunged almost 16% to 1.87 million, its lowest level this year. Days to cover declined to less than two. Short interest is less than 1% of the float.

Short interest in Celgene Corp. (NASDAQ: CELG) sank 18.8% to 4.91 million shares, its lowest level in a year. Days to cover fell to just under two.

Dendreon Corp. (NASDAQ: DNDN) short interest increased 6.4% to 45.83 million shares. This was 30.4% of the float. But days to cover dropped to less than nine from more than 11 in the previous period.

Gilead Sciences Inc. (NASDAQ: GILD) short interest jumped 26.3% to 26.05 million shares. Average daily volume was the highest it has been since February.

Illumina Inc. (NASDAQ: ILMN) saw short interest rise 7.7% to 27.75 million shares. That was the sixth consecutive period of increasing short interest. Short interest was 22.9% of the float.

Onyx Pharmaceuticals Inc. (NASDAQ: ONXX) shares sold short increased 8.5% to 5.98 million. Days to cover fell to about three. Short interest was 9.3% of the float.

Pharmacyclics Inc. (NASDAQ: PCYC) short interest retreated 8.4% to 4.49 million shares. That was the lowest level this year. Short interest was 8.2% of the float.

Regeneron Pharmaceuticals Inc. (NASDAQ: REGN) short interest fell about 4% to 4.98 million shares. Short interest was 6.7% of the float.

Short interest in Vertex Pharmaceuticals Inc. (NASDAQ: VRTX) grew 9.4% to 7.08 million shares, its highest level since June. Days to cover is less than four.

VIVUS Inc. (NASDAQ: VVUS) short interest was down 5.4% to 15.19 million shares. Days to cover fell from about 10 in the previous period to less than four. Short interest was 15.2% of the float.

To show how the sector has held up: the iShares Nasdaq Biotechnology (NASDAQ: IBB) ETF closed at $142.59 on the same date. Its current 52-week range is $92.15 to $148.54.

Trey Thoelcke

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