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.

The short interest in biotech stocks is mixed, judging by the short interest changes between the October 15 and October 31 settlement dates. There were notable swings in shares sold short from Amgen, Biogen Idec, Celgene and Vertex. Short interest also was mixed in the previous period.

Alexion Pharmaceuticals Inc. (NASDAQ: ALXN) short interest fell 2.9% between October 15 and October 31 to 5.01 million shares. Days to cover dwindled to 2.3.

Amgen Inc. (NASDAQ: AMGN) saw short interest jump 12.8% to 15.01 million shares. That was the highest number of shares since March. Days to cover slipped to about three.

Arena Pharmaceuticals Inc. (NASDAQ: ARNA) short interest rose 7.4% to 50.94 million shares. That is the highest level this year. Short interest was more than 23% of the float.

Biogen Idec Inc. (NASDAQ: BIIB) shares sold short grew about 30% to 2.19 million, after declines in the previous two periods. Days to cover is about 1.5. But short interest is about 1% of the float.

Short interest in Celgene Corp. (NASDAQ: CELG) fell 11.5% to 4.66 million shares, its lowest level this year. Days to cover remains about two.

Dendreon Corp. (NASDAQ: DNDN) short interest ticked up 0.6% to 47.16 million shares. This was about 31% of the float. Days to cover is more than 14.

Gilead Sciences Inc. (NASDAQ: GILD) short interest increased 4.0% to 30.29 million shares. This was the fourth consecutive period of rising short interest.

Illumina Inc. (NASDAQ: ILMN) saw short interest rise 3.6% to 29.41 million shares. That was the eighth consecutive period of increasing short interest. Short interest rose to 24.3% of the float.

Onyx Pharmaceuticals Inc. (NASDAQ: ONXX) shares sold short declined 3.2% to 5.91 million. Days to cover is less than five. Short interest slipped to 8.9% of the float.

Pharmacyclics Inc. (NASDAQ: PCYC) short interest retreated 2.5% to 4.29 million shares. That was the lowest level this year. Days to cover fell from about 12 in the previous period to about eight.

Regeneron Pharmaceuticals Inc. (NASDAQ: REGN) short interest fell 3.6% to 4.99 million shares, on the highest average daily volume since July. Shares sold short ticked down to 6.7% of the float.

Short interest in Vertex Pharmaceuticals Inc. (NASDAQ: VRTX) shrank 16.2% to 5.59 million shares. That was its lowest level since July. Days to cover rose to almost 2.5.

VIVUS Inc. (NASDAQ: VVUS) short interest was 4.1% higher to 17.63 million shares. Days to cover rose to about six. Short interest rose to 17.6% of the float.

To show how the sector has held up: the iShares Nasdaq Biotechnology (NASDAQ: IBB) ETF pulled back 7.3% to $131.87 during that period. Its current 52-week range is $92.15 to $148.54.

Trey Thoelcke

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