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Updated from 11: 46 a.m. EDT

Merger and acquisition news emanating from the pharmaceutical sector boosted many biotech and health care indices to start the week.

Vaccine maker

MedImmune

( MEDI) said it will be acquired by pharmaceutical-maker

AstraZeneca

(AZN) - Get Astrazeneca PLC Sponsored ADR Report

for more than $15 billion. MedImmune surged 17.8% to $56.57, while AstraZeneca slumped 5.3% to $55.94.

The jump in MedImmune's shares powered the Amex Biotechnology Index by 1.5% and the Nasdaq Biotechnology Index by 1.3%.

The acquisition wasn't the only news AstraZeneca was making Monday. Shares of

AtheroGenics

( AGIX) slid more than 15% after AstraZeneca ended its collaboration in the development of the cardiovascular therapy AGI-1067. In March, AtheroGenics said the treatment failed to lower heart disease risk in a phase III clinical trial.

Recently, AtheroGenics was falling 56 cents, or 15.3%, to $3.09.

Among other winners,

Avanir Pharmaceuticals

(AVNR)

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continued its run higher. Avanir said Monday that it now expects a fiscal second-quarter loss of $11 million, compared with a year-ago loss of $13.9 million. Avanir, which surged more than 265% last week, was up another 12 cents, or 2.7%, to $4.52.

Elsewhere,

Dendreon

(DNDN)

climbed 11.2% to $16.78,

Biogen-Idec

(BIIB) - Get Biogen Inc. Report

was higher by 4.8% at $48.86, and

Elan

(ELN)

added 2% to $15.24.

Several ratings changes were sinking pharmaceutical names. Prudential cut its rating for

Pfizer

(PFE) - Get Pfizer Inc. Report

to neutral from overweight. Pfizer closed down 2.3% to $26.35.

Vertex Pharmaceuticals

(VRTX) - Get Vertex Pharmaceuticals Incorporated Report

eased 1.1% after Needham & Co. downgraded the stock to hold from buy.

Among other losers,

Quest Diagnostics

(DGX) - Get Quest Diagnostics Incorporated Report

lost 0.7% to $49.60,

Eli Lilly

(LLY) - Get Eli Lilly and Company Report

shed 1.5% to $59.63, and

Boston Scientific

(BSX) - Get Boston Scientific Corporation Report

was down 0.7% to $16.09.