New Data Presented at ECTRIMS Congress Show TYSABRI(R) Has Sustained Effect on Relapse Rate in Multiple Sclerosis Patients Treated for up to Three Years

Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN)
announced today new data that show TYSABRI(R) (natalizumab) has a
sustained effect on relapse rate in multiple sclerosis (MS) patients
treated for up to three years. These data, presented at the 22nd
Congress of the European Committee for Treatment and Research in
Multiple Sclerosis (ECTRIMS) in Madrid, Spain, were part of long-term
follow-up of TYSABRI clinical trial patients.

Patients who participated in the Phase III TYSABRI program were
eligible to enroll in an open-label extension study that evaluated the
therapy's long-term effects. Approximately 1,900 patients and over 200
sites worldwide participated in the extension study. Approximately 250
of these patients remained on TYSABRI monotherapy for nearly three
years. The annualized relapse rate for these patients over the
three-year period was 0.23, translating into an average of one relapse
every 4.3 years. This was consistent with the 0.23 annualized relapse
rate seen in the two-year AFFIRM study, which represented a 68%
relative reduction when compared to the two-year placebo annualized
relapse rate of 0.73, as published in the New England Journal of
Medicine.

"Data from this long-term follow-up study show that TYSABRI has a
sustained and compelling effect on relapse rates beyond two years of
treatment. The efficacy benefit of TYSABRI when considered with the
management of its known risks, offers an important therapeutic option
for many patients living with the debilitating effects of MS," said
Paul O'Connor, MD, St. Michael's Hospital, Toronto, Ontario, Canada,
lead investigator of the extension study.

About TYSABRI

In the US, TYSABRI is approved as a monotherapy treatment for
relapsing forms of MS. TYSABRI increases the risk of progressive
multifocal leukoencephalopathy (PML), an opportunistic viral infection
of the brain that usually leads to death or severe disability.
Patients should be monitored at regular intervals for any new or
worsening signs or symptoms suggestive of PML. Because of the
increased risk of PML, TYSABRI is generally recommended for patients
who have had an inadequate response to, or are unable to tolerate,
alternate MS therapies. It is available in the US only through a
restricted distribution program called the TOUCH Prescribing Program.
According to product labeling, after two years, TYSABRI treatment led
to a 67% relative reduction (p<0.001) in the annualized relapse rate
compared to placebo and reduced the relative risk of disability
progression by 42% (p<0.001). TYSABRI treatment also resulted in
sustained and statistically significant reductions in brain lesion
activity as measured by MRI. Changes in MRI findings often do not
correlate with changes in the clinical status of patients (e.g.,
disability progression). The prognostic significance of the MRI
findings in these studies has not been evaluated.

In the European Union, TYSABRI is indicated as a single
disease-modifying therapy in highly active relapsing-remitting MS
patients. Because of the increased risk of PML, it is for patients
with high disease activity despite treatment with a beta-interferon or
in patients with rapidly evolving severe relapsing-remitting MS.
According to product labeling in the EU, after two years, TYSABRI
treatment led to a 68% relative reduction (p<0.001) in the annualized
relapse rate compared to placebo and reduced the relative risk of
disability progression by 42-54% (p<0.001).

Serious adverse events that occurred in TYSABRI-treated patients
included hypersensitivity reactions (e.g., anaphylaxis), infections,
depression and gallstones. In MS trials, the incidence and rate of
other serious and common adverse events, including the overall
incidence and rate of infections, were balanced between treatment
groups. Herpes infections were slightly more common in patients
treated with TYSABRI. Serious opportunistic and other atypical
infections have been observed in TYSABRI-treated patients, some of
whom were receiving concurrent immunosuppressants. Common adverse
events reported in TYSABRI-treated patients include headache, fatigue,
infusion reactions, urinary tract infections, joint and limb pain,
lower respiratory infections, rash, gastroenteritis, abdominal
discomfort, vaginitis, and diarrhea.

For more information about TYSABRI please visit www.tysabri.com,
www.biogenidec.com or www.elan.com, or call 1-800-456-2255.

About Biogen Idec

Biogen Idec creates new standards of care in oncology, neurology
and immunology. As a global leader in the development, manufacturing,
and commercialization of novel therapies, Biogen Idec transforms
scientific discoveries into advances in human healthcare. For product
labeling, press releases and additional information about the company,
please visit www.biogenidec.com.

About Elan

Elan Corporation, plc is a neuroscience-based biotechnology
company committed to making a difference in the lives of patients and
their families by dedicating itself to bringing innovations in science
to fill significant unmet medical needs that continue to exist around
the world. Elan shares trade on the New York, London and Dublin Stock
Exchanges. For additional information about the company, please visit
www.elan.com.

Safe Harbor/Forward Looking Statements

This press release contains forward-looking statements regarding
TYSABRI. These statements are based on the companies' current beliefs
and expectations. The commercial potential of TYSABRI is subject to a
number of risks and uncertainties. Factors which could cause actual
results to differ materially from the companies' current expectations
include the risk that we may be unable to adequately address concerns
or questions raised by FDA or other regulatory authorities, that
concerns may arise from additional data, that the incidence and/or
risk of PML or other opportunistic infections in patients treated with
TYSABRI may be higher than observed in clinical trials, or that the
companies may encounter other unexpected hurdles. Drug development and
commercialization involves a high degree of risk. For more detailed
information on the risks and uncertainties associated with the
companies' drug development and other activities, see the periodic and
current reports that Biogen Idec and Elan have filed with the
Securities and Exchange Commission. The companies assume no obligation
to update any forward-looking statements, whether as a result of new
information, future events or otherwise.