The primary objective of the study is to evaluate the safety of natalizumab use in the pediatric multiple sclerosis (MS) population.

All available retrospective and prospective data from pediatric MS participants who have received at least 1 dose of natalizumab before turning 18 years old and before 31Mar2015 will be used in this study. Prospective data are defined as data to be collected in the future, i.e., from new participants not included in the first data transfer or additional data from existing participants that were not included in the first data transfer. Existing biospecimen samples will be evaluated but not collected as part of this study.

Key Inclusion Criteria:

• All available retrospective and prospective data from pediatric MS patients who have received at least 1 dose of natalizumab before turning 18 years old and before 31 March 2015.
• In addition, patients must be registered in TOUCH (US patients only), enrolled in a Biogen sponsored postmarketing observational study (e.g., TOP (NCT00493298) or TYGRIS (NCT00477113,NCT00483847)), or in a country-specific TYSABRI registry.
• Adequate data received by Biogen by 30 September 2015 will be used for this meta-analysis including data collected after a patient may have turned 18 years old.

Key Exclusion Criteria:

• Data received by Biogen after 30 September 2015 will not be included in the statistical analyses

NOTE: Other protocol-defined inclusion/exclusion Criteria May Apply

Pediatric MS patients who have received at least 1 dose of natalizumab prior to the age of 18 years where the first dose was administered prior to 31 March 2015.