This is a retrospective, observational, non-interventional drug study using de-identified data from two administrative claims datasets. The study design and analysis will reflect the perspective of the commercially insured. The objectives of this study are twofold:

1. To compare the rates of re-hospitalization among patients treated with either linezolid orally or IV, or vancomycin IV for complicated skin and skin structure infections (cSSSI) or pneumonia hospitalization.
2. To compare the total direct medical costs of patients treated with linezolid orally or IV, or vancomycin IV for cSSSI or pneumonia hospitalization.

A sampling method is not used in this study since it is a retrospective, non-interventional claims database analysis. Pre-specified inclusion/exclusion criteria and ICD-9, NDC, HCPC codes are applied to identify the study cohorts.

Inclusion Criteria:

Subjects must meet all of the following inclusion criteria to be eligible for enrollment into the study:

• Patients with a hospitalization for cSSSI or pneumonia for which the discharge date (i.e. index date) is between January 1, 2007 and September 30, 2009.
• Patient must have a claim for either vancomycin (IV) or linezolid (IV or oral) within 7 days following their cSSSI or pneumonia hospitalization discharge date (i.e. index date). We will exclude patients who initially receive both vancomycin and linezolid on the same day. We will apply an intention-to-treat approach and use the first drug as the index drug, even if the "comparator drug" (e.g. vancomycin for linezolid and vice-versa) is used subsequently on a future date. If there are both oral and IV formulations of the same index drug on the same index date, the patient will be kept and categorized as an IV user. As with all duplicate claims, for two claims that have both oral and IV index drug as well as the same IV and oral costs on the same index date, the claim with the oral drug will be removed, and the patient will be called an IV user.
• 18-64 years of age at index date.
• Continuous eligibility for six months prior to the index hospitalization date. No minimum of post-index continuous enrolment is required to assure that early mortality patients are included (maximum of 180 days after the index vancomycin or linezolid claim).

Exclusion Criteria:

Subjects presenting with any of the following will not be included in the study:

• Patients who were enrolled in Medicare.
• Patients who were over age 65 or younger than 18.
• Patients with fewer than 3 days of oral therapy.
• Patients with index hospitalization of greater than 30 days.
• Patients with osteomyelitis, infective endocarditis, meningitis, joint infections, necrotizing fasciitis, gangrene, prosthetic joint infection, or prosthetic implant/ device infection during index hospitalization.

The study population will consist of patients with a hospitalization for complicated skin and skin structure infections (cSSSI) or pneumonia. We will identify cases from January 1, 2007, to September 30, 2009, with additional 6 months pre-period and up to 6 months of post-index follow-up period.