This drug is approved around the world for the treatment of chronic hepatitis C (including patients with HIV co-infection, cirrhosis, 'normal' levels of ALT) and has recently been approved (in the EU, U.S., China and many other countries) for the treatment of chronic hepatitis B.
Peginterferon alfa-2a is a long acting interferon. Interferons are proteins released in the body in response to viral infections. Interferons are important for fighting viruses in the body, for regulating reproduction of cells, and for regulating the immune system.
For genotype 1 hepatitis C treated with pegylated interferon-alpha-2a or pegylated interferon-alpha-2b (brand names Pegasys or PEG-Intron) combined with ribavirin, it has been shown that genetic polymorphisms near the human IL28B gene, encoding interferon lambda 3, are associated with significant differences in response to the treatment. This finding, originally reported in Nature, showed genotype 1 hepatitis C patients carrying certain genetic variant alleles near the IL28B gene are more likely to achieve sustained virological response after the treatment than others. Later report from Nature  demonstrated the same genetic variants are also associated with the natural clearance of the genotype 1 hepatitis C virus.
^Ge D, Fellay J, Thompson AJ et al. (2009). "Genetic variation in IL28B predicts hepatitis C treatment-induced viral clearance". Nature461 (7262): 399–401. doi:10.1038/nature08309. PMID19684573.CS1 maint: Explicit use of et al. (link)