pegylated interferon is formulated in such a manner that it lasts longer and shots don't have to be taken so often.
Peg-interferon is interferon attached to Poly Ethylene Gylcol which slows it's release and gives a more consistent dose.
It is taken as 1 shot per week.
The older interferon (also called consensus interferon or brand name Infergen) is commonly given 3 times per week and the blood serum levels fluctuate much more (highest when yo take it, to lowest just before the next shot.