I am a professional biologist, an amateur Spanish enthusiast, and have had a parent who has undergone this type of treatment for HCV, and this is what I understand of the subject.
Se les sometió a un nuevo tratamiento a base de interferón pegilado y ribavirina en dosis ajustadas al peso."
They participated in a new treatment of pegylated interferon and ribavarin with dosing based upon the weight of the subject.
The synergistic drug combination of pegylated interferon and ribavarin is the currently preferred method of treating the hepatitis C virus (HCV). Previously, the disease had been treated with interferon alone, but adding a polyethelyne glycol side chain to the interferon backbone - a process known as pegylation - was found to increase the drug's bioavailability (in terms of longevity within the body). This allows a decrease in the need for injections from 3 times a week to only once a week. This is an important aspect of the newer treatment in that, often, the side effects following injections are often severe in terms of nausea and depression.
Additionally, treatments combining Peg-Interferon with Ribavarin have been shown to increase the response rates for most populations (works well on the most common viral genotypes found in the population that cause the disease).
The portion regarding the dosing based on the weight may have something to do with an interest in mediating the often debilitating side effects that these drugs can have.