Complex sentence: However, studies on the effectiveness of flu vaccines in the real world are uniquely difficult; vaccines may be imperfectly matched, virus prevalence varies widely between years, and influenza is often confused with other influenza-like illnesses.
Split into simpler sentences: However, studies on the effectiveness of flu vaccines in the real world are uniquely difficult. The vaccine may not be matched to the viruses in circulation that year; virus prevalence varies widely between years, and influenza is often confused with other influenza-like illnesses.