However, studies on the effectiveness of flu vaccines in the real world are difficult; vaccines may be imperfectly matched, virus prevalence varies widely between years, and influenza is often confused with other influenza-like illnesses.
Namun, studi tentang efektivitas vaksin flu di dunia nyata itu lebih sulit; Vaksin mungkin tidak sempurna, prevalensi virus sangat bervariasi setiap tahun, dan influenza sering bingung dengan adanya penyakit mirip influenza.