Studies that compared any influenza vaccine with placebo or no treatment were eligible for inclusion. Any vaccine dose, preparation or time schedule were eligible. Most of the included studies used a dose of 15 μg of vaccine. All of the vaccines evaluated in the review included more than one viral strain.
Studies of HIV-infected individuals of all ages and from any geographical location were eligible for inclusion. The participants included were aged from 20 to 78 years and the majority were male. In all but one study the participants were not severely immunocompromised (median CD4 count above 400 cells/μL). The proportion of participants receiving highly active antiretroviral therapy ranged from 56 to 96% across the included studies. All of the included studies were conducted in 'developed' countries.
The primary outcomes were the occurrence of culture- or seriologically-confirmed influenza or clinical influenza-like illness during the influenza season.
Randomised controlled trials (RCTs), cohort and case-control studies were eligible for inclusion. To be eligible, studies had to report sufficient data to calculate effectiveness. The follow-up period of the included studies ranged from 3 months to 2 years.
Two authors independently selected the studies, with any disagreements resolved by consensus.