Immunogenicity and Safety of Intradermal Influenza Vaccine in the Elderly: A Meta-Analysis of Randomized Controlled Trials

Introduction Immunosenescence makes the elderly more susceptible to influenza complications and less responsive to vaccination. An intradermal formulation (IDflu) is one of several strategies being investigated to increase the immunogenicity of influenza vaccines. Objective The overall goal of the study was to assess the safety and immunogenicity of IDflu compared with the intramuscular route (IMflu) in the elderly. Methods A meta-analysis of randomized controlled trials (RCTs) was performed. Included articles met the following criteria: RCTs; primary studies, not re-analyses or reviews; enrolment of elderly people; comparing the immunogenicity and/or safety of IDflu with IMflu; measuring seroprotection and/or seroconversion rate to assess immunogenicity; measuring local reactions and/or general symptoms and/or other mild local reactions that could affect acceptability of vaccine as safety indicators, according to the European Medicines Agency (EMA) criteria; published through January 2015. Results The results of our meta-analysis on seroprotection showed that IDflu is comparable to IMflu for each strain (A/H1N1: risk ratio [RR] 1.02, 95 % confidence interval [CI] 0.98-1.07; A/H3N2: RR 1.01, 95 % CI 0.99-1.04; B 1.02, 95 % CI 0.98-1.08). The seroconversion rate achieved with IDflu was comparable to that of the control group (A/H1N1: RR 1.08, 95 % CI 0.97-1.2; A/H3N2: RR 1.08, 95 % CI 0.96-1.21; B: RR 1.21, 95 % CI 1-1.45). Systemic reactogenicity appeared similar in the two groups, while local reactions were significantly more frequent in the IDflu group. Conclusions The novel IDflu appears to have the adequate balance between immunogenicity and safety in the elderly compared with IMflu, and its utilization may be considered among the possible strategies to enhance the control of seasonal influenza outbreaks according to the existing policy recommendations in the elderly.