Interventions for preventing influenza: An overview of Cochrane systematic reviews and a Bayesian network meta-analysis

Background: Influenza places a heavy public health burden in numerous countries every year. In addition to vaccines, there are some interventions that are effective in preventing influenza. Objective: This overview of systematic reviews (SRs) aimed to evaluate the efficacy and safety of interventions for influenza prevention. Search strategy: We searched the Cochrane Database of Systematic Reviews, 2020, Issue 1 for relevant Cochrane SRs using the keywords “common cold,” “influenza,” and “flu.” Inclusion criteria: Cochrane SRs that investigated the prevention of influenza were included. Participants included the general population without influenza or influenza-like symptoms, who were treated with preventative interventions and compared to individuals receiving no treatment or placebo. Data extraction and analysis: Two reviewers independently screened citations against pre-defined inclusion criteria and extracted data. The methodological quality of these SRs was evaluated using the Assessing the Methodological Quality of Systematic Reviews-II (AMSTAR-II) guidelines. The primary outcome of our analysis was the incidence of influenza, and the secondary outcomes were the incidence of influenza-like illness and hospitalization. In addition to the narrative summary of SR findings, we also pooled data from homogeneous trials among these SRs and produced evidence mapping. We conducted a network meta-analysis to compare the effect across interventions and used the Cochrane approach to grading of recommendations, assessment, development, and evaluation (GRADE) to assess the quality of evidence. Results: Eleven Cochrane SRs were included, covering five medications, eleven vaccinations and four complementary therapies. Among these SRs, 73% scored “high” quality on AMSTAR-II rating. We found that eight interventions, including amantadine, garlic, and six different vaccines, were beneficial for reducing the incidence of influenza compared to placebo, while oseltamivir, zanamivir, Ganmao capsule, Echinacea, and another three types of vaccine were probably beneficial. Ganmao capsule ranked highest for influenza prevention in the network meta-analysis, followed by amantadine, garlic, and vaccines of all types. Monovalent inactivated parenteral vaccine was found to be beneficial in reducing the incidence of influenza-like illness. None of the interventions reduced the hospitalization rate. Conclusion: High-quality evidence showed that garlic or vaccine had advantages in preventing influenza, and that vitamin C is not effective. The effect of other interventions needs to be further verified with high-quality evidence.