Abstract: Nine trials were included in the first published version of this review, and nine further trials have been included in four updates. The included studies cover a wide diversity of people, settings and types of influenza vaccination, and we pooled data from the studies that employed similar vaccines. Protective effects of inactivated influenza vaccine during the influenza season A single parallel-group trial, involving 696 children, was able to assess the protective effects of influenza vaccination. There was no significant reduction in the number, duration or severity of influenza-related asthma exacerbations. There was no difference in the forced expiratory volume in one second (FEV1) although children who had been vaccinated had better symptom scores during influenza-positive weeks. Two parallel-group trials in adults did not contribute data to these outcomes due to very low levels of confirmed influenza infection. Adverse effects of inactivated influenza vaccine in the first two weeks following vaccination Two cross-over trials involving 1526 adults and 712 children (over three years old) with asthma compared inactivated trivalent split-virus influenza vaccine with a placebo injection. These trials excluded any clinically important increase in asthma exacerbations in the two weeks following influenza vaccination (risk difference 0.014; 95% confidence interval -0.010 to 0.037). However, there was significant heterogeneity between the findings of two trials involving 1104 adults in terms of asthma exacerbations in the first three days after vaccination with split-virus or surface-antigen inactivated vaccines. There was no significant difference in measures of healthcare utilisation, days off school/symptom-free days, mean lung function or medication usage. Effects of live attenuated (intranasal) influenza vaccination There were no significant differences found in exacerbations or measures of lung function following live attenuated cold recombinant vaccine versus placebo in two small studies on 17 adults and 48 children. There were no significant differences in asthma exacerbations found for the comparison live attenuated vaccine (intranasal) versus trivalent inactivated vaccine (intramuscular) in one study on 2229 children (over six years of age). Uncertainty remains about the degree of protection that vaccination affords against asthma exacerbations that are related to influenza infection. Evidence from more recently published randomised trials of inactivated split-virus influenza vaccination indicates that there is no significant increase in asthma exacerbations immediately after vaccination in adults or children over three years of age. We were unable to address concerns regarding possible increased wheezing and hospital admissions in infants given live intranasal vaccination.

Plain-language summary: One trial in 696 children assessed the benefits of injecting inactivated influenza vaccine (inactivated virus vaccines are the type currently used in the US and UK and cannot cause flu). There were no significant differences in the number of people experiencing an asthma attack (worsening of symptoms); however, there were better symptom scores (people reporting fewer asthma symptoms) in weeks in which children had a positive test for influenza, in those who had received the jab compared to those who did not. Two trials involved 1526 adults and 712 children who were given inactivated influenza vaccination, examined the harmful effects caused immediately after injection. These studies ruled out the likelihood of any more than four out of 100 people having a resultant asthma attack in the first two weeks after getting their flu jab. There was not enough information to compare different vaccination types.