IMPORTANCE
Recent observational studies report conflicting results regarding the effectiveness of live attenuated influenza vaccine (LAIV), particularly against influenza A(H1N1)pdm09.

OBJECTIVE
To compare the effectiveness of LAIV and inactivated influenza vaccine (IIV) against laboratory-confirmed influenza.

DESIGN, SETTING, AND PARTICIPANTS
A test-negative study to estimate influenza vaccine effectiveness (VE) using population-based, linked, individual-level laboratory, health administrative, and immunization data. Data were obtained from 10 169 children and adolescents aged 2 to 17 years (children) who were tested for influenza in inpatient or outpatient settings during periods when influenza was circulating based on a threshold level of 5%weekly test positivity for the province during the 4 influenza seasons spanning from November 11, 2012, to April 30, 2016, in Alberta, Canada. Logistic regression was used to estimate VE by vaccine type, influenza season, and influenza type and subtype. The relative effectiveness of each vaccine type was assessed by comparing the odds of laboratory confirmed influenza infection for LAIV recipients with that for IIV recipients.

EXPOSURES
The primary exposure was receipt of LAIV or IIV before testing for influenza.

MAIN OUTCOMES AND MEASURES
The primary outcome was influenza case status as determined by reverse-transcriptase polymerase chain reaction testing.

RESULTS
A total of 10 779 respiratory specimens (from 10 169 children) collected and tested for influenza during the 4 influenza seasons were included, with 53.4%from males; the mean (SD) age was 7.0 (4.6) years. Across the 4 influenza seasons, 3161 children tested positive for influenza. Combining the 4 influenza seasons, the adjusted VE against influenza A(H1N1)pdm09 was 69%(95%CI, 56%-78%) for LAIV compared with 79% (95%CI, 70%-86%) for IIV. Vaccine effectiveness against influenza A(H3N2) was 36%(95%CI, 14%-53%) for LAIV and 43%(95%CI, 22%-59%) for IIV. Against influenza B, VE was 74% (95%CI, 62%-82%) for LAIV and 56%(95%CI, 41%-66%) for IIV. There were no significant differences in the odds of influenza infection for LAIV recipients compared with IIV recipients except for influenza B during the 2015-2016 season, when LAIV recipients had lower odds of infection than IIV recipients (odds ratio, 0.36; 95%CI, 0.17-0.76).

CONCLUSIONS AND RELEVANCE
There was no evidence to support the lack of effectiveness of LAIV against influenza A(H1N1)pdm09. These results support administration of either vaccine type in this age group.