Influenza Vaccine Effectiveness Against Laboratory-confirmed Influenza-like Illness in the Community, Israel, 2017-2018

Background: Influenza viruses undergo frequent genetic and antigenic changes. Therefore, the composition of the influenza vaccine is determined yearly. This situation calls for assessing the influenza vaccine effectiveness (VE) every season.

Objectives: To estimate the effectiveness of influenza vaccine against laboratory-confirmed influenza in the community for the 2017/2018 season.

Methods: Influenza virus detection and typing was done by Real-time PCR from nose-throat samples of patients presenting with influenza-like illness (ILI) to sentinel clinics throughout Israel. Epidemiological and influenza vaccination data were recorded.

VE was estimated using the test-negative design for patients aged 0.5 years and over. VE estimates were adjusted for age, time from disease onset to sample collection, calendar period and underlying diseases.

Results: A total of 1319 patients were included in this analysis. A total of 580 (44%) were influenza-positive, of which 177 (30.5%) were influenza A and 404 (69.6%) influenza B (one sample was positive for both influenza viruses). Most influenza B viruses belonged to the B/Yamagata Lineage, while only influenza B/Victoria lineage was represented in the trivalent vaccine. Of the 18.6% participants who were vaccinated against influenza, 90.6% received the trivalent inactivated vaccine and only 9.4% received the quadrivalent inactivated vaccine.

Adjusted VE was 46.8% (95% CI 4.1 to 64.1) for influenza A and 23.2% (-10.1 to 46.4) for influenza B. The highest VE estimates for influenza A was observed among infants and children. Specifically, adjusted VE estimates was 72.8% (-19.8 to 93.8) for the 0.5-4 years age group, and 65.6% (95 CI 3.3% to 87.8) for the 5-17 years age group.

Conclusions: The moderate-high VE against influenza A in children during the 2017-2018 season supports the importance of vaccinating children against influenza to prevent illness and transmission. The VE against influenza B supports the introduction of the quadrivalent influenza vaccine for use in Israel.