Seasonal influenza vaccine effectiveness in preventing laboratory-confirmed influenza in primary care in Israel 2016-2017 season: insights into novel age-specific analysis

Abstract

Background

The 2016-2017 influenza season in Israel was dominated by the circulation of influenza A(H3N2). Influenza vaccine is recommended for the entire population in Israel over the age of six months. The inactivated influenza vaccine was chosen for use this season.

Methods

We estimated the 2016-2017 end-of-season influenza vaccine effectiveness (VE) in preventing influenza-like illness (ILI) due to influenza A(H3N2), using the test-negative design. Age-specific VE was estimated using a moving age window and weekly analysis.

Results

During the 2016-2017 season 1267 samples were collected; 467 (36.9%) were positive for influenza, with 97.9% A(H3N2), 0.2% A(H1N1)pdm09, and 1.9% B.  A total of 1088 individuals were found eligible to be included in VE assessment. All vaccinated individuals included in the VE assessment received the inactivated influenza vaccine. Adjusted VE against influenza A(H3N2) was 29.0% [95% confidence interval (CI) 0.3% to 49.5%]. Age group-specific adjusted VE demonstrated VE of 69.2 [95% CI 19.4 to 88.3] for children aged 5-17, and VE of 58.8 [95% CI 0.81 to 82.9] for adults ages 45- 64. Other age groups demonstrated lower VE estimates that were not statistically significant. Adjusted VE estimates using a moving window of 15 years and weekly VE analyses provided a more defined understanding of age-specific VE during the 2016-2017 season.

Conclusion

Estimating VE using a moving age window, and weekly VE analysis may provide more detailed information regarding the relationship between VE and age.