Mid-Season Estimates of Influenza Vaccine Effectiveness against XXXX (H3N2) Hospitalization in the Elderly in Quebec, Canada, January 2015.

@entity1 @entity683 @entity3082
BACKGROUND: The 2014/15 @entity683 season in Canada was characterized by an @entity3082 due to vaccine-mismatched @entity683 , disproportionately affecting elderly individuals >= 65-years-old. We assessed vaccine effectiveness (VE) against A(H3N2) hospitalization among elderly individuals during the peak weeks of the 2014/15 epidemic in Quebec, Canada. METHODS: Nasal specimens and clinical/epidemiological data were collected within 7 days of illness onset from elderly @entity1 admitted with respiratory symptoms to one of four participating hospitals between November 30, 2014 and January 13, 2015. Cases tested RT-PCR positive for @entity683 A(H3N2) and controls tested negative for any @entity683 . VE was assessed by test-negative case-control design. RESULTS: There were 314 @entity1 including 186 cases (62% vaccinated) and 128 controls (59% vaccinated) included in primary VE analysis. Median age was 81.5 years, two-thirds were admitted from the community and 91% had underlying comorbidity. Crude VE against A(H3N2) hospitalization was -17% (95%CI: -86% to 26%), decreasing to -23% (95%CI: -99 to 23%) with adjustment for age and comorbidity, and to -39% (95%CI: -142 to 20%) with additional adjustment for specimen collection interval, calendar time, type of residence and hospital. In sensitivity analyses, VE estimates were improved toward the null with restriction to @entity1 admitted from the community (-2%; 95%CI: -105 to 49%) or with specimen collection  <=4 days since illness onset (- 8%; 95%CI: -104 to 43%) but further from the null with restriction to @entity1 with comorbidity (-51%; 95%CI: -169 to 15%). CONCLUSION: The 2014/15 mismatched @entity683 vaccine provided elderly @entity1 with no cross-protection against hospitalization with the A(H3N2) epidemic strain, reinforcing the need for adjunct protective measures among high-risk individuals and improved vaccine options.
