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Background During 2017–2018, influenza vaccine effectiveness (VE) against A(H3N2) illness was highest among children Methods Outpatient children aged 6 months–17 years with acute respiratory illness with cough were enrolled in the United States Influenza VE Network and tested for influenza infection by RT–PCR. Vaccination status was derived through medical records and immunization registries. Children with partial or unknown vaccination status were excluded. We used a test-negative design to estimate VE and 95% confidence intervals (CI) from logistic regression, adjusting for potential confounders. Cohorts were defined by birth after or before June 2014; we assumed exposure to the new A(H3N2) virus among children born after June 2014. Results During 2016–2017, among 2,545 children, 445 (18%) tested positive for A(H3N2) and 1,809 (71%) tested negative. VE against A(H3N2) did not differ among children born after June 2014 and among those born before June 2014 [49% (95% CI: −12%, 77%) vs. 43% (27%, 55%); interaction P < 0.75]. During 2017–2018, among 2,936 patients, 631 (22%) tested positive for A(H3N2), and 1,852 (63%) tested negative. VE against A(H3N2) was 59% (36%, 74%) among children born after June 2014 vs. 20% (−1%, 37%) among those born before June 2014 (interaction P < 0.01). Conclusion We did not consistently see differences in VE against A(H3N2) between children potentially exposed to different A(H3N2) viruses. However, error in exposure assignment to A(H3N2) viruses and few seasons since the emergence of the new A(H3N2) viruses limit our interpretation. Future study will include additional A(H3N2) seasons as initial exposures to current circulating viruses increase among young children. Alternative explanations for age-related differences will also be explored, such as prior seasonal vaccination. Disclosures All authors: No reported disclosures. |
