2731. Does Last Season’s Influenza Vaccination Affect Current Season’s Vaccine Effectiveness in Young Children?

Background We evaluated influenza VE over two consecutive years vs. the current season against influenza illness during two H3N2-predominant seasons in children receiving care at emergency/urgent care (ED/UC) facilities in metropolitan Colorado. Methods We conducted a test-negative case–control analysis of 1478 children aged 6 months to 8 years enrolled at Children’s Hospital Colorado ED/UC with influenza like illness during the 2016–2017 and 2017–2018 influenza seasons. The primary outcome was PCR-confirmed influenza and vaccination status was confirmed using electronic medical record and parental interviews. Vaccination status was defined as completely vaccinated (all doses of influenza vaccine according to child’s age); partially or not vaccinated children were defined as unvaccinated. Multivariable logistic regression models adjusted for high-risk medical condition, age, race and insurance status were used to calculate odds ratios (OR) and 95% confidence intervals. Vaccine effectiveness was calculated as (1 − OR) × 100. Results Of the 1224 (82.8%) children enrolled in the study with known vaccination status for both seasons, 361 (29%) tested positive for influenza. Overall, VE against influenza was 49% (95% CI, 33–61%) after adjusting for other covariates in the model. VE did not differ significantly between those vaccinated in both seasons and those vaccinated in only the current season (VE 69%, 95% CI 41–115) (Table 1). Conclusion Our estimates of influenza VE for two predominantly H3N2-influenza seasons in Colorado are comparable to the CDC -VE for children 6 months to 8 years. VE against ED or UC-attended influenza illness in children did not vary significantly by prior seasons’ vaccination status. Disclosures All authors: No reported disclosures.