Table 5.

Unadjusted and Adjusted Vaccine Effectiveness, Stratified by Combinations of Prior (2010–2011) and Current (2011–2012) Influenza Vaccination Status Among Patients Aged ≥9 Years

	Influenza-Positive Cases		Influenza-Negative Controls		Unadjusted		Adjusteda
	No. Cases/ Row Total	Row %	No. Controls/ Row Total	Row %	VE %	(95% CI)	VE %	(95% CI)
Vaccinated current 2011–2012b only	42/512	8.2	470/512	91.8	61	(45 to 72)	56	(37 to 69)
Vaccinated current 2011–2012b and prior 2010–2011c	106/895	11.8	789/895	88.2	41	(26 to 54)	45	(27 to 58)
Vaccinated prior 2010–2011c only	45/277	16.3	232/277	83.8	15	(−19 to 40)	18	(−20 to 43)
Not vaccinated either 2010–2011 or 2011–2012	298/1597	18.7	1299/1597	81.3	Reference		Reference

Vaccine effectiveness (100 × [1 − odds ratio]) was estimated by calculating the ratio of the odds of a specific vaccine exposure (current only, both current and prior, and prior only) among influenza positive cases to the odds of that vaccine exposure among influenza negative controls, relative to those unvaccinated in both years, in logistic regression models. The P value for the interaction of prior (2010–2011) and current (2011–2012) season vaccination status for patients aged ≥9 years was .03.

Abbreviations: CI, confidence interval; VE, vaccine effectiveness

^a Models were adjusted for network center, subject age in months, sex, race/ethnicity categories, presence of high-risk health conditions, self-rated health status, time (days) between illness onset and specimen collection, and calendar time.

^b Subjects were considered vaccinated in 2011–2012 if they had documented medical record or immunization registry evidence of receipt of at least 1 dose of influenza vaccine for the current (2011–2012) season ≥14 days before illness onset.

^c Subjects were considered vaccinated in 2010–2011 if they had documented medical record or immunization registry evidence of receipt of at least 1 dose of influenza vaccine for the 2010–2011 season.