Table 4.

Vaccine Effectiveness Against Influenza A(H3N2) and B Virus Related Illness According to Combinations of Prior Season (2015–2016) and Current Season (2016–2017) Influenza Vaccination Status Among Patients Aged ≥9 Years

	Influenza-positive Cases		Influenza-negative Controls		Unadjusted		Adjusteda
	No. Cases/Row Total	(%)	No. Controls/Row Total	(%)	VE %	(95% CI)	VE %	(95% CI)
Influenza A(H3N2)b
Vaccinated current 2016–2017 only	130/708	18.4	578/708	81.6	35%	(19 to 48)	35%	(18 to 48)
Vaccinated current 2016–2017 and prior 2015–2016	399/1804	22.1	1405/1804	77.9	18%	(4 to 29)	26%	(12 to 38)
Vaccinated prior 2015–2016 only	106/444	23.9	338/444	76.1	9%	(−16 to 29)	14%	(−10 to 33)
Not vaccinated either 2015–2016 or 2016–2017	467/1820	25.7	1353/1820	74.3	REF		REF
Influenza B c
Vaccinated current 2016–2017 only	49/627	7.8	578/627	92.2	53%	(35 to 66)	54%	(35 to 68)
Vaccinated current 2016–2017 and prior 2015–2016	151/1556	9.7	1405/1556	90.3	40%	(26 to 52)	42%	(26 to 55)
Vaccinated prior 2015–2016 only	47/385	12.2	338/385	87.8	23%	(−8 to 45)	22%	(−12 to 46)
Not vaccinated either 2015–2016 or 2016–2017	243/1596	15.2	1353/1596	84.8	REF		REF

Vaccine effectiveness was estimated by comparing the odds of influenza for each category of current or prior vaccination versus the odds of influenza among enrollees not vaccinated in either current (2016–2017) or prior season (2015–2016), and calculated as 100 × (1 – odds ratio) in logistic regression models. Vaccination includes any inactivated influenza vaccine for each season.

Abbreviations: CI, confidence interval; REF, referent group (1.0); VE, vaccine effectiveness.

^aModels were adjusted for study site, patient age in months, presence of any high-risk health condition and calendar time (two week intervals).

^bThe P-value for interaction of prior (2015–2016) and current (2016–2017) season vaccination was 0.10.

^cThe P-value for interaction of prior (2015–2016) and current (2016–2017) season vaccination was 0.07.