Table 4.

Comparison of Influenza Virologic Surveillance by Birth Cohort, 2000–2001 Through 2016–2017 - Relative Risk of Influenza A Positivity During H3 Compared to H1 Predominant Seasons

	Prepandemic (2000–2001 to 2008–2009 Seasons)			Pandemic and Later (2009–2010 to 2016–2017 Seasons)
	Percent A Positive (No. Positive/No. Tested)		H3 vs HI Predominant Seasons	Percent A Positive (No. Positive/No. Tested)		H3 vs HI Predominant Seasons
Cohort	H1 Predominant Seasons	H3 Predominant Seasons	Relative Risk (95% Confidence Interval)	H1 Predominant Seasons	H3 Predominant Seasons	Relative Risk (95% Confidence Interval)
1918–1922	10.07 (44/437)	32.16 (380/1026)	3.19 (1.65–11.07)	7.73 (236/3054)	47.66 (3426/7188)	6.17 (3.11–14.27)
1933–1937	14.52 (54/372)	37.65 (285/757)	2.59 (1.58–4.46)	15.21 (1166/7664)	41.50 (5838/14 069)	2.73 (1.73–4.76)
1947–1951	18.69 (117/626)	33.20 (408/1229)	1.78 (1.08–3.07)	24.63 (2853/11 582)	34.86 (5261/15 091)	1.42 (0.95–2.18)
1957–1961	27.50 (253/920)	36.15 (548/1516)	1.31 (0.87–2.10)	32.78 (4561/13 913)	33.62 (5027/14 954)	1.03 (0.70–1.49)
1968–1972	33.63 (307/913)	37.55 (620/1651)	1.12 (0.73–1.82)	35.00 (3763/10 752)	34.25 (4022/11 744)	0.98 (0.67–1.48)
1977–1981	26.46 (463/1750)	41.13 (985/2383)	1.56 (0.94–2.88)	37.17 (4267/11 479)	37.62 (4840/12 864)	1.01 (0.70–1.54)
1985–1989	28.68 (689/2434)	40.41 (1316/3257)	1.41 (0.93–2.10)	40.60 (5719/14 087)	37.70 (6122/16 237)	0.93 (0.61–1.40)

The bold indicates the value is statistically significant.