Interseason waning of vaccine-induced hemagglutination inhibition antibody titers and contributing factors to pre-existing humoral immunity against influenza in community-dwelling older adults 75 years and older

Table 6 Ratios of pre-vaccination HAI antibody titers measured using current season vaccine strain antigens over those using prior season vaccine strain antigens among individuals who participated in respective two consecutive study seasons^a

Vaccine strain	Individual study season	Unadjusted	Adjusted^b
IAV H3N2	2015 (n = 49)	0.39 (0.32–0.47) (p < .01)	0.36 (0.36–0.50) (p < .01)
IAV H3N2	2016 (n = 52)	0.64 (0.50–0.81) (p < .01)	0.56 (0.33–0.94) (p = .03)
IBV	2015 (n = 49)	0.48 (0.41–0.55) (p < .01)	0.41 (0.32–0.51) (p < .01)
IBV	2016 (n = 52)	1.77 (1.32–2.37) (p < .01)	0.84 (0.47–1.49) (p = .55)
IAV H1N1	2017 (n = 78)	0.86 (0.79–0.94) (p < .01)	0.86 (0.75–0.99) (p = .04)

^aRatios of pre-vaccination HAI titers measured using current study season vaccine strain antigens over those measured using prior season vaccine strain antigens among individuals who participated in the same two consecutive study seasons as the main analysis
^bAdjusted for age at the study season, sex, race, and education

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