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 2 Ratios of pre-vaccination HAI titers over prior season post-vaccination HAI titers among participants who completed the study in the corresponding two consecutive seasons when corresponding virus strains in the vaccine formula changed^a

Vaccine strain	Consecutive two seasons	Unadjusted	Adjusted^b
IAV-H3N2	2014–2015 (n = 49)	0.69 (0.53–0.89) (p < .01)	0.82 (0.51–1.34) (p = .43)
IAV-H3N2	2015–2016 (n = 52)	0.50 (0.41–0.60) (p < .01)	0.42 (0.28–0.62) (p < .01)
IBV	2014–2015 (n = 49)	0.37 (0.32–0.42) (p < .01)	0.39 (0.30–0.50) (p < .01)
IBV	2015–2016 (n = 52)	0.75 (0.64–0.87) (p < .01)	0.78 (0.56–1.08) (p = .13)
IAV-H1N1	2016–2017 (n = 78)	0.70 (0.60–0.82) (p < .01)	0.65 (0.49–0.85) (p < .01)

^aPre-vaccination and prior season post-vaccination HAI antibody titers were both measured using the respective prior season vaccine strain antigens
^bAdjusted for age at prior study season, sex, race, and education

ISSN: 1742-4933