Table 2.

Pre- and post-TIV immunization HI titers and seroprotection or seroversion rates to H1N1, H3N2, and B vaccine strains in all study subjects and across the nonfrail, prefrail, and frail study groups.

HI antibody responses	All subjects (n = 71)	Nonfrail (n = 22)	Prefrail (n = 32)	Frail (n = 17)	p values
H1N1
Pre-immunization HI titersa	174±2.1	201±2.0	157±2.2	149±1.9	.17*
Post-immunization HI titersa	308±2.1	387±2.0	282±2.3	201±2.1	.03*
Pre- vs. post- p valuesb	.001	<.001	.01	.43	.86**
Seroprotection ratesc	94%	95%	94%	96%	ND
Seroconversion ratesd	7%	13%	6%	0
H3N2
Pre-immunization HI titersa	279±2.2	309±1.6	278±2.1	255±2.0	.16
Post-immunization HI titersa	408±2.6	497±1.9	388±2.4	307±2.3	.02*
Pre- vs. post- p valuesb	.01	.001	.01	.17	.71**
Seroprotection ratesc	92%	91%	94%	88%	.05**
Seroconversion ratesd	13%	27%	6%	6%
B
Pre-immunization HI titersa	78±1.7	88±1.4	78±1.6	65±2.0	.19*
Post-immunization HI titersa	85±1.8	105±1.5	81±1.3	67±2.1	.05*
Pre- vs. post- p valuesb	.005	.01	.23	.33	.87**
Seroprotection ratesc	82%	86%	78%	82%	ND
Seroconversion ratesd	1%	5%	0	0

ND: not done.

^*These p values derived from linear regression analysis adjusted for age.

^**These p values derived from Fisher exact test between nonfrail and frail groups.

^aMean±geometric SD of geometric mean titers (GMT).

^bThese p values derived from paired t tests of the pre–post immunization difference in log transformed HI titers.

^cSeroprotection defined by post-vaccination HI titer≥1:40.

^dSeroconversion defined by 4-fold or higher HI titer increase after TIV immunization or post- over pre-vaccination HI titer ratio ≥4.