Post-infection geometric mean HI titers were significantly higher for virologically confirmed H3N2 cases compared to H1N1 cases (p < 0.001) with values of 218 (95%CI 113–421) and 40 (95%CI 26–62), respectively. A number of participants with virologically confirmed H1N1 that did not seroconvert, according to our pre-defined criteria, exhibited a 2-fold increase in titer or a 4-fold increase from 5 to 20. The proportion of participants with HI antibody titers of 20 or more in pre-season plasma ranged between 11% and 48% for seasonal influenza strains but was only 2.3% for pandemic A/California/04/2009-like

virus. The effect of pre-season serum/plasma HI titer on subsequent homosubtypic infection was investigated for www.selleckchem.com/products/ipilimumab.html each subtype and season. Log2 GSK126 in vitro titers were modeled to affect the log-odds of the risk of infection linearly with adjustment for age (Table 2). There was a significant linear effect of HI titer on the risk of infection for H3N2 in S2 and influenza B (Yamagata lineage) in S1 and S2 but not for H1N1 in S1, S2 or S3. There was no evidence for a non-linear (quadratic) association

for any of the analyses (all p > 0.1), except for H1N1 in S2 (p = 0.01), where there was evidence that titers ≥ 80 may decrease the risk of infection. After adjusting for HI titer, age was independently associated with decreasing risk of infection for H1N1 in S1 (p = 0.08), S2 (p < 0.0001), and pandemic S3 (p < 0.0001) and for H3N2 in S2 (p = 0.03), however there was no significant age effect for influenza B (Yamagata lineage) (p > 0.6 in S1 and S2). This is concordant with age effects, unadjusted for titer, discussed in detail in our previous report. 21 There was no evidence for titer–age interactions (all p > 0.3), except for H3N2 in S1 (p = 0.06). To examine whether the relation between HI titer and protection is significantly different for H1N1 compared to H3N2 and B, the association between infection with a strain and the HI titer against that strain

was modeled with an interaction with other strains. The Interleukin-3 receptor effect of HI titer was significantly different for H3N2 and B versus H1N1, but this was mainly due to differences during season 2 (Table 2). The effect of including titer rises from 5 (<10) to 20 in the definition of seroconversion and hence infection was examined (Supplementary, Table S3). All associations that were significant using the original definition of infection remained significant. In addition, unadjusted and age-adjusted associations between pre-season H3N2 titer and infection in season 1 were significant with the new definition, and other significant effect sizes were greater, reflecting increases in the numbers defined as infected amongst participants whose pre-season titer was 5.