Proportion (%) considered immune by age category, virus and antibody threshold.

SPR = Sero-protection rate – defined as the proportion (%) considered sero-protected on the basis of having met or exceeded the specified antibody titre threshold.
pre-H1N1pdm09: 2009 H1N1 pandemic virus; SPR presented based on PRE-pandemic antibody levels measured in 2009 or earlier.
post-H1N1pdm09: 2009 H1N1 pandemic virus; SPR presented based on POST-pandemic antibody levels measured in fall 2010.
H3N2v: swine-origin H3N2 variant strain; SPR presented based on antibody levels measured in sera collected in fall 2010.
post-Brisbane: a contemporary seasonal human influenza H3N2 virus; SPR presented based on post-circulation antibody levels in sera collected in fall 2010.
SPR40: the proportion considered sero-protected according to the standard hemagglutination inhibition (HI) titre threshold of 40.
½SPR40: assumes half the individuals meeting SPR40 are considered sero-protected.
SPR80: the proportion considered sero-protected according to a hemagglutination inhibition (HI) titre threshold of 80.
Note that a blended composite of sero-protection based on a gradient of immunity defined as ¼SPR20, ½SPR40, ¾SPR80 and 100% SPR160 was also explored (see Table S1 and narrative).
1Author unpublished data.
2The proportion with titre ≥160 was 2% (95%CI 1–3%) overall, highest in those 18–24 years of age (10%; 95%CI 4–17%).
3Lower post-pandemic estimate for this age group within expected error of laboratory assay method and sampling variability.
4Overall age-standardized SPRs are displayed for interest but were not used in generating model-based AR estimates.