Geometric mean hemagglutinin inhibition (HAI) antibody titers over time for 26 adolescents and adults aged <50 years, 13 adults aged 50–64 years, and 9 adults aged ≥65 years hospitalized with critical illness due to influenza A(H1N1)pdm09 virus infection — Canada, 2009–2011. Size of dot is proportionate to the number patients with a blood draw in the time interval. Wilcoxon rank sum test comparing the median antibody titer between patients aged <50 years and patients aged ≥65 years: P = .64 at 0–10 days, P = .09 at 11–20 days, P = .29 at 21–30 days, P = .21 at 31–40 days, and P = .60 at >40 days from illness onset.