Background: The kinetics of the antibody response during severe influenza are not well documented.

Methods: Critically ill patients with influenza A(H1N1)pdm09 virus infection, confirmed by RT-PCR or seroconversion (≥4-fold rise in titers), during 2009-2011 in Canada were prospectively studied. Antibody titers in serially collected sera were determined using hemagglutinin inhibition (HAI) and microneutralization assays. Average antibody curves were estimated using linear mixed effects models and compared by patient outcome, age, and corticosteroid treatment.

Results: Of 47 patients with A(H1N1)pdm09 virus infection (median age 47 years), 59% had baseline HAI and 68% had baseline neutralizing titers <40. Antibody titers rose quickly after symptom onset and, by day 14, 83% of patients had HAI and 80% had neutralizing titers ≥40. Baseline HAI titers were significantly higher in patients who died compared with patients who survived; however, the antibody kinetics were similar by patient outcome and corticosteroid treatment. Geometric mean titers over time in older patients were lower compared with younger patients.

Conclusions: Critically ill patients with influenza A(H1N1)pdm09 virus infection had strong HAI and neutralizing antibody responses during their illness. Antibody kinetics differed by age but were not associated with patient outcome.