The immunologic factors underlying severe influenza are poorly understood. To address this, we compared the immune responses of influenza-confirmed hospitalized individuals with severe acute respiratory illness (SARI) to those of nonhospitalized individuals with influenza-like illness (ILI).

Peripheral blood lymphocytes were collected from 27 patients with ILI and 27 with SARI, at time of enrollment and then 2 weeks later. Innate and adaptive cellular immune responses were assessed by flow cytometry, and serum cytokine levels were assessed by a bead-based assay.

During the acute phase, SARI was associated with significantly reduced numbers of circulating myeloid dendritic cells, CD192⁺ monocytes, and influenza virus–specific CD8⁺ and CD4⁺ T cells as compared to ILI. By the convalescent phase, however, most SARI cases displayed continued immune activation characterized by increased numbers of CD16⁺ monocytes and proliferating, and influenza virus–specific, CD8⁺ T cells as compared to ILI cases. SARI was also associated with reduced amounts of cytokines that regulate T-cell responses (ie, interleukin 4, interleukin 13, interleukin 12, interleukin 10, and tumor necrosis factor β) and hematopoiesis (interleukin 3 and granulocyte-macrophage colony-stimulating factor) but increased amounts of a proinflammatory cytokine (tumor necrosis factor α), chemotactic cytokines (MDC, MCP-1, GRO, and fractalkine), and growth-promoting cytokines (PDGFBB/AA, VEGF, and EGF) as compared to ILI.

Severe influenza cases showed a delay in the peripheral immune activation that likely led prolonged inflammation, compared with mild influenza cases.