Severe swine influenza A (H1N1) versus severe human seasonal influenza A (H3N2): clinical comparisons

At the beginning of the swine influenza (H1N1) pandemic in the spring of 2009, there were still stories of human seasonal influenza A circulating in the New York area. Adult patients admitted with influenza-like illnesses (ILIs) (fever > 102°F, dry cough, and myalgias) presented diagnostic problems. First, clinicians had to differentiate ILIs from influenza, and then differentiate human seasonal influenza A from H1N1 in hospitalized adults with ILIs and negative chest films (no focal segmental/lobar infiltrates). Human seasonal influenza A was diagnosed by rapid influenza diagnostic tests (RIDTs), but H1N1 was often RIDT negative. Reverse transcriptase-polymerase chain reaction for H1N1 was restricted or not available. The Winthrop-University Hospital Infectious Disease Division developed clinical diagnostic criteria (a diagnostic weighted point score system) to rapidly and clinically diagnose H1N1 in patients with negative RIDTs. The point score system was modified and shortened for ease of use, that is, the diagnostic H1N1 triad (any 3 of 4) (ILI, see above) plus thrombocytopenia, relative lymphopenia, elevated serum transaminases, or an elevated creatine phosphokinase. Our clinical experience during the pandemic allowed us to develop the swine diagnostic H1N1 triad. In the process, similarities and differences between human seasonal influenza A and H1N1 were noted. We present 2 illustrative cases of severe influenza, one due to human seasonal influenza A and one due to H1N1, for clinical consideration reflective of our experiences early in the H1N1 pandemic in 2009.