INTRODUCTION

Influenza is an acute respiratory disease caused by influenza viruses transmitted primarily by droplets expelled during coughing and sneezing. Influenza type A and B virus infections can cause substantial human disease and mortality worldwide. Patients present with variable signs and symptoms depending on age and the presence of underlying chronic disease. Seasonal winter influenza epidemics in temperate countries can have a substantial impact on the emergency department (ED), but travelers may present with influenza illness acquired in other countries year-round. Rarely, the emergence of a novel influenza A subtype virus can lead to a global influenza pandemic.

Influenza viruses are single-stranded negative sense RNA viruses of the family Orthomyxoviridae. Three types (A, B, and C) of influenza viruses infect humans. Type A and B viruses are known to cause significant human disease. The genome contains eight gene segments that code for 11 proteins, including the two main surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). Type A viruses are further classified into subtypes based on their HA and NA proteins. Currently circulating human influenza A subtypes include A (H1N1) and A (H3N2) viruses. Human influenza viruses bind to and replicate primarily in epithelial cells of the upper respiratory tract.

Influenza viruses are evolving continuously through a process called “antigenic drift” in which random point mutations in the HA gene result in changes to the HA surface protein.