Influenza Virus Infections

Influenza, commonly known as flu, is a contagious, febrile viral disease. Common symptoms are chills, fever, sore throat, coughing, sneezing, muscle pains, headache, myalgia, etc. Influenza viruses (IVs) belonging to orthomyxovirus group are of three types (types A, B, and C). IVs are enveloped, segmented, negative-stranded RNA viruses containing eight distinct RNA segments (types A and B) and seven RNA segments (type C). Only type A viruses exhibit different subtypes. Currently, 18 hemagglutination (HA) subtypes (H1 to H18) and 11 neuraminidase (NA) subtypes (N1 to N11) are known in humans and other warm-blooded animals. Flu outbreaks occur as annual epidemics in fall and winter and infrequently, as worldwide pandemics. Annual epidemics are caused by antigenic drift due to mutations in HA and NA antigens, whereas pandemics are caused by antigenic shift due to genetic reassortment among different subtype viruses. The first pandemic of the twenty-first century, declared by the World Health Organization (WHO) in 2009, was caused by an H1N1 virus, derived from quadruple reassortment among swine, avian, and human IVs. Currently, two types of antiinfluenza drugs, based on either blocking the M2 proton channel (amantadine and rimantadine) or inhibiting NA (oseltamivir or Tamiflu by Roche and zanamivir or Relanza by GlaxoSmithKline) are available. The virus is commonly spread by aerosol droplets and by fingertips. Therefore, washing hands and avoiding contact with infected persons are the first lines of defense. Indoor living and closeness of people in an enclosed environment in winter facilitate virus transmission leading to epidemics. Annual vaccination is the best preventative measure. Currently, two types of vaccines (subunit vaccine by injection and live attenuated vaccine by nasal spray) are available and recommended by WHO and Centers for Disease Control.