A person's immunity to the surface antigens, especially hemagglutinin, reduces the likelihood of infection and severity of disease if infection occurs. Antibody against one influenza virus type or subtype confers little or no protection against another virus. Furthermore, antibody to one antigenic variant of influenza virus might not protect against a new antigenic variant of the same type or subtype. Frequent development of antigenic variants through antigenic drift is the virological basis for seasonal epidemics and the reason for the usual incorporation of one or more new strains in each year's influenza vaccine.¹

Formal subclassification utilizing neuraminidase antigens has not been done for influenza B viruses.

The incubation period for influenza is 1- 4 days with an average of 2 days. Adults typically are infectious from the day before symptoms begin through approximately 5 days after illness onset. Children can be infectious for ≥ 10 days, and young children can shed virus for ≤ 6 days before their illness onset. Severely immunocompromised persons can shed virus for weeks or months.

Uncomplicated influenza illness is characterized by the abrupt onset of constitutional and respiratory signs and symptoms (eg, fever, myalgia, headache, severe malaise, nonproductive cough, sore throat, and rhinitis). Among children, otitis media, nausea and vomiting are also commonly reported with influenza illness.

Influenza illness typically resolves after a limited number of days for the majority of persons, although cough and malaise can persist for > 2 weeks. Among certain persons, influenza can exacerbate underlying medical conditions (eg, pulmonary or cardiac disease), lead to secondary bacterial pneumonia or primary influenza viral pneumonia, or occur as part of a coinfection with other viral or bacterial pathogens. Young children with influenza infection can have initial symptoms mimicking bacterial sepsis with high fevers and ≤ 20% of children hospitalized with influenza can have febrile seizures. Influenza infection has also been associated with encephalopathy, transverse myelitis, Reye syndrome, myositis, myocarditis, and pericarditis.¹

The risks for complications, hospitalizations, and deaths from influenza are higher among persons aged ≥ 65 years, young children, and persons of any age with certain underlying health conditions than among healthy older children and younger adults.¹ Among children aged 0 - 4 years, hospitalization rates have ranged from approximately 500/100,000 children for those with high-risk medical conditions to 100/100,000 children for those without high-risk medical conditions, and are comparable to rates reported among persons aged ≥ 65 years.¹ In addition, influenza is a leading cause of death in young children and, along with pneumonia, is the sixth leading cause of death in those 1-4 years of age.²

During influenza epidemics from 1969-1970 through 1994-1995, the estimated overall number, for all ages, of influenza-associated hospitalizations in the US has ranged from approximately 16,000 to 220,000/epidemic. An average of approximately 114,000 influenza-related excess hospitalizations occurred per year, with 57% of all hospitalizations occurring among persons aged < 65 years. Since the 1968 influenza A (H3N2) virus pandemic, the greatest numbers of influenza-associated hospitalizations have occurred during epidemics caused by type A (H3N2) viruses, with an estimated average of 142,000 influenza-associated hospitalizations per year.¹

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