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.¹ Influenza infection has also been associated with encephalopathy, transverse myelitis, Reye syndrome, myositis, myocarditis, and pericarditis.¹

Hospitalizations and deaths from influenza

The risks for complications, hospitalization, and deaths from influenza are higher among persons aged ≥ 65 years, very young children, and persons of any age with certain underlying health conditions than among healthy older children and younger adults. Estimated rates of influenza-associated hospitalizations have varied substantially by age group in studies conducted during different influenza epidemics.¹

Among children aged 0-4 years, hospitalization rates have ranged from approximately 500 per 100,000 population for those with high-risk medical conditions to 100 per 100,000 population for those without high- risk medical conditions. Within the 0-4 age group, hospitalization rates are highest among children aged 0-1 years and are comparable to rates found among persons ≥ 65 years.

During influenza epidemics from 1969-1970 through 1994-1995, the estimated overall number of influenza- associated hospitalizations in the United States has ranged from approximately 16,000 to 220,000 per 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

Influenza-related deaths can result from pneumonia as well as from exacerbations of cardiopulmonary conditions and other chronic diseases.

Older adults account for ≥ 90% of deaths attributed to pneumonia and influenza.¹ In a recent study of influenza epidemics, approximately 19,000 influenza-associated pulmonary and circulatory deaths per influenza season occurred during 1976-1990, compared with approximately 36,000 deaths during 1990- 1999.¹ Estimated rates of influenza-associated pulmonary and circulatory deaths per 100,000 persons were 0.4-0.6 among persons aged 0-49 years, 7.5 among persons aged 50-64 years, and 98.3 among persons aged ≥ 65 years.¹ In the United States, the number of influenza-associated deaths might be increasing in part because the number of older persons is increasing.¹ In addition, influenza seasons in which influenza A (H3N2) viruses predominate are associated with higher mortality;¹ influenza A (H3N2) viruses predominated in 90% of influenza seasons from 1990-1999, compared with 57% of seasons from 1976-1990.¹

Deaths from influenza are uncommon among children with and without high risk conditions, but do occur.¹ A study that modeled influenza-related deaths estimated that an average of 92 deaths occurred among children aged < 5 years annually during the 1990s compared with 35,274 deaths among adults aged ≥ 50 years.¹ Preliminary reports of laboratory-confirmed pediatric deaths during the 2003-04 influenza season indicated that among these 143 influenza-related deaths (as of April 10, 2004), 58 (41%) were aged < 2 years and, of those aged 2-17 years, 65 (45%) did not have an underlying medical condition traditionally considered to place a person at risk for influenza-related complications.¹ Further information is needed on the risk of severe influenza-complications and optimal strategies for minimizing severe disease and death among children.¹

Options for controlling influenza

In the United States, the primary option for reducing the effect of influenza is immunoprophylaxis with vaccine. Vaccinating persons at high risk for complications and their contacts each year before seasonal increases in influenza virus circulation is the most effective means of reducing the effect of influenza. Vaccination coverage can be increased by administering vaccine to persons during hospitalizations or routine health-care visits before the influenza season, making special visits to physicians' offices or clinics unnecessary. When vaccine and epidemic strains are well matched, achieving increased vaccination rates among persons living in closed settings (e.g., nursing homes and other chronic care facilities) and among staff can reduce the risk for outbreaks by inducing herd immunity. Vaccination of health-care workers and other persons in close contact with persons at increased risk for severe influenza illness can also reduce transmission of influenza and subsequent influenza-related complications.¹ Antiviral drugs used for chemoprophylaxis or treatment of influenza are a key adjunct to vaccine. However, antiviral medications are not a substitute for vaccination.

Influenza vaccine composition

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