Severe pediatric influenza in California, 2003-2005: implications for immunization recommendations

The 2003-2004 influenza season was marked by both the emergence of a new drift "Fujian" strain of influenza A virus and prominent reports of increased influenza-related deaths in children in the absence of baseline data for comparison. In December 2003, the California Department of Health Services initiated surveillance of children who were hospitalized in California with severe influenza in an attempt to measure its impact and to identify additional preventive measures.

From December 2003 to May 2005, surveillance of children who were hospitalized in PICUs or dying in the hospital with laboratory evidence of influenza was performed by hospital infection control practitioners and local public health departments using a standardized case definition and reporting form.

In the 2003-2004 and 2004-2005 influenza seasons, 125 and 35 cases, respectively, of severe influenza in children were identified in California. The mean and median age of cases were 3.1 years and 1.5 years, with breakdown as follows: < 6 months, 39 (24%); 6 to 23 months, 53 (33%); 2 to 4 years, 40 (25%); 5 to 11 years, 15 (9%); and 12 to 17 years, 13 (8%). Fifty-three percent (85 of 160) had an underlying medical condition(s), including a neurologic disorder (n = 36), chronic pulmonary disease (n = 26), genetic disorder (n = 19), cardiac disease (n = 18), prematurity (n = 14), immunocompromised status (n = 12), endocrine/renal disease (n = 2), and other (n = 1). Only 16% (15 of 96) of all patients had received influenza vaccination. Thirty-seven patients had an underlying illness that met existing Advisory Committee on Immunization Practices (ACIP) or American Academy of Pediatrics (AAP) recommendations for immunization, but only 8 had been vaccinated.

More than 3 times as many children were reported to be hospitalized in intensive care with influenza in California during the 2003-2004 season compared with the 2004-2005 season. Because children who are younger than 6 months remain at highest risk for severe influenza yet cannot currently be immunized, development and validation of preventive measures for them (eg, maternal immunization, breastfeeding, immunization of young infants and their close contacts) are urgently needed. ACIP's recent recommendation for influenza vaccination of children with conditions that can compromise respiratory function (eg, cognitive dysfunction, spinal cord injuries, seizure disorders, other neuromuscular disorders) is further supported by the frequency of underlying neurologic disease in these cases of severe influenza. A significant proportion of children with severe influenza in California, including children who are aged 2 to 4 years or have underlying genetic syndromes or prematurity, would not have been routinely recommended for influenza vaccination in 2005-2006 ACIP and AAP recommendations, calling into question whether such guidelines should be expanded. Continued surveillance for severe influenza-related morbidity and mortality is important to measure the impact of influenza on children.