Characteristics of Infants <6 Months of Age with Severe or Fatal Influenza Infections — California, April 2009–September 2014

BACKGROUND: Infants <6 months of age are at greater risk for severe or fatal influenza, but are too young to receive influenza vaccine. Influenza vaccine is recommended during pregnancy and may help protect young infants through transplacental transfer of influenza antibodies. We described the demographic and clinical characteristics of infants <6 months of age with severe (intensive care unit [ICU] admission) or fatal influenza infections reported to the California Department of Public Health (CDPH).

METHODS: All infants <6 months of age with severe or fatal influenza infections reported to CDPH with illness onset during April 2009 through September 2014 were included. Demographic, clinical, and laboratory information were obtained from surveillance data and medical records. Birth information was obtained from birth certificates for infants born during 2008–2013.

RESULTS: One hundred forty-two infants <6 months of age with an influenza-associated ICU admission (128) or death (14) were reported during April 2009 through September 2014. Eight (57%) deaths occurred without hospital admission. Median age at illness onset was 11 weeks, 55% were male, and 55% were Hispanic (matching census data). One hundred twenty-four (87%) infants had influenza A, 16 (11%) had influenza B, and 2 (1%) had an unknown influenza type. Ninety-four (76%) infants with influenza A had subtyping performed; 81 (86%) were 2009 H1N1, including all 62 cases occurring during the 2009 pandemic, and 13 (14%) were H3N2. Of the 111 (78%) infants with known medical histories, 60 (54%) had an underlying medical condition; the most common were prematurity (34%) and pulmonary disorders (27%). Fifteen (11%) infants had healthcare-associated infections. Excluding infants born during the 2009 pandemic, 60% were born during October–December.

CONCLUSIONS: Influenza causes severe disease and death in infants, and maternal vaccination during pregnancy may help protect infants too young to be vaccinated. A study conducted during the 2013–2014 influenza season found that only 35% of women who were pregnant during influenza season were vaccinated during pregnancy. Improving influenza vaccine uptake among pregnant women is important, both to protect pregnant women and also their infants. Efforts are underway to improve uptake of Tdap vaccine among pregnant women and improving influenza vaccine uptake could be done concurrently. Given that a disproportionate number of infected infants were born during October–December, providers should prioritize and encourage women with expected delivery dates during these months to receive influenza vaccine, particularly in the early fall when vaccine quantities may be limited.