Infants hospitalized in intensive care units with 2009 H1N1 influenza infection, California, 2009-2010.

Background: The 2009 H1N1 influenza virus emerged in April 2009 and primarily affected children and young adults. Few reports describe 2009 H1N1 influenza infection in infants. This report describes the clinical and epidemiologic features of 2009 H1N1 influenza in critically ill infants younger than 1 year of age.

Methods: Laboratory-confirmed cases were reported to the California Department of Public Health as part of public health surveillance for 2009 H1N1 influenza. Data were collected using standardized report forms and medical-chart abstractions.

Results: From April 23, 2009 through May 1, 2010, 82 cases of infants hospitalized in the intensive care unit with 2009 H1N1 influenza were reported in California. Medical charts were available for 77 of the infants, whose median age was 109 days (range: 1-361 days). Twenty-seven (35%) infants had a gestational age of 36 weeks or less. More than half (46; 60%) of the infants had at least 1 reported chronic medical condition. Thirty-five (45%) infants required mechanical ventilation; 7 (9%) died. Five infants were hospitalized since birth and acquired influenza infection during their admission; 2 (40%) of these infants died.

Conclusions: Infants who are premature or with chronic conditions seem to be at increased risk for developing severe 2009 H1N1 influenza infection. We encourage clinicians to maintain high suspicion for influenza in infants when influenza viruses are circulating. Vaccination should be encouraged among contacts of infants <6 months of age, who are too young to be immunized or treated with licensed antivirals. Infection control measures should also be implemented in hospital settings to reduce nosocomial transmission.