Pandemic (H1N1) 2009 Outbreak at Camp for Children with Hematologic and Oncologic Conditions

An outbreak of influenza A pandemic (H1N1) 2009 occurred among campers and staff at a summer camp attended by children with hematologic and oncologic conditions. The overall attack rate was 36% and was highest among children and adolescents (43%), persons with cancer (48%), and persons with sickle cell disease (82%).

On day 2, fever developed in a healthy camper in patrol G3 and promptly subsided. Fever also developed in a second camper (patrol B2) with sickle cell disease (SCD). This camper was seen at the local hospital, had a negative rapid infl uenza test (RIT) result, and was sent home. On day 3, fever and cough developed in 4 children (2 from B2); 1 was tested by RIT with a negative result. Evaluation of the entire B2 patrol found no one else symptomatic. On day 4, fever developed in1 child with cancer (B2) and 1 with SCD (G2 patrol); each had positive test results for infl uenza A virus.
The number of episodes of fever was considered excessive, and because additional campers reported having fever the camp was closed. At the time of dismissal, all campers and staff were provided masks and instructions about cough etiquette and handwashing; a dose of oseltamivir was also administered, and a prescription for oseltamivir was provided.
Ten days after the camp closed, all attendees were contacted (by email, telephone, and regular mail) to gather information about their outcomes. Clinical signs and symptoms of interest were fever (measured or subjective) or chills; cough or sore throat; muscle pain; and nausea, vomiting, or diarrhea. On the basis of an adaptation of the defi nition by the Centers for Disease Control and Prevention (www.cdc.gov/h1n1fl u/clinicians), persons with fever or chills and symptoms in >2 other categories were classifi ed as having infl uenza-like illness (ILI); persons with fever alone (without an explanation) and symptoms in 1 other category, or no documented fever but symptoms in >2 other categories, were classifi ed as having probable ILI (P-ILI). A case-patient was defi ned as a person in whom ILI or P-ILI developed within 10 days of closing of the camp.
Probably because of small numbers, no difference was evident in the attack rate between staff assigned and unassigned to a patrol, between boy and girl campers, between patrols, or between those who recalled or did not recall being in contact with a case-patient. Twenty-six case-patients sought medical attention, and 12 (2 staff and 10 campers) were hospitalized; 8 had SCD and 4 were in active cancer treatment; all were regular-fl oor admissions; none required oxygen support or other intensive care; mean length of stay was 4.25 (range 3-6) days; and all recovered uneventfully.
Twenty-fi ve persons (18 campers and 7 staff) were tested for infl uenza: 13 were tested at an outside facility, of whom 3 had positive results (method of testing unknown); 12 were tested at Children's Hospital by RIT (BinaxNOW; Invernass Medical, Waltham, MA, USA) and direct immunofl uorescence assay (D 3 Ultra; Diagnostic Hybrids, Athens, OH, USA), and 5 (42%) had positive test results for infl uenza A virus. Of the 5 positive specimens obtained from patients, 4 were confi rmed as pandemic (H1N1) 2009 virus by real-time reverse transcription-PCR.

Conclusions
Limited preliminary information is available about summer camp outbreaks of pandemic (H1N1) 2009 (2). The closest scenarios (school outbreaks) reported attack rates of 3%-33% in the United States (3-5) and 2%-31% in the United Kingdom (6,7). Differences between studies and groups probably refl ect different levels of exposure and surveillance defi nitions. In the outbreak reported here, the overall attack rate was 36%; infection was more common among campers (47%), children and adolescents (43%), those with cancer (48%), and those with SCD (82%). Because of their underlying condition, the threshold for evaluation and intervention is lower for children with hematologic or oncologic processes, which might account for some increased reporting of symptoms, hospitalization, and extended stay.
Children with SCD were disproportionately affected; 82% of them reported symptoms, and 89% of those symptomatic were hospitalized because of fever or pain crisis. For seasonal infl uenza, the hospitalization rate is 56× high-er for children with SCD than those without SCD (8); the same trend seems true for pandemic (H1N1) 2009 infection. Among those with no underlying condition, the infl uenza attack rate was lower (28%), few (11%) persons sought medical attention, and none were hospitalized. For persons with no underlying condition, the reported symptoms suggest an illness no more severe than seasonal infl uenza.
A few limitations should be recognized. First, 25% of attendees did not return the questionnaire. Second, we used a clinical defi nition of ILI; however, although the limitations of the defi nition are well recognized, it is a well-accepted tool for outbreak investigation. Third, data are based on recall and report by participants. Fourth, only a few persons were tested to determine the causative agent. However, of the 12 patients we tested at Children's Hospital, 5 (42%) had infl uenza A virus and 4 were confi rmed as having pandemic (H1N1) 2009 virus infection. This level of positivity is in accordance with sensitivity reported for available tests (9,10) and suggests that most, if not all, cases of ILI identifi ed were caused by pandemic (H1N1) 2009 virus.