Epidemiological investigation of a novel influenza A (H1N1) outbreak in Fukuoka city, 2009---preliminary report


Fukuoka City Office and Infectious Disease Surveillance Center, National Institute of Infectious Diseases


From June 9 to 20, 2009, a field investigation team consisting of the Infections Disease Surveillance Center, National Institute of Infectious Diseases and the Field Epidemiology Training Program was dispatched to investigate a novel influenza outbreak in Fukuoka city, in collaboration with the Fukuoka City government, following the request from Fukuoka city and the Ministry of Health and Welfare. This report focuses on the descriptive epidemiology of confirmed cases.


Background

On July 6, 2009, a 1^st grade student of junior-high school A, who attended one of the sentinel clinics for influenza in Kasuga city, Fukuoka prefecture, was laboratory confirmed as novel influenza infection by RT-PCR at the Fukuoka prefectural laboratory. This was the second case of novel influenza in Fukuoka prefecture, followed by one American male case diagnosed on May 25. The male American case stayed in Shime-cho, Fukuoka prefecture.

On July 7, 5 additional cases were confirmed among students in junior-high school A and six cases were confirmed among students in elementary school B. Junior-high school A and elementary school B are in the same school district C in Fukuoka city.

Laboratory surveillance had been enhanced since May 19, 2009 in Fukuoka prefecture, in addition to case finding according to the national guidelines initiated by epidemiological linkage to endemic areas. The enhanced surveillance in Fukuoka prefecture included RT- PCR testing of novel influenza on all rapid influenza A positive cases, from all 198 sentinel clinics for seasonal influenza. The first case of junior-high school A was detected by this enhanced surveillance system. On June 11, Fukuoka city expanded the enhanced surveillance to cover all clinics following the occurrence of the outbreak.


Case definition and active case finding

Cases were defined as residents in Fukuoka city who showed influenza-like symptoms from May 19 to July 30, 2009 with laboratory confirmation as novel influenza by RT-PCR. They were interviewed by public health nurses. Medical charts were reviewed for inpatient cases. Laboratory confirmation was performed at the municipal or prefectural laboratory.


Descriptive epidemiology

Forty male and 31 female cases were detected. The cases were 2 to 43 years old (median; 11).

An outbreak affecting 63 cases occurred in the restricted area: school district C and the neighboring areas. None had a travel history to endemic countries or endemic areas in Japan within two weeks before developing symptoms. Elementary school B had 33 student cases: 4 cases in the 3^rd, 14 in the 4^th, and 15 in the 6^th grades. Junior-high school A had 15 student and 2 teacher cases; among them 11 were 1^st grade students. In addition, 4 elementary schools were involved; they had 5 student cases in total. Other than school-related cases, small clusters were found in this area: 4 cases related to a nursery school and 3 cases related to a restaurant. One sporadic adult case was found among the residents in school district C.

Only one cluster was detected in other areas of Fukuoka city; 4 cases related to a bank without travel history to endemic areas. Four additional cases were suspected to have acquired infections in Canada or the US.

Symptoms of 70 cases were as follows: fever more than 37.0 °C 70 / 70 cases(38.0 to less than 39.0 °C - 32 cases, 39.0 to less than 40.0 °C - 23 cases, 40.0 °C or more - 6 cases), cough 59 / 70 cases, sore throat 37 / 68 cases, general fatigue 34 / 68 cases, diarrhea or soft stool 9 / 67 cases, headache 27 / 30 cases, rhinorrhea 30 / 45 cases, muscle pain or joint pain 17 / 24 cases, abdominal pain 8 / 13 cases, nausea 7 / 11 cases, vomiting 2 / 9 cases, congestion of conjunctiva 4 /10 cases, eruption 1 case. Symptoms of 57 cases aged under 15 years old were as follows: fever more than 37.0 °C 57 / 57 cases(38.0 to less than 39.0 °C - 29 cases, 39.0 to less than 40.0 °C - 19 cases, 40.0 °C or more - 6 cases), cough 50 / 57 cases, sore throat 28 / 56 cases, general fatigue 27 / 56 cases, diarrhea or soft stool 9 / 56 cases, headache 21 / 23 cases, rhinorrhea 27 / 40 cases, muscle pain or joint pain 11 / 15 cases, abdominal pain 7 / 9 cases, nausea 6 / 8 cases, vomiting 2 / 6 cases, congestion of conjunctiva 4/ 7cases, eruption 1 case.

Although 47 cases were hospitalized at the beginning of the outbreak mainly because of the Infectious Disease Control Law requirement to prevent further transmission, all cases were discharged as of June 23 without any complications. Seven cases had asthma and 2 had febrile convulsions as a past history or underlying condition.

The date of fever onset more than 37.0 °C ranged from June 3 to 22 for all cases , June 3 - 13 among junior-high School A student and teacher cases, June 4 - 10 among elementary school B student cases, June 9 - 30 among the others. Despite continuing enhanced laboratory surveillance targeting all clinics, no additional cases without travel history to endemic countries have been found after the case with June 17 onset among residents of Fukuoka city.