Isolation of influenza A(H1) virus from an influenza outbreak at a primary school, January 2001 - Shimane
(IASR 2001; 22: 60-60)

On January 19, 2001, an outbreak of influenza-like illness at a grade 4 class (9-10 years of age) was reported from Hamada City. Of 22 children, 12 developed illness, 11 were absent from school, and the class was closed for a few days. Major symptoms included fever (37-39C), headache, runny nose, cough, and joint pain.

Throat garglings were collected from 8 patients (not vaccinated with the current vaccine), and were inoculated onto MDCK cells. Influenza A/H1N1 virus was isolated from 6 patients.

The isolate showed a hemagglutination titer of 1:16 - 1:48 with guinea pig RBCs. The hemagglutination inhibition (HI) titers were 1:160 (homo-titer 1:160) for A/New Caledonia/20/99 (H1N1), 1:10 (1:80) for A/Moscow/13/98 (H1N1), and <1:10 (1:160) for A/Panama/2007/99 (H3N2).

Paired sera at the acute and convalescent phases were collected from 5 patients (3 virus isolation positive, 2 negative). HI antibody titers increased by 4 folds or more against A/New Caledonia/20/99 (H1N1) and A/Moscow/13/98 (H1N1) in all of 3 culture-positive patients and 1 out of 2 negatives. In contrast, HI antibody titers remained low (=<1:10) against A/Panama/2007/99 (H3N2), B/Yamanashi/166/98, and B/Shangdong/07/97.

In the previous season, this school was closed for a short period of time due to the concurrent epidemics of influenza A/H3N2 and A/H1N1 which caused absenteeism from school in 15% of the whole children. In this season there has been no influenza outbreak that expanded more than one class. We speculate that this outbreak occurred among children who had not acquired A/H1N1 antibodies.

The start of influenza-like illness in Shimane Prefecture was delayed for more than a month in this season. Influenza B virus, as well as influenza A virus, was isolated from some cluster cases. The numbers of patients, however, remain at 5-10% of those in the preceding years.

Reported by: Y. Akiba, Y. Matsuda, S. Iizuka, T. Takeda, and A. Itagaki, Infectious Diseases and Epidemiology, Shimane Prefectural Institute of Public Health and Environmental Science; Shimane Hamada Health and Welfare Center

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