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From June to July 1999 an outbreak of acute respiratory illness occurred in the town of Iporanga. Out of a total of 4,837 inhabitants, 324 cases were notified to the Regional Surveillance Service. Influenza virus was isolated from 57.1% of the collected samples and 100% seroconversion to influenza A (H1N1) was obtained in 20 paired sera tested. The isolates were related to the A/Bayern/07/95 strain (H1N1). The percentages of cases notified during the outbreak were 28.4%, 29.0%, 20.7%, 6.2% and 15.7% in the age groups of 0-4, 5-9, 10-14, 15-19 and older than 20 years, respectively. The highest proportion of positives was observed among children younger than 14 years and no cases were notified in people older than 65 years, none of whom had been recently vaccinated against influenza. These findings suggest a significant vaccine protection against A/Bayern/7/95, the H1 component included in the 1997-98 influenza vaccine for elderly people. This viral strain is antigenically and genetically related to A/Beijing/262/95, the H1 component of the 1999 vaccine. Vaccines containing A/Beijing/262/95 (H1N1) stimulated post-immunization hemagglutination inhibition antibodies equivalent in frequency and titre to both A/Beijing/262/95-like and A/Bayern/7/95-like viruses. Thus, this investigation demonstrates the effectiveness of vaccination against influenza virus in the elderly. Durante os meses de junho e julho de 1999, foram notificados 324 casos de doença respiratória aguda no Município de Iporanga-SP. O isolamento do vírus da influenza do tipo A/Bayern/07/95 (H1N1) e a conversão sorológica para a estirpe viral (H1N1) foram de 57,1% e 100%, respectivamente. A porcentagem de casos com diagnóstico clínico notificados durante a epidemia foi de 28,4%, 29,0%, 20,7%, 6,2% e 15,7%, nas faixas etárias de 0-4, 5-9, 10-14, 15-19 anos e indivíduos acima de 20 anos de idade, respectivamente. Observou-se maior incidência da doença entre os indivíduos menores de 14 anos. Atribui-se a ausência de notificação de casos em indivíduos maiores de 65 anos à campanha de vacinação, na população idosa de Iporanga, que em 1999 atingiu 72,4%. O virus isolado é genética e antigenicamente semelhante à estirpe A/Beijing/262/95 (H1N1), o componente H1 da vacina de 1999. Vacinas contendo a estirpe A/Beijing/262/95 (H1N1) estimularam, após imunização, anticorpos inibidores da hemaglutinação, os quais foram equivalentes em freqüência e título para ambas as estirpes: A/Bayern/07/95 e A/Beijing/262/95.