RESUMO
INTRODUCTION: Pandemic A/H1N1 influenza emerged in Mexico at the end of March 2009. Since then, it is still important to provide evidences that contributed to the international spread of the virus and to ascertain the attack rate of this new strain of influenza among the first cases in Spain that led to identify the first transmission in Europe. METHODS: Three pandemic A/H1N1 influenza groups related to an overseas flight were studied: 71 student group, 94 remaining passengers, and 68 contacts of confirmed cases. The attack rate with their 95% confidence interval (CI) among the student group and contacts was calculated. On April 26th, when the first cases were notified, strong preventive measures were implemented among the student group and the contacts of the confirmed cases. RESULTS: On 27th April, the first pandemic A/H1N1 influenza cases confirmed in Spain were three students that came back from Mexico by airplane. A student generated the first native case in Spain and one of the first cases in Europe. Similar attack rates were found between the student group (14.1%; CI: 12.1-16.1) and their contacts (13.2%; CI: 4.4-22.0), but no cases among remaining passengers were detected, suggesting low transmission risk during air travel. CONCLUSION: The first cases of pandemic A/H1N1 influenza in Spain were imported by airplane from Mexico. Preventive efforts to reduce the impact of the influenza influenced that primary and secondary rates were lower than first estimations by WHO.