Introduction This study aims to determine if the A/H1N1 influenza outbreak could have been earlier detected through changes in morbidity and mortality patterns analysed from a health information system (HIS).

Methods Specific data subsets were created to compare the burden of influenza during the epidemiological week (EW) 21 to 26 for years 2007 to 2009 among 150 000 Health Maintenance Organization members in Buenos Aires. The threshold for identifying an epidemic was considered met when the weekly influenza-like illness (ILI) rate exceeded 200 per 100 visits. Mortality rates of severe acute respiratory infection (SARI) from 2007 to 2009 were compared. Case fatality and mortality rates for A/H1N1 influenza 2009 also were estimated.

Results The HIS detected the outbreak in EW 23 while the government Ministry of Health (MoH) gave a national epidemic alert during EW 25. The number of visits for ILI increased more than fourfold when comparing 2009 to the 2007–2008. SARI mortality rate in 2009 was higher than in 2008 (RR 2.8; 95% CI 1.18 to 6.63) and similar to that of 2007 (RR 1.05; 95% CI 0.56 to 1.49). 2009 was the first year with mortalities younger than 65 years attributable to SARI. The estimated A/H1N1 case fatality rate for SARI was 6.2% (95% CI 2.5 to 15.5). The estimated A/H1N1 mortality rate was 6 per 100 000 (95% CI 0 to 11.6).

Conclusions the outbreak was detected 2 weeks before than the MoH gave a national alert suggesting that with a private-public integration a more real-time outbreak and disease surveillance system could be implemented.