Our findings suggest that influenza A(H1N1)pdm09 caused a significant burden of disease in Argentina during 2009. If we assume that influenza activity was similar throughout country, we could multiply the age specific influenza-associated rates from Santa Fe, Tucumán, and Mar del Plata and their 95% confidence intervals by the census population of Argentina (3,240,001 persons aged <5 years; 25,055,187 aged 5–44 years; 7,717,549 aged 45–64; and 4,121,684 aged ≥65 years) to estimate that approximately 1,300,000 (95% CI 10,000,000–1,800,000) persons visited clinicians for ILI, 10,000 (95% CI 13,000–7,200) were hospitalized, and 990 died (95% CI 1,100–970) throughout the country as a result of the 2009 pandemic.
Our mortality pandemic rates were more conservative than those estimated using linear models of 2009 Argentina pneumonia and influenza mortality data (8.4/100,000py (95% CI 6.5–10.3/100,000py) a method that is not well suited to differentiate the impact of influenza from that or RSV 
. Our estimates are similar, however, to age-adjusted influenza A(H1N1)pdm09-associated respiratory and cardiovascular rates for the southern cone countries (2.1–4.0/100,000py) 
. Indeed, our estimates were similar to those of diverse countries such as Bangladesh (4/100,000py) 
and subpopulations within the United States (0.9–3.7/100,000py) 
Influenza A(H1N1)pdm09 mortality and hospitalization rates were also similar to those of seasonal influenza 
. Influenza A(H1N1)pdm09 mortality was similar to Argentina's seasonal influenza mortality estimated using pneumonia and influenza diagnostic codes and Serfling models (2.3–10.6/100,000 person-years during 2002–2009) 
and auto-regressive integrated moving averages models (0–4.6/100,000py during 1992–2002) 
. Our findings, therefore, suggest that early case-fatality proportions overestimated the actual burden of influenza A(H1N1)pdm09. Plausible explanations for this early overestimation include clinicians' preferentially identifying, sampling, and reporting severely ill case-patients. Nevertheless, it is important to note that while the overall influenza rates may be have been similar during 2009 and previous years, the years to life lost were likely greater during the pandemic because a greater proportion of ill persons were aged <65 years than during a typical influenza season.
Estimates of national influenza burden
Influenza A(H1N1)pdm09-associated hospitalizations were similar to those reported in Australia during 2009 (23/100,000py) 
but higher to those of low income countries such as Bangladesh (13/100,000py hospitalizations) 
. The similarities between Argentina's medically attended ILI rates and those of Bangladesh (6.6/100py) 
, however, suggests that differences in hospitalization rates may be a factor of health utilization.). As with influenza mortality, Argentina's pandemic and seasonal influenza hospitalization rates were similar (20/100,000 influenza-associated pneumonia and influenza hospitalizations and 60/100,000 influenza-associated respiratory and cardiac hospitalizations during 2005–2008 
A higher proportion of influenza decedents were pregnant, obese, or diabetic when compared with surviving ILI case-patients 
. Such findings suggest that ministries of health may be justified in exploring the burden of seasonal influenza in these groups and whether measures used to control and prevent influenza during the pandemic 
would be applicable to prevent and mitigate disease among subpopulations at high risk of complications from seasonal influenza illness 
Our study found that the proportion of decedents with positive 2009 H1N1 samples (66%) was significantly higher than that of hospitalized SARI case-patients (42%) and ILI case-patients (25%). Such a finding suggests that while influenza was identified in a fraction of ILI case-patients 
and community acquired pneumonias 
, influenza can be identified among a significant proportion (1/3–2/3) of severe hospitalized illness case-patients and decedents in during epidemic periods. If consistent among other surveillance platforms, such findings could have implications for countries exploring whether to empirically treat SARI case-patients with oseltamivir or other antivirals during influenza epidemic periods 
This study had several important limitations. We assumed that after accounting for case-definition, age-group, and epidemiologic week, the proportion of tested and untested case-patients with influenza A(H1N1)pdm09 was likely similar. This may be incorrect if physicians were more likely to test severely ill younger case-patients without pre-existing medical conditions, if the laboratory used one assay preferentially to test severely ill case-patients, and if the probability of testing positive for influenza A(H1N1)pdm09 was greater among severely ill case-patients. Although improbable, it is mathematically feasible that all un-tested case-patients had influenza A(H1N1)pdm09 infection, (a theoretical scenario where our influenza A(H1N1)pdm09-associated ILI rates would have been 50/100py, the hospitalization rates 48/100,000py, and the mortality rates 3.3/100,000py). Conversely, all un-tested case-patients may have been infected with other pathogens and not influenza (a theoretical scenario where our influenza A(H1N1)pdm09-associated ILI rates would have been 1/100py, the hospitalization rates 11/100,000py, and the mortality rates 2.0/100,000py). Last, we assumed that proportion of persons seeking care multiple times for ILI was similar during the 2010 and 2009 epidemic periods.