Pandemic (H1N1) 2009 Cases, Buenos Aires, Argentina

To determine clinical and virologic characteristics of pandemic (H1N1) 2009 in Buenos Aires, Argentina, we conducted real-time reverse transcription–PCR on samples from patients with influenza-like illness, June 11–30, 2009. Of 513 patients tested, 54% were positive for influenza virus subtype H1N1. Infection rate was lowest for patients ≥60 years of age.

To determine clinical and virologic characteristics of pandemic (H1N1) 2009 in Buenos Aires, Argentina, we conducted real-time reverse transcription-PCR on samples from patients with infl uenza-like illness, June 11-30, 2009. Of 513 patients tested, 54% were positive for infl uenza virus subtype H1N1. Infection rate was lowest for patients ≥60 years of age.

A novel infl uenza A (H1N1) virus emerged in mid-
April 2009 and spread rapidly among humans worldwide (1,2). The virus, pandemic (H1N1) 2009 virus, is a unique combination of gene segments from North American and Eurasian swine lineages and ancestral genes derived from avian species and humans (3,4). In Buenos Aires, Argentina, the fi rst laboratory-confi rmed case of pandemic (H1N1) 2009 was documented May 16, 2009, in a patient who had traveled to the United States. This index case seeded an elementary school outbreak in Buenos Aires, and, within days, several schools reported increasing numbers of cases. The public health response included closure of schools with laboratory-confi rmed infl uenza subtype H1N1 cases, voluntary self-isolation and oseltamivir treatment of suspected and confi rmed patients, and recommended chemoprophylaxis for contacts. Despite these interventions, cases increased among the school-age and general population (5).
In early June, the number of confi rmed cases increased among young adults in Buenos Aires. In addition, cases more severe than those previously seen in school students were being identifi ed among young adults and pregnant women, some of whom required hospitalization in intensive care units. On June 11, 2009, the World Health Organization declared a phase 6 pandemic of this subtype H1N1 virus (6).
Argentina's National Reference Laboratory (Instituto Nacional de Enfermedades Infecciosas, Administración Nacional de Laboratorios e Institutos de Salud, ANLIS, "Dr. Carlos G. Malbrán") was initially designated as the only approved laboratory for diagnosing pandemic (H1N1) 2009 virus infections. After the rapid increase in the number of cases and the elevation of the pandemic alert level, the virology laboratory at Centro de Educación Médica e Investigaciones Clínicas (CEMIC) University Hospital in Buenos Aires also began diagnosing infections. This report describes clinical and virologic fi ndings for the initial cases of pandemic (H1N1) 2009 diagnosed at CEMIC.

The Study
Study patients were enrolled from June 11 through 30, 2009, a time that corresponds to the end of autumn and the beginning of winter in the southern hemisphere. Patients seeking care at CEMIC University Hospital and other healthcare providers in Buenos Aires (Federal District) were enrolled if they met the criteria for an infl uenza-like illness. During the study period, respiratory samples from 513 patients were submitted to CEMIC's virology laboratory for pandemic (H1N1) 2009 testing.
Combined nasal and pharyngeal swab specimens or nasopharyngeal or tracheal aspirates were obtained from patients and shipped to CEMIC in viral transport media. Epidemiologic and demographic parameters and information on clinical signs, underlying disease, chest radiographs, and medication use were obtained for each patient.
A laboratory diagnosis of infl uenza infection was determined by real-time reverse transcription-PCR (RT-PCR), by using the Centers for Disease Control and Prevention's protocol, on a Smart Cycler II (Cepheid, Sunnyvale, CA, USA) (7). The QIAamp Viral RNA Mini Kit (QIAGEN, Valencia, CA, USA) was used to extract RNA. Statistical analyses were performed by using χ 2 and Fisher exact tests.
Pandemic (H1N1) 2009 was detected most frequently among patients <5 years of age (59%), followed by those 19-59 years of age (57%). The lowest prevalence (37%) was among those >60 years of age; the difference in prevalence among those in this group and those in the groups <5 and 19-59 years of age was signifi cant (p = 0.019 and p = 0.003, respectively) ( Table 1). The highest percentage of hospitalizations occurred among patients <5 years of age (51%), followed by those >60 years of age (36%). Among patients 19-59 years of age, 24% required hospitalization ( Table 1).
The A total of 78 case-patients were hospitalized at CEMIC and other institutions in Buenos Aires. Six patients died, of whom 5 had an underlying condition: 3 were immunocompromised, 1 was obese and a chronic user of tobacco, and 1 had pulmonary and heart disease. Three patients were >60 years of age, 2 were in the 19-59-year age group, and 1 was an infant. Two hospitalized patients died at CEMIC. Both were >60 years of age, had underlying health conditions, and later developed bacterial sepsis; 1 also had bilateral pneumonia.
Underlying conditions were evaluated for all 275 patients with laboratory-confi rmed infl uenza virus subtype H1N1 infection. For evaluation, hospitalized and ambulatory patients were divided into 2 age groups: <5 and >5 years of age. For patients >5 years of age, the presence of an underlying condition was signifi cantly associated with a higher rate of hospitalization (p = 0.040). Of the 78 hospitalized patients, 56 (72%) had no underlying condition ( Table 2).

Conclusions
From June 11-30, 2009, pandemic (H1N1) 2009 virus was detected in 54% of respiratory samples submitted to CEMIC's virology laboratory. This percentage is similar to the overall percentage reported by Argentina's National Reference Laboratory during the same period (8). However, initial reports from Mexico showed a lower positivity rate (29%, 2,582/8,817) (9). This difference may be partly caused by the different clinical criteria for respiratory specimen collection, site restrictions, or seasonal factors. The percentage of infl uenza subtype H1N1 cases detected during this study (54%) is apparently higher than that for seasonal infl uenza A strains (H1N1 and H3N2) detected at CEMIC University Hospital during the same epidemiologic weeks in previous years (19% and 26% in children and adults, respectively, as confi rmed by immunofl uorescence testing) (10) (M. Echavarría, unpub. data).
The high frequency of infection with pandemic (H1N1) 2009 virus likely resulted from the lack of population immunity to this antigenically novel H1N1 subtype. The lowest rate of infection was seen for patients >60 years of age. This fi nding may suggest that persons in this age group were previously exposed, through infection or vaccination, to an infl uenza A (H1N1) virus that  is genetically and antigenically more closely related to pandemic (H1N1) 2009 than to other recent infl uenza A viruses (11). Our fi nding that 66% of respiratory samples tested during this study were from patients 19-59 years of age is similar to fi ndings in previous pandemic infl uenza situations. Furthermore, 49% of hospitalized patients in this study were 19-59 years of age. Most case-patients hospitalized with pandemic (H1N1) 2009 infection (72%) had no underlying medical condition, but a severe degree of lung involvement was observed in these patients: one third required mechanical ventilation support.
We report a mortality rate of 2% among the 275 confi rmed pandemic (H1N1) 2009 case-patients followed in this study. A similar rate (2.3%) was reported by the National Ministry of Health during the same period (8). From May through December 4, 2009, a total of 11,234 cases were confi rmed in Argentina, and 613 (5.5%) case-patients died (5). In the Americas, Argentina currently has the fourth highest number of deaths associated with infl uenza virus subtype H1N1, after the United States, Brazil, and Mexico (12). The reasons for these unusual epidemiologic features are the focus of ongoing investigations.