Correlation between virologic profile and clinical features of patients infected by influenza virus provides important information for epidemiological control and clinical management of future disease outbreaks. Samples from patients in Southern Brazil, from June to December 2009, were examined and the viral load was correlated with epidemiological data. All samples were analyzed by qRT-PCR for detection of the 2009-pandemic Influenza A (H1N1). Relative viral loads were assessed based on the 2(-ΔCT) method and epidemiological data were obtained for each patient, following ethical policies. A total of 933 samples were positive for pH1N1 (2009) influenza; 172 were positive for seasonal influenza A; 13 were undetermined; 1992 samples were negative for influenza A. Combined molecular and epidemiological data were available for 38 seasonal and 198 pandemic samples. The median viral load was higher in pandemic than in seasonal influenza samples; in patients infected with pH1N1 (2009), viral load associated positively with chills, myalgia and rhinorrhea, and negatively with dyspnea, but no association was observed with other symptoms, nor with clinical conditions such as pregnancy, smoking, immunodepression and co-morbidities. Regarding patients infected with seasonal influenza, viral loads did not show statistically significant association with any of the symptoms. This is the first study in Brazil that examines epidemiological and molecular data from the 2009 influenza pandemic. The results may serve as a basis for developing strategies to control human-to-human infection and viral dissemination, and for implementing effective measures and public health policies against future novel disease outbreaks.

Influenza viruses are highly contagious and circulate in all geographical regions. During the 2009 pandemics caused by influenza A(H1N1), the State of Rio Grande do Sul (RS) was the first to detected A(H1N1) cases. In 2010, a broad vaccination program was applied in RS when 44.9% of the population joined the program. During the 2011, a total of 1,433 samples were sent to the Central Laboratory in Porto Alegre (LACEN-RS) for viral detection by qRT-PCR. Only 107 (7.5%) cases of the A(H1N1) virus were confirmed versus 182 (12.7%) cases of seasonal influenza A. The incidence of both influenza types virus was higher in patients aged 0-10 years old. The median viral load was higher in patients infected with seasonal, in comparison to those infected with A(H1N1) virus contrary of pandemic period. In 2011 most of the patients that were infected by influenza A virus (79%, p<0.001), did not receive vaccine. The presence of fever, cough, dyspnea, myalgia and rhinorrhea were the most frequent symptoms (positivity >60%). Furthermore in 2011 only patients infected by pandemic virus died (12.9%, p=0.001) in contrast with 2009 pandemic period when 6% of patients infected by pandemic virus died. In other hand in the whole population (5.3%) the mortality rate was similar that observed in pandemic period (5.9%). These analyses about epidemiological and molecular data provide important scenery about the characteristics of the host-pathogen interaction after massive exposure during pandemic period.