Influenza epidemics occur worldwide annually. The incidence of influenza shows a seasonal pattern in temperate areas, but little is known about influenza seasonality in tropical regions. The objective of this study was to determine the prevalence and the seasonal pattern of influenza infections in children living in the city of Fortaleza in northeastern Brazil. An indirect immunofluorescence assay was performed on nasopharyngeal aspirates collected from children attending in ambulatories, emergency rooms, and wards of the Hospital Infantil Albert Sabin with suspicion of acute respiratory infection during 7 consecutive years (2001-2007). Influenza viruses were detected in 6.3% (234/3,708) of specimens. Laboratory-based surveillance data showed a clear annual epidemic cycle of influenza, with a peak usually occurring in the rainy periods. In Fortaleza, flu infections occurred at a low level throughout the year but exhibit a marked seasonal increase during the rainy season.

The seasonality of influenza is very diverse in tropical countries. Recent studies on the subject confirm the year-round activity of these viruses in many countries and the regular occurrence of epidemic outbreaks in others. The use of molecular diagnostic methods has led to prevalence rates comparable to those seen in temperate countries. Phylogenetic analysis of hemagglutinin of influenza isolates allows understanding of the evolution and divergence of avian (H5N1) and human influenza viruses (A/H1N1, A/H3N2, and B) in tropical settings. Influenza viruses resistant to oseltamivir and zanamivir have been detected in tropical countries despite the rare use of these drugs in the treatment of influenza in people living in these areas.

Background Although acute respiratory infections (ARIs) are the global leading cause of pediatric morbidity and mortality, the relative impact of viral pathogens on pediatric ARIs is still poorly understood, especially in equatorial settings. Long-term studies of multiple viruses concurrently circulating in these regions are still lacking. Here we report the results of a systematic prospective surveillance of multiple respiratory viruses conducted every weekday for nearly a decade in an equatorial city in Brazil. Methods We analyze the relative burden of influenza, parainfluenza, respiratory syncytial virus (RSV), adenovirus and metapneumovirus, their seasonality and their association with climatic and demographic factors, ARI diagnosis and pediatric mortality. Results and Conclusions RSV was the primary driver of severe childhood respiratory infections, including pneumonia. RSV was also the virus most strongly associated with respiratory-associated deaths, with RSV circulation and pediatric mortality being in phase. Influenza was the second most common cause of childhood ARIs but, similarly to parainfluenza, adenovirus and metapneumovirus, it was mostly associated to upper tract infections, and peaked much earlier than mortality. The results also show that viral circulation can be strongly seasonal even in equatorial regions, which lack seasons with low temperatures: while parainfluenza predominantly circulated in the dry season, RSV and influenza were concentrated in the rainy season. The consistent epidemiological patterns observed can be used for an effective adjustment of the timing of therapeutic and prophylactic interventions in this and potentially other equatorial regions.

A cohort of children attending a day care center in Salvador (Bahia, Brazil) was studied prospectively to determine the incidence of viral respiratory infectious episodes and to identify the viruses associated with them. Two hundred seventy-one nasopharyngeal samples were collected over a 1-year period for examination, using indirect immunofluorescence with monoclonal antibodies against adenovirus, influenza A and B, parainfluenzae 1-3, and respiratory syncytial virus, and reverse transcriptase-polymerase chain reaction for picornavirus. Examination yielded positive results in 116 samples (42.8%). Rhinovirus was identified alone in 56 samples (48.3%) and was observed along with other viruses in 11 additional samples. Incidence density of viral respiratory infectious episodes was 7.66 episodes/1,000 child-days.