There are limited data from Thailand on the aetiology of LRTI but no data on mortality of hospitalised children. Thai children < 1 year accounted for circa one third of LRTIs in children who were treated as out or inpatients in whom influenza was isolated in 6 (2.7%) of 271 children and RSV in 44 (20%). At the Queen Sirikit hospital, Bangkok, influenza A and B and RSV accounted for approximately 11% (9/80), 2.5% (2/80) and 6% (5/80) of children < 1 year, respectively. This study included children with underlying diseases like congenital heart disease and chronic lung disease. A small laboratory series of 110 children at Siriraj hospital with LRTIs infections (Pilaipan Puthavathana, personal communication) identified RSV A/B (17%), metapneumovirus (14%), parainfluenza 1 (12%) and adenovirus (12%), influenza B (6%), influenza A (4%), coronaviruses (3%), Parainfluenza 3 (2%) and 2 (0%). The number of drugs registered for treating influenza is limited to oral Oseltamivir, amantadine and rimantadine and inhaled zanamivir. As a result of the 2009 influenza A/H1N1, clinical guidelines have been updated to include children less than one years old . However, regulatory studies of oseltamivir excluded children under 1 year based on preclinical data in rats in which there were deaths in young rats (7 days old) but none in 14 days old rats given large doses of Oseltamivir. Higher concentrations of Oseltamivir were found in the brains of the younger rats which was thought to be due immaturity of the blood brain barrier. There is, however, some clinical experience with Oseltamivir in the under ones from Japan, Thailand, Germany , the USA , and additional experience with 2009 pH1N1 . The doses used were 2 mg/kg bid which is consistent with the dose recommended in the UK for children who weigh less than 15 kg (30 mg bid for 5 days). At the Queen Sirikit hospital, Oseltamivir has been given to a very small number of children < 1 year with severe influenza with good effect (T. Chotpitayasunondh, unpublished observations). This experience is similar to that of others i.e. good clinical outcomes and apparently good tolerability. An Oseltamivir pharmacokinetic study in children age 1-5 years showed that the dose of 2 mg/kg resulted in plasma-concentration time curves (AUC) similar to the AUC accepted in adult. However, the younger the child, the lower the AUC level; never the less, there are still insufficient pharmacokinetics data in children under one year . The clinical significance of reduced in vitro sensitivity is unclear owing to the paucity of human data but these mutations are likely to result in reduced antiviral efficacy of Oseltamivir and the adamantanes against H1N1. Furthermore, amantadine treatment of influenza frequently results in the rapid development of amantadine resistance in both H1N1 and H2N3 viruses, resulting in continued virus replication, thus, making this drug less than ideal for treating influenza. Currently, there is limited adamantane resistant H1N1 but widespread adamantane resistant in H3N2. H3N2 and influenza B remain sensitive to Oseltamivir. The adamantanes have no activity against influenza B. The emergence of resistance poses difficulties for the treatment of influenza in children less than one but oseltamivir represents at present the optimal choice for treating such children. Therefore, this protocol will assess the effect of oral Oseltamivir at doses recommended by the WHO to see if they are applicable to Thai children.

Currently, there is no treatment for children less than one year of age with influenza related lower respiratory tract infection that is either considered standard or registered in any country. This dismal scenario exists even though influenza related LRTI is a significant illness causing morbidity and mortality, especially in children less than 6 months of age. Avian influenza has been reported rarely in children less than one. There are no data in Vietnam and very few data in Thailand on the burden of influenza in children less than one. This young age group suffers high mortality. Oseltamivir may be beneficial in such children. This is basis of this trial.