Oseltamivir, commonly referred to by its trade name Tamiflu, featured prominently in 2009 during the swine flu epidemic.
Several side-effects were also reported, however, including significantly increased risks of nausea (3.7%) and vomiting (4.7%) when compared with placebo.
The study, published in The Lancet, follows the publication of a meta-analysis last year conducted by Cochrane - an independent global network of health practitioners and researchers, renowned for high-quality systematic reviews.
The previous analysis found that, while Tamiflu did reduce the risk of developing influenza, there was not enough evidence to suggest the drug reduced complications of influenza such as pneumonia. Cochrane also reported an increased risk of adverse effects, including nausea, vomiting and psychiatric effects.
However, the authors of the new study state that among adults with influenza, use of Tamiflu alleviates symptoms more swiftly, reduces the risk of respiratory tract complications and reduces the risk of patients requiring hospitalization.
Sales of Tamiflu saw huge increases in 2009 as a result of the H1N1 swine flu epidemic. Both the US and UK governments were reported to have spent millions of dollars in stockpiling supplies of the antiviral drug in response.
While the Cochrane analysis utilized clinical trial study reports, the new analysis made use of individual patient data from not only the published adult treatment trials but previously unpublished ones as well. Access to the unpublished trials was provided by Roche Pharmaceuticals, the manufacturer of Tamiflu.
The authors examined nine randomized placebo-controlled double-blind trials in their analysis. These trials examined the effects of the licensed 75 mg twice a day dose of Tamiflu among 4,328 adults with seasonal influenza between 1997 and 2001.
In their new analysis, the authors write that treating influenza with Tamiflu reduced the duration of symptoms from 123 hours to 98 hours - a reduction of 21% - compared with placebo.
Tamiflu also reduced the risk of lower respiratory tract infections requiring antibiotics over 48 hours after being assigned the drug or placebo. The authors report that the risk was reduced by 44%, with 4.9% of participants receiving Tamiflu becoming infected compared with 8.7% of participants on placebo.
Hospitalizations also reduced with the prescription of Tamiflu. A total of 0.6% of participants with influenza were hospitalized after being given the drug, compared with 1.7% of those given placebo - a reduction of 63%.
No benefits were observed among participants that were found to not have influenza.
"Our meta-analysis provides compelling evidence that oseltamivir therapy reduces by one day the typical length of illness in adults infected with influenza and also prevents complications and reduces the number of people needing hospital treatment," states study author Prof. Arnold Monto of the University of Michigan School of Public Health.
Symptoms improve, but at what cost?
In an accompanying editorial, Heath Kelly of the Australian National University in Canberra and Benjamin Cowling of the University of Hong Kong write that the increased risk of nausea and vomiting necessitates caution.
"In view of the risk of nausea and vomiting in all patients who receive the drug, confirmation of the diagnosis of influenza before treatment is advisable," they write.
Funding for the study was provided by the Multiparty Group for Advice on Science (MUGAS), through an unrestricted grant from Roche. Outside of the trial, Prof. Monto also receives fees from Roche. The authors state that Roche played no role in the analysis, interpretation or reporting of the study.
An immediate concern presented by the new study is whether the reduction of symptoms by one day is enough considering the risk of side-effects confirmed by the researchers.
"Whether the magnitude of these benefits outweigh the harms of nausea and vomiting needs careful consideration," the authors conclude.
Dr. Tom Jefferson, an epidemiologist at the Cochrane Acute Respiratory Infections Review Group, was critical of the new study's findings, stating that the only reasons that the conclusion of the two studies differ is due to the manner in which the data have been interpreted.
"Our interpretations were driven by our access to over 100,000 pages of Tamiflu clinical study reports, something these authors did not access according to their methods section," he states.