It's not clear whether the anti-influenza drug Tamiflu prevents infection or reduces complications, a new analysis suggests.

Researchers combed through more than 16,000 pages of clinical trial data and documents used by regulators to approve oseltamivir, sold as Tamiflu.

Governments worldwide spent billions of dollars stockpiling antivirals such as Tamiflu (CBC)

While the drug did reduce the time to first alleviation of symptoms by an average of 21 hours, it did not reduce the number of people who went on to need hospital treatment, Dr. Tom Jefferson, an epidemiologist based in Rome, and his team concluded in the Cochrane Library, an international publication that scrutinizes health information.

Governments worldwide spent billions of dollars stockpiling antivirals such as Tamiflu and Relenza (zanamivir) as the World Health Organization recommended in 2002.

The drugs aim to limit the spread of viruses within an infected person to reduce the duration of symptoms and the chances of passing on the infection to others.

When the team compared published data with more complete unpublished clinical trial records, they said they found inconsistencies in the serious adverse events and disparities in the numbers of influenza-infected people in the treatment and control groups.

"The disparity is important because in oseltamivir trials, primary efficacy outcomes were analyzed on the influenza infected subpopulation, but it is not clear that these groups were in fact comparable," Jefferson said in a release.

"We believe that until more is known about the mode of action of neuraminidase inhibitors health professionals, patients and other decision makers need to reflect on the findings of this review before making any decision about the use of the drug," he added.

Stockpile 'essential' to flu plan

Tamiflu's maker, Roche, maintains it provided the Cochrane team with access to 3,200 pages of information to conduct the evaluation.

The Public Health Agency of Canada said it is aware of the publication and is reviewing it closely.

"This review comments on the effectiveness of antivirals for the prevention and treatment of influenza in healthy adults and children," an agency spokesperson wrote in an email. "It does not address the effectiveness of antivirals for the treatment of pandemic influenza, particularly in individuals at high risk of complications due to influenza, such as pregnant women, young children, individuals with chronic medical conditions and others."

"Multiple observational studies conducted during the 2009 H1N1 pandemic have supported the effectiveness of antivirals in minimizing illness and deaths from pandemic influenza," said Sylwia Gomes. "That’s why the stockpiling of antivirals is an essential component of the Canadian Pandemic Influenza Plan."

One infectious disease expert said he has some questions about the review.

"It's kind of presented a bit like a 'gotcha' kind of paper, and it's not clear in my mind how valid that is," said Dr. Michael Gardam, the director of infection prevention and control for Toronto's University Health Network.

He said health officials have to "live in the real world" and "get data from wherever we can get data."

"Some of it is better than others, but I think in general most people who work in these fields believe that these drugs have an impact."

On the amount of money the government spent to stockpile Tamiflu, Gardam said: "So what's the alternative? Not buy it. Do you feel comfortable doing that?"

The Cochrane review was funded by the National Institute for Health Research Health Technology Assessment program.