Google Flu Trends overestimated peak flu levels in the U.S. this winter, a science journal reports.

Nature News compared three different methods of measuring the extent of flu in the U.S. population this flu season.

Google reports trends in search terms used on its site and can correlate them with other data such as geographic locations. For example, rising requests for information about a subject can be a leading indicator of real events, such as an outbreak of influenza.

Traditional estimates of flu levels are based on how many people report symptoms to doctors. (iStock)

A graph accompanying the Nature News article shows the search engine's peak for last month at more than 10 per cent, compared with about six per cent using traditional data from the U.S. Centers for Disease Control and Prevention.

Traditionally, countries use networks of doctors who report cases of patients showing flu symptoms such as high fever.

Google Flu Trends compiles records of flu-related search terms to estimate requests for information about the flu. Researchers at the CDC have said it can act as an early warning, and other researchers have confirmed its estimates are accurate.

The overestimate may have been fuelled by media coverage of this year's early and severe flu season in the U.S., which in turn triggered searches for flu information by people who weren't sick, the journal said.