The main circulating flu virus this winter is influenza A type H3N2, which affects the elderly in particular.
The US Centres of Disease Control has warned that half of H3N2 viruses tested there were "drifted strains" which were different to the one protected by the flu jab.
"There has been a significant increase in flu and flu-like infections in Graz and Tyrol", the head of Vienna Medical University’s Virology Department, Franz X. Heinz, told the Austrian Press Agency. He said there had also been an increase of cases in Vienna, affecting the elderly, schoolchildren and teachers especially.
“Our data indicates that we are on the threshold of a flu outbreak,” he said.
Doctors can prescribe anti-viral drugs, which if taken shortly after the onset of symptoms (such as a high fever and body aches) can shorten the duration and severity of the virus.