ATHENS, Jan. 17 (Xinhua) -- Greek health authorities have recorded the fourth death from complications caused by flu, according to weekly data from the Hellenic Center for Disease Control and Prevention (KEELPNO) released on Thursday. Three of the deaths were reported within a week.

The current epidemiological surveillance of KEELPNO, which recorded data for seasonal influenza in the week from Jan. 7 to Jan. 13, showed the number of serious cases of influenza introduced into intensive care units (ICUs), due to complications, has risen as well from 12 cases to 35.

Of these, 21 are men and 14 are women, aged 39 to 90 years. It is noteworthy that only three of them were vaccinated, although most of them belong to high-risk groups and are therefore prone to complications.

In other words only 9 percent of certified influenza cases who were hospitalized, have been vaccinated this year, according to health experts.

"The period of increased flu activity has begun in Greece. The predominant subtype is A (H1N1) which also caused the 2009 epidemic," experts say in their report and note that flu vaccination "is the best way to prevent the disease".

KEELPNO said that infections usually rise gradually ahead of Christmas, peak at the end of January and February and then start to subside in March.

The European Union recommends that 75 percent of vulnerable groups -- such as the elderly, pregnant women and those with chronic medical conditions -- should be covered by seasonal flu vaccinations.

Each year, there are as many as 50 million cases of seasonal influenza in the EU, with an estimated 40,000 deaths.