VILNIUS, Feb. 13 (Xinhua) -- Lithuania suffers from one of the most severe flu outbreaks in recent years, with epidemic levels officially stated in 31 out of 60 country's municipal districts, Lithuania's Centre of Infectious Diseases (ULAC) announced on Tuesday.

According to the statement released by ULAC, during the sixth week of this year, Feb. 5-11, 185.2 cases of flu per 10,000 residents were recorded, compared to 108.8 cases in the same period a year earlier.

"Due to increased sickness rate of flu and other infectious respiratory diseases, residents are urged to avoid mass gathering places and contacts with infected people," ULAC said in the statement.

More than 500,000 sick-leave certificates were issued from Jan. 1 to Feb. 13 in the Baltic country with around 2.9 million residents, Lithuanian state-run social insurance fund Sodra data showed. The authority forecasts further growth of sick-leave cases, local website 15min.lt reported.

According to 15min.lt, several Lithuanian employers already faced problems due to epidemic levels of flu. Small Planet Airlines, Lithuanian charter airline company, was forced to urgently search for staff in neighboring Poland after two crew members fell ill with flu.

Some companies encourage employees to work at home in order to avoid the flu, 15min.lt reported.

According to the recommendations from the World Health Organization, vaccination has been acknowledged as the most effective preventive measure as regards spread of influenza.