A flu epidemic is raging nationwide with the number of patients in a single week estimated to have reached a record high 2.83 million, the health ministry said Friday.

The average number of patients per medical institution reported in the week through last Sunday stood at 51.93, the highest since comparable data became available in 1999, it said. The figure for the same week last year was 28.66.

The illness has hit young people particularly hard, causing the temporary closure of 108 schools and 5,737 classes across the nation.

The data is based on reports by some 5,000 clinics throughout the country. The average number of flu patients per clinic nearly doubled from 26.44 in the previous week to surpass the benchmark of 30, prompting authorities to issue a warning, according to the Health, Labor and Welfare Ministry.

This season a delay in the domestic production of flu vaccines has caused a shortage, and the ministry urged those aged 13 and above to only take a single shot. The current supply still exceeds last season's demand, but health minister Katsunobu Kato admitted that some regions are experiencing a shortage.

The most common strains detected in the last five weeks were Type A and Type B, with the former being a relatively recent version of the strain that was responsible for the 2009 swine flu pandemic. Cases of Type B flu usually start increasing in February, but this season the number of patients has already surged, the ministry said.

There were an estimated 590,000 flu patients aged 5 to 9, followed by 400,000 cases among those aged 10 to 19, 290,000 for people in their 40s, and 270,000 for children under 5.

By prefecture, Kagoshima had the highest number of flu patients per clinic at 86.53, followed by Miyazaki at 84.97, Fukuoka at 83.99, Oita at 82.40 and Saga at 69.64.