Illinois has seen a spike in flu cases, with the state raising its activity level from low to moderate in just a week’s time.

The latest data gathered shows hospital admissions for the flu more than doubled from Dec. 13 to Dec. 20, according to the Illinois Department of Public Health. The most recent data has not included Christmas week activity.

Emergency room visits due to the flu also rose sharply, the state reported.

The biggest increase in hospital admissions for the flu is children ages 5 to 17, who accounted for more than 10% of cases reported Dec. 14 to Dec. 20, the Department of Public Health said. Children also make up the largest demographic for acute respiratory cases at 22.7%.

Hospital reports for flu patients in intensive care units rose to 51 in one week, far outpacing other seasonal respiratory illnesses, including COVID-19, the Department of Public Health said.

Only 1 in 5 Illinois residents has received the flu vaccine, and only 5% have received the updated COVID-19 shot, according to the latest data from the Illinois Department of Public Health.

The number of residents who have received the flu shot through the end of November was at 21.9%, down 4% from this time a year ago and reflecting a consistent downward trend in people getting vaccinated over the past five years.

The highest rate of people being immunized against the flu by the end of November was 30.2% in 2020, in the middle of the COVID-19 pandemic.

The majority of those who have received the flu vaccine – 47.5% – are people age 65 and older. The next group with the highest rate is children ages 6 months to 4 years old, at 23.2%, followed by adults ages 50 to 64 at 21%.

Nationally, 39% of adults to date have received the flu shot, according to the latest data provided by the Centers for Disease Control and Prevention.

Last season, flu vaccination reduced the risk of hospitalization by up to 78% for children, COVID-19 vaccines cut hospitalizations among seniors almost in half, and RSV protection prevented almost 90% of infant hospitalizations, according to a news release from the IDPH.