HPA figures show that 40 patients were admitted to ICU in the week ending 31 December, with five deaths. The previous week had seen 21 admissions and no deaths.
GPs in Northern Ireland saw the busiest time over the Christmas period, carrying out 43.7 consultations for flu-like illness per 100,000 people.
Calls to NHS Direct also rose and outbreaks were recorded in seven care homes, three schools and two hospitals in a single week.
The HPA said that the figures may underestimate the true incidence as GP surgeries were open for only three days that week.
Although flu activity remains below the level expected in a typical flu season, officials said people should continue to protect themselves by having the flu vaccine and by staying at home if they contract flu.
However, vaccination rates in the elderly continue to improve: 71.7% of patients aged 65 or over have had the vaccine this winter, close to the DH target of 75%.
Dr Richard Pebody, head of seasonal flu surveillance at the HPA, said: 'Although unpleasant, flu is a self-limiting illness and if you have flu it is best to stay at home until you are well. If people in at risk groups develop symptoms consistent with flu, or if anyone’s symptoms persist or become more severe, we advise they seek medical advice.'