NEW YORK (AP) — U.S. flu infections surged over the holidays, prompting health officials to call it a severe season that is likely to worsen.

Recent government data showed that by some measures, this season surpasses last year's flu epidemic, acknowledged as one of the harshest on record. In addition, COVID-19 infections have also seen an uptick, although they remain less frequent than flu cases.

Forty-five states reported high or very high flu activity during Christmas week, an increase from 30 states the previous week. Public health experts attribute the escalating numbers, in part, to the A H3N2 flu virus, which has a history of causing severe outcomes, particularly among older adults. Alarmingly, over 90% of the reported H3N2 cases this season stem from a variant known as subclade K, which does not match the strain included in this year’s vaccinations.

Typically, flu seasons do not reach their peak until January or February, leaving uncertainty regarding the impact of the vaccine mismatch.

Last year's flu season saw its overall hospitalization rate reach the highest levels since the H1N1 pandemic over 15 years ago, with child flu deaths hitting a record of 288. So far this season, there have been nine pediatric deaths reported, and emergency department visits for flu has surpassed previous highs.

The CDC estimates at least 11 million illnesses, 120,000 hospitalizations, and 5,000 deaths from the flu have already occurred this year. Authorities recommend that everyone aged six months and older who has yet to be vaccinated this season should do so to mitigate risks associated with the illness.