Visits to hospital emergency departments, urgent care centers and other outpatient clinics by people with flu symptoms have been skyrocketing for several weeks. As of mid-January, such visits had surpassed every flu season except 2009-10, when a new flu strain caused a global pandemic. The dominant strain this season, H3N2, is particularly virulent, and the vaccine isn’t very effective against it.

NYC Health & Hospitals has seen a 40% increase in the number of people being tested for the flu compared with this time last year, according to Sean Studer, deputy chief medical officer of the group, which runs New York City’s public hospitals, clinics and long-term care facilities.