University Hospitals Tees has reported more than 100 flu-related admissions per week in December.
Group chief medical officer Mike Stewart said: “Flu can be a really serious illness.
“It’s not just a cold.
“We have seen hundreds of patients over the last few weeks with flu, most requiring oxygen therapy and some so ill, often with pneumonia as a complication, they require treatment in our high dependency and intensive care units.
“This is a continuing trend and we are expecting this pattern to continue throughout January so this is far from over.”
To prevent further cases, Mr Stewart encouraged the public to get the vaccine.
He said: “If you haven’t been vaccinated yet, I would urge you to get the vaccine as soon as possible.
“It’s not too late to benefit from this.
“It’s safe, effective and for many people it’s free of charge.
“Even if you have to pay for it, it’s a small price to pay to avoid this potentially life threatening illness.
“The vaccine is the best way to protect yourself and those around you.”
Pharmacies offering vaccinations can be found by calling 119 free of charge until January 31.
Mr Stewart said: “Hospitals and emergency departments throughout the region are facing some real pressures, as are our colleagues in the ambulance services.
“We will always be here for seriously unwell patients but we do ask that for anything non-life threatening people first try their local community pharmacy, 111 online or by phone, an urgent treatment centre or GP practice.”
As a measure to curb infections, patients are also being asked to wear masks.
Mr Stewart added: “When in hospital, we have been asking some patients if they would be willing to wear a mask to help protect themselves as well as help prevent the further spread of infection.
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“This is one of a number of measures we ask staff, patients and visitors to carry out to help stop infections.
“Wearing a mask is not mandatory of course and our clinical teams will always ensure all other precautions are also taken.”
For further information on flu, the public can visit the NHS website.