"severe flu" is correct and usable in written English.
You can use it to refer to a particularly bad case of the flu (influenza). For example: "I had a severe flu last winter that lingered for months."
In his last letter to me, in March 1996, when he was 83, he told me of his recovery from severe flu: "I'm back to normal and have several projects.
Thus we are entering what looks like a severe flu season with less than optimal protection.
Last year was a particularly severe flu season, mainly because of the Type A Sydney strain.
A patient with severe flu could be given the wrong drug and die of pneumonia before test results come in.
Each year in the United States, about 200,000 people are hospitalized with severe flu symptoms, and 36,000 die.
The vaccine was less effective in macaques, but the animals had less severe flu symptoms after receiving the jabs.
Symptoms are typically a severe flu-like illness with severe headache, muscle pain and a rash.
Robert Air, a builder, died on Tuesday, hours after arriving at hospital complaining of severe flu-like symptoms.
I was brilliantly well, really looking forward to it, then as soon as I was on the plane I went down with severe flu-like symptoms.
Even classic dengue _ known as "bonebreak fever" _ can cause severe flu-like symptoms, excruciating joint pain, high fever, nausea and rashes.
Patients with dengue fever, a mosquito-borne illness that causes a severe flu-like condition, typically start out with similar symptoms, but as the disease progresses, some wind up with life-threatening infections, while others mend fine on their own.
