'serious symptoms' is correct and usable in written English.
You could use it to describe a medical condition, for example: "The patient suffered from a severe fever and other serious symptoms."
More than 20 people died or suffered serious symptoms.
The bite, though rarely fatal, can cause serious symptoms.
For men with serious symptoms, testosterone deficiency can be debilitating.
Usually people are hospitalized if they have more serious symptoms.
More serious symptoms can include rapid breathing, unresponsiveness and disorientation.
Around 30% of patients given antibiotics get diarrhoea, with potentially serious symptoms.
That has lead them to prescribe "uppers" to try and head off any more serious symptoms.
She did not show serious symptoms and was discharged without medication or surgery.
Less common but more serious symptoms include complete paralysis or coma.
However, those who experience serious symptoms would benefit from immediate medical attention.
Less common, but more serious symptoms, include complete paralysis or coma.
