Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Get emergency medical help for:
Yellow skin or yellowing of the eyes
Vomiting blood or passing black or bloody stool
Bleeding that is difficult to stop
Confusion, unusual drowsiness, seizures, or collapse
Severe abdominal pain or repeated vomiting
Very little urine
Trouble breathing
Cold, clammy skin, fainting, or other signs of shock
And for anyone, someone who:
has had a seizure that will not stop, or does not wake up afterwards
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek prompt medical advice if you develop fever, severe headache, muscle pain, nausea, or vomiting during travel in a yellow fever risk area or soon after returning.
Tell the healthcare service exactly where you travelled, when you returned, and whether you were vaccinated.
And for anyone, someone who:
has a fever with a stiff neck, a rash that does not fade under pressure, or severe headache
3
Does any of this apply?
Book an appointment
Routine follow-up is appropriate after recovery if you were advised to repeat liver or kidney tests, or if tiredness is slowly improving without new warning signs.
Seek earlier review if symptoms worsen, jaundice persists, or your urine output falls.
The article also says
Do not rely on vaccination history to rule out every possibility.
Vaccination is highly effective, but no vaccine prevents every case, and another travel-related infection may be causing the symptoms.
Every sign above is quoted from the sofidex article on Yellow fever. If you are not sure, err upward — it is always reasonable to be checked.