Hi, I had gone through your question and understand your concerns. Ii is sad but coincidence that your niece is having high fever with congestion after H1N1 shot. Fever is the most common systemic reaction after vaccination. Fever due to vaccination does not usually last for more than 48 hours and any fever persisting beyond this time should be evaluated for other causes . You have not mentioned her age , I assume she received inactive influenza vaccine , more ever live attenuated influenza ( nasal) is given after 2 years of age . Some children and young adults 2 years through 17 years of age have reported experiencing mild reactions after getting the nasal spray flu vaccine, including runny nose, nasal congestion or cough, chills, tiredness/weakness, sore throat and headache but live attenuated virus is made from weakened virus and does not cause flu. So it seems to be flu like illness but a coincidence , not related to her H1N1 shot . Hope this answers your question. If you have additional questions then please do not hesitate in writing to us. Wishing her speedy recovery. • DISCLAIMER: - All the information provided here is for information purpose only , it is not a substitute for the advice of a physician after physical examination , it is not intended to replace your relationship with your doctor. This information in no way establishes a doctor-patient relationship. Before acting anything based on this information do consult your doctor. I recommend that online users seek the advice of a physician who can perform an in-person physical examination