People at higher risk of serious complications include:

1-people age 65 and older
2-Children younger than 5 years old
3-Children with neurodevelopmental conditions.
4-Pregnant women (see also Is there a treatment)
5-People of any age with underlying medical conditions, such as:

-Asthma
-Diabetes &obesity
-Heart disease.
-People with weakened immune system (e.g., taking -immunosuppressive medications

The following are the most worrisome symptoms that calls for immediate seek of professional medical care:

For adults, teens, and kids aged 3 to 12 years old

1-Shortness of breath
2-Persistent vomiting
3-Confusion  or dizziness

For children younger than 2 years old

1- Very rapid breathing
2- Not interacting normally, not eating or drinking normally, being unusually irritable, or appearing unusually sleepy
3- High fever and rash
4- A bluish color of the lips and skin