If your one-year-old child has been experiencing a fever between 38.7 and 39.1 degrees Celsius since December 25th, it is recommended to consult a healthcare professional if the fever persists for more than 3 days. Some vaccines can cause fever about 5 to 12 days after administration, but it should typically not last longer than 3 to 4 days. Consult a healthcare professional if the fever persists beyond this timeframe.