Is a temperature of 100.1 degrees Fahrenheit, indicating mild fever, a normal reaction in a 4-month-old infant after vaccination, and when should parents contact emergency services (911) or a physician?

From the Guidelines

A temperature of 100.1 degrees Fahrenheit, indicating mild fever, is a normal reaction in a 4-month-old infant after vaccination.

Key Points to Consider

• Fever after vaccination is a common adverse event, with studies showing that approximately 5-27% of children develop fever after vaccination, depending on the vaccine and age group 1.
• Mild fever is generally defined as a temperature less than 102°F (38.9°C), and is usually self-limiting and resolves on its own within 1-2 days.
• Severe reactions, such as high fever (greater than 103°F or 39.4°C), seizures, or difficulty breathing, are rare but require immediate medical attention.
• When to contact emergency services (911) or a physician:
  • If the infant's temperature exceeds 104°F (40°C)
  • If the infant shows signs of severe distress, such as difficulty breathing, seizures, or lethargy
  • If the infant has a history of severe reactions to vaccines or has a compromised immune system
  • If the fever persists for more than 2 days or is accompanied by other concerning symptoms, such as vomiting, diarrhea, or rash.

Vaccine-Specific Information

• DTaP vaccine: 5.5% of children experienced fever >101 F (>38.3 C) within 72 hours of vaccination 1.
• MMR vaccine: approximately 5% of children develop a temperature of greater than or equal to 103 F (greater than or equal to 39.4 C) after MMR vaccination, usually occurring 7-12 days after vaccination 1.
• Influenza vaccine: postvaccination fever was noted among 11.5% of children aged 1--5 years, 4.6% among children aged 6--10 years, and 5.1% among children aged 11--15 years 1.
• MMRV vaccine: fever (>102°F [>38.9°C] oral equivalent) was reported at a statistically significantly greater rate in persons who received MMRV vaccine than in persons who received the two vaccines administered concomitantly at separate injection sites 1.

From the Research

Fever Reaction in Infants After Vaccination

• A temperature of 100.1 degrees Fahrenheit, indicating mild fever, can be a normal reaction in a 4-month-old infant after vaccination 2, 3.
• Fever is a frequent systemic adverse event following immunization, especially in infants and young children, and can be caused by the immunization or may coincide temporally as an indication of underlying disease 2.
• The time pattern of fever attributable to immunization has characteristic features depending on the vaccine used, and comparability of fever rates associated with different vaccines is frequently hampered by the use of different definitions and/or assessment techniques for fever 2.

When to Contact Emergency Services or a Physician

• Parents should contact emergency services (911) or a physician if the infant's fever is extremely high (usually above 104°F) or if the infant shows signs of distress, such as difficulty breathing, lethargy, or seizures 4, 5.
• If the infant has a fever after vaccination, parents should monitor the infant's temperature and watch for any signs of complications, such as febrile seizures, and seek medical attention if necessary 4, 5.
• It is essential to note that fever after vaccination is generally a benign clinical sign, but it can be a concern in certain cases, and parents should consult with their physician if they have any concerns about their infant's reaction to vaccination 3, 6.