Timing of Fever After MMR Vaccine
Fever typically occurs 5-12 days after MMR vaccination, with peak incidence at 7-10 days post-vaccination, coinciding with the replication of the live attenuated measles virus. 1
Primary Fever Window
The established risk period for fever is 5-12 days after vaccination, when the live attenuated measles virus reaches peak replication in the body. 1
The Advisory Committee on Immunization Practices (ACIP) defines this risk period variably across studies as 8-14 days, 7-12 days, or 5-12 days, but the most consistent evidence points to the 5-12 day window. 1
Peak viral replication occurs specifically at 6-12 days post-vaccination, which directly correlates with the timing of fever development. 2
Incidence and Characteristics
Fever (≥102°F or ≥39°C) occurs in approximately 21.5% of children receiving MMRV vaccine and 14.9% of those receiving separate MMR and varicella vaccines during the 0-42 day period after vaccination. 1
The fever typically resolves spontaneously without sequelae and represents a normal immune response to viral replication rather than a complication. 1
Approximately 5% of vaccinated persons develop measles-like symptoms including fever 7-10 days after vaccination, which can include rash and conjunctivitis. 3, 4, 5
Clinical Context and Mechanism
The timing corresponds directly to when the live attenuated measles virus replicates most vigorously, generating the immune response necessary for protection. 2
Children who develop fever during this window demonstrate higher measles antibody titers at 6 weeks post-vaccination, indicating a more robust immune response. 2, 6
The fever risk is concentrated in the 6-12 day window when viral replication peaks, which is also when febrile seizure risk is elevated. 2
Important Clinical Caveats
Any fever occurring 5-14 days after MMR vaccination in a child without other risk factors for measles should be presumed vaccine-related, not wild-type measles infection, given the current low incidence of natural measles. 4, 5
PCR testing during this period will detect measles vaccine virus, not wild-type measles, and does not indicate transmissible infection or require public health response in recently vaccinated children. 5
Children with prior history of medically-attended fever, previous seizures, or those receiving MMRV (rather than separate MMR and varicella vaccines) have increased risk of developing fever during this window. 7
Fever occurring outside the 5-14 day window should prompt evaluation for other causes, as it is unlikely to be vaccine-related. 1