Influenza Vaccine (Influvac Tetra) is More Likely to Cause Same-Day Fever
Influvac Tetra (inactivated influenza vaccine) is significantly more likely to produce same-day onset fever in a 6-month-old infant compared to Typhoid Conjugate Vaccine (TCV). This is because influenza vaccines have well-documented rapid onset of fever within 24 hours in young infants, whereas TCV data for this age group and timeframe is extremely limited.
Evidence for Influenza Vaccine Same-Day Fever
Fever occurs within 24 hours after immunization in approximately 10% to 35% of children younger than 2 years following inactivated influenza vaccine (IIV) administration 1. This represents a substantially higher rate than in older children and adults, where fever rarely occurs 1.
Timing and Mechanism
- Systemic reactions from influenza vaccines, including fever, typically begin 6-12 hours after vaccination and can persist for 1-2 days 1, 2
- The fever represents an immune response to the vaccine antigens, not actual influenza infection, as IIVs contain only inactivated virus 1, 3
- Among high-risk infants aged 6 months to 4 years, fever rates can reach up to 27% 4, 2
Limited Evidence for TCV Same-Day Fever in 6-Month-Olds
TCV is typically not administered to 6-month-old infants as it is generally recommended for children ≥6 months in endemic areas, but more commonly started at 9-12 months or older 4. The available evidence does not provide specific data on same-day fever onset in 6-month-old infants receiving TCV.
Available TCV Safety Data
- In a Malawian study of children aged 9 months to 12 years, TCV recipients had similar solicited local reactions (3%, 95% CI 1.3-5.1) and systemic reactions (9%, 95% CI 6.2-12.6) compared to control vaccine 5
- The study did not specifically report timing of fever onset or stratify fever rates by the youngest age group 5
- TCV is safe and well-tolerated, but the temporal pattern of fever development in young infants is not well-characterized in the literature 6, 7
Clinical Reasoning for This Case
Given that both vaccines were administered on the same day and fever developed by evening:
- The influenza vaccine is the most probable cause based on its well-documented pattern of fever onset within 6-24 hours in this age group 1, 4
- The 10-35% incidence rate of same-day fever after influenza vaccination in infants under 2 years is clinically significant 1
- TCV lacks comparable data demonstrating same-day fever onset in 6-month-old infants 4
Important Clinical Caveats
- Acetaminophen 10-15 mg/kg every 4-6 hours can be used for fever management, but routine prophylactic use is not recommended for healthy infants 4
- Prophylactic antipyretics are only recommended for infants with a history of seizures or family history of convulsions 4
- Fever ≥40.5°C (≥105°F) within 48 hours constitutes a severe reaction requiring VAERS reporting 4
- When influenza vaccine is given concomitantly with other vaccines (such as PCV13 or DTaP), there is a small increased risk of febrile seizures, with a maximum additional 30 cases per 100,000 children vaccinated 1
- The benefits of timely vaccination with same-day administration outweigh the risk of febrile seizures, which rarely have long-term sequelae 1