Second Dose of Influenza Vaccine for 18-Month-Old Child
Yes, the 18-month-old child is due for the second dose of influenza vaccine now, provided at least 4 weeks have elapsed since the first dose. This child requires two doses during their first season of influenza vaccination to achieve adequate immune protection.
Two-Dose Requirement for Young Children
Children aged 6 months through 8 years receiving influenza vaccine for the first time require two doses administered at least 4 weeks apart. 1, 2, 3 This recommendation applies to:
- Children receiving influenza vaccine for the first time 1
- Children who have received only 1 dose before July 1 of the current season 1
- Children whose vaccination history is unknown 1
The rationale for two doses is immunologic: vaccine-naïve children do not produce adequate protective antibody responses after a single dose. 4, 5 Research demonstrates that children receiving only one dose in their first vaccination season have significantly lower antibody levels and are less likely to achieve protective antibody titers compared to those receiving two doses. 3, 4
Timing Between Doses
The minimum interval between the first and second doses is 4 weeks (28 days). 1, 2, 3, 4 Key timing considerations include:
- Doses administered up to 4 days before the minimum 4-week interval are considered acceptable 1, 4
- There is no maximum interval between doses—if the second dose is delayed beyond 4 weeks, it should still be administered as soon as possible 4
- Both doses should ideally be completed by the end of October to ensure protection before peak influenza activity 1, 2, 3, 4
Practical Implementation
For this 18-month-old child:
- Verify that at least 4 weeks have elapsed since the first dose before administering the second dose 1, 3
- The two doses do not need to be the same brand or formulation, as long as both are age-appropriate 1, 4
- The child may receive a combination of different inactivated influenza vaccines if appropriate for age 1, 4
- The recommended dose volume for an 18-month-old is either 0.25 mL or 0.5 mL depending on the specific vaccine product 1, 2
Common Pitfall to Avoid
Do not assume that one dose is sufficient for young children in their first vaccination season. Poor two-dose compliance has been documented, with only 63-68% of children aged 6-23 months completing the two-dose series across multiple seasons. 6 Among children aged 2-4 years requiring two doses, compliance drops to only 12-19%. 6 This incomplete vaccination leaves children inadequately protected against influenza.
In the 2014-15 season, among children who required and received 2 doses, only half completed the series before influenza activity peaked. 6 This underscores the importance of administering the first dose early in the season and ensuring timely follow-up for the second dose.
Future Seasons
After completing the two-dose series this season, this child will require only one dose annually in subsequent influenza seasons (until age 9 years), provided they have received at least 2 total doses of influenza vaccine (any formulation) administered at least 4 weeks apart at any time before July 1 of future seasons. 1, 2, 3 The prior doses need not be from the same or consecutive seasons. 1