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When “Up-To-Date” Isn’t – The Impact of Birth Order on Vaccine Status

September 9, 2022

In a recently released Research Brief in Pediatrics, we learn that being “up-to-date” (UTD) on vaccines, versus not being UTD, appears to be associated with whether a child is first-born or not (10.1542/peds.2022-056883). The authors analyzed data from the CDC-administered National Immunization Survey-Child (NIS-Child), a nationally representative survey covering US children from 19-35 months, for the years 2016-2019. All vaccines studied were intended to have been given prior to 19 months. By combining these four years of data, the authors were able to complete a cross-sectional analysis of vaccine status for 24,582 first-born and 37,761 non-first-born children and were able to examine the impact of multiple possible confounding variables on UTD status, including child’s age, sex, race/ethnicity, mother’s age and level of education, household poverty status, insurance coverage, and number of children in the household.

The authors carefully define UTD as per the CDC, and this is detailed in the footnotes for Table 1 which beautifully presents the study results. For readers not familiar with this lingo, “The 4:3:1:3:3:1:4 vaccine series indicates 4+ DT containing, 3+ polio, 1+ measles, 3+ Hib, 3+ Hepatitis B, 1+ varicella, and 4+ pneumococcal vaccines.” Most of us recall when there were Haemophilus influenzae type b shortages, and these are incorporated into the definitions also. Overall, and with adjustment for the confounding variables noted above, being first-born is associated with greater likelihood of being UTD on almost every vaccine and combination of vaccines. Interestingly, when number of children in the household was also added to the analysis, this result was slightly diminished: the authors suggest that family size may play a role in this phenomenon. Please see the article for more details.

The authors suggest several common-sense reasons for their findings, but rather than just speculating on this topic, I like the “what’s next” approach – what can we do about this clearly defined issue? There are many times when I am seeing one child in the family and the parent asks me whether the others are due for shots or well care – this seems to me a “golden moment” to collaborate with the parent to make sure each of their children is appropriately scheduled as possible, and the parent knows when each should be seen next. Possibly parents could be provided a small wallet-sized card from their medical home that has each child’s name, date of birth and, next planned appointment on it – this could be routinely updated at check out when any child in the family is seen. Another strategy might be adding on a sibling for “shots only” if they are there when another child in the family is being seen, and then scheduling their preventive health care for a future time. Maybe your practice is already doing all of this or more – let us know how you try to keep families with more than one child on track with vaccines for all of their children.

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