A Hidden Vulnerable Population: Young Children Up-to-Date on Vaccine Series Recommendations Except Influenza Vaccines

"...longstanding hesitancy specific to the influenza vaccine."

Children under the age of under 2 years ("very young") are at high risk for influenza complications, but their uptake of influenza vaccination is substantially lower than other routinely recommended vaccines in the United States (US). This study has the goal of addressing limitations of prior studies on very young child influenza vaccine uptake, which include: Few examine provider-verified influenza vaccination (relying instead on parental report), few contain parental vaccine attitudes variables (known to be crucial to vaccine uptake), and none, to the researchers' knowledge, consider intersectionality of social disadvantage or how influenza vaccine determinants differ from those of other recommended vaccines.

The nationally representative study examined provider-verified data on 7,246 children aged 6-23 months from the 2011 National Immunization Survey (NIS) to include the Parental Concerns (PC) module, focusing on children who are up to date (UTD) on a series of vaccines (the 4:3:1:3:3:1:4 series) but not influenza vaccines (what they call "hidden vulnerability to influenza").

The researchers divide determinants into 3 factors: (i) predisposing (e.g., child's race/ethnicity, parental vaccine attitudes and beliefs); (ii) enabling (e.g., family income, health insurance); and (iii) need (e.g., functional state, need for medical care). The model also accounts for intermediate-level health behaviours influencing health services use (e.g., personal health practices). Seven variables represent contextual-level factors (family- or medical-practice-level) predisposing, enabling, or creating need for influenza vaccination and other health services use; 7 variables represent parental perceptions and beliefs surrounding vaccines and vaccine-preventable diseases; 5 variables represent individual (child)-level factors; and 1 variable represents the child's personal health practices: whether they were ever fed breast milk.

The study found that approximately 71% of children were UTD on the series, yet only 33% were UTD on influenza vaccine recommendations by their second birthday; 44% had hidden vulnerability to influenza. Independent of parental history of vaccine refusal and a myriad of health services use factors (referenced above), no parental history of delaying vaccination was associated with 7.5% (2.6-12.5) higher probability of hidden vulnerability to influenza despite being associated with 15.5% (10.8-20.2) lower probability of being UTD on neither the series nor influenza vaccines. Thus, parental compliance with broad child vaccine recommendations and lack of vaccine hesitancy may not indicate choice to vaccinate children against influenza. In other words, parents with children UTD on most vaccines, who thus appear to support the concept of vaccination, seem to be uniquely hesitant or skeptical about the influenza vaccine. This supports the theory that vaccine hesitancy is highly context-dependent and functions differently comparing influenza to other vaccines.

Examination of intersectionality - a theory positing that social statuses like race/ethnicity, gender, and social class reinforce each other in producing and maintaining health outcomes across the life span - suggests another unique predictor of hidden vulnerability to influenza: Maternal college degree attainment was associated with higher uptake of all vaccines studied except among non-Hispanic Black and Hispanic children, for whom it was instead associated with hidden vulnerability to influenza. That is to say, higher parental education, which is generally associated higher vaccine uptake in US children, appears to have different implications by race/ethnicity, particularly with regards to health behaviour. This finding is in line with recognition on the part of public health and health policy researchers that "health equity can only occur by incorporating health into upstream decision-making, such as social and economic policy (e.g., the 'Health in All Policies' approach)."

In light of the findings, the researchers recommend that paediatricians talk with parents about their history of refusing or delaying vaccination, seeking to understand their vaccine hesitancy broadly and specific to influenza, regardless of the child's general vaccine history. They also suggest that policymakers and researchers from public health, sociology, and other sectors "collaborate to examine both how preventive health services use functions in the context of interacting social disadvantage, and how upstream social and economic policies lead to equitable health."