Association of Seasonal Severity and Vaccine Effectiveness With Influenza Vaccination Rates in Children

Abstract

This study examines the associations of seasonal influenza severity as well as current and prior year’s vaccine effectiveness with yearly influenza vaccination rates stratified by age groups.

Influenza vaccination rates among children remain lower than other pediatric and adolescent vaccines.^1,2 Many variables may affect whether a child is vaccinated against influenza.³ However, to what extent, if any, the severity of an influenza season and either current or prior year’s vaccine effectiveness (VE) may affect vaccination rates has yet to be studied, to our knowledge. This information may be important in understanding how to message severity and VE data to parents. Therefore, using publicly available data, we examined the associations of seasonal influenza severity as well as current and prior year’s VE with yearly vaccination rates within age groups.

Methods

Publicly available data from the Centers for Disease Control and Prevention were abstracted for the 2010 to 2017 seasons, including (1) influenza vaccination rates per age group (6-23 months, 2-4 years, 5-12 years, and 13-17 years),⁴ (2) yearly Centers for Disease Control and Prevention influenza severity designations for pediatric patients (low, moderate, high, and very high),⁵ and (3) VE. Graphical tools were used to display vaccination rate trends stratified by age group. Data points in each year were denoted using different shapes to distinguish the 4 categories of influenza season severity and using different bubble sizes to demonstrate VE. Negative binomial models were used to assess the associations of vaccination rates with current and prior year’s VE and seasonal influenza severity, adjusting for age groups and linear and quadratic terms of years. All statistical tests were 2-sided, and a P value less than .05 was considered statistically significant. All analyses were performed using SAS version 9.4 (SAS Institute). Plots were created using ggplot2 in R version 3.4.2 (The R Foundation). This study was approved by the Columbia University Institutional Review Board. Publicly available deidentified data were used; therefore, informed consent was not required.

Results

While there had previously been a general trend of significantly increasing vaccination rates over time, in recent years, there has been a downward trend in vaccination rates in all age groups (Figure 1). Severity of the current influenza season was not associated with vaccination rates across age groups and years (high/very high severity vs low severity: rate ratio, 1.02; 95% CI, 0.93-1.12; P = .63; moderate severity vs low severity: rate ratio, 1.03; 95% CI, 0.99-1.07; P = .21), nor was severity of the prior season. Vaccine effectiveness varied largely across years, ranging from 19% to 60%. Neither current nor prior year’s VE was significantly associated with vaccination rates, with rate ratios close to 1, although a slight decline in vaccination rates was observed in 2015 after the very low VE of 19% in 2014 (Figure 2).

Figure 1. Vaccination Rate by Year and Flu Season Severity Stratified by Age Group.

Figure 2. Vaccination Rate by Year and Vaccine Effectiveness (VE) Stratified by Age Group.

Size of the bubble denotes percentage of VE, with larger bubbles representing higher effectiveness.

Discussion

Families are often made aware of a particularly severe influenza season through the media, which frequently reports on the number of deaths or other indicators of influenza severity. Thus, it is reasonable to assume that most parents will have some knowledge of the general severity of influenza infection each yearly cycle. However, from our analysis, it appears that for the general population, this may not affect vaccination rates. Similarly, current and prior season’s VE also do not appear to affect vaccination, although there was a slight decline in vaccination rates from 2015 following a very low VE during the 2014-2015 flu season. With sparse data, no causal conclusion can be made. This study also only assessed the association of current and prior season’s severity and VE with vaccination rates. It is possible that the effects of season severity and VE may be cumulative, reducing uptake if trends continued for multiple years in succession.

Still, alarmingly, there appears to be a general decline in influenza vaccination rates across age groups over recent years. The decrease in influenza vaccinations in children and adolescents could be a reflection of an increasingly popular antivaccine movement. In addition, influenza vaccine hesitancy can be a mix of issues of complacency reflecting misperception of influenza risk and severity as well as lack of confidence in vaccination effectiveness and safety, including adverse effects (ie, belief that the flu vaccine causes the flu), and in health care authorities.⁶ Further research understanding what factors affect vaccination decisions that could be targeted in future interventions may be warranted.