Previous Flu Severity, Vaccine Effectiveness, Does Not Affect Vaccination Rates

Melissa Stockwell, MD, MPH

The severity of disease burden and hospitalization from an influenza (flu) season does not directly affect the rate of vaccinations, according to a new study.

The reported effectiveness of vaccines, as well as past flu season severity reports, do very little to influence what is already a decreasing rate of vaccinations among children and teenagers in the US.

A team of investigators, led by Melissa Stockwell, MD, MPH, of the Columbia University Irving Medical Center, believe the findings are counterintuitive to the narrative that families are heavily influenced by the media’s coverage of flu severity and annual vaccines success rates.

The New York-based investigators used publicly-available US Centers for Disease Control and Prevention (CDC) data from the 2010-2017 flu seasons to assess for flu vaccination rates, flu severity designation rates per pediatric patients, and vaccine effectiveness.

Vaccinated persons were stratified by age groups: 6-23 months, 2-4 years, 5-12 years, and 13-17 years old. Flu severity designations were split as low, moderate, high, and very high.

The team used negative binomial models to assess the associations of vaccination rates with current and prior year’s vaccine effectiveness and seasonal flu severity, with adjustments for age groups and linear and quadratic terms of years.

Though recent general trends show a significantly increased rate of vaccination over time, all pediatric and adolescent age groups reported decreased rates. Current flu season severity was not found to be associated with vaccination rates, across age groups and in various flu seasons (high/very high severity vs low severity RR, 1.02; 95% CI, 0.93-1.12; P = .63)—even in instances of moderate versus low flu severity (RR, 1.03; 95% CI, 0.99-1.07; P= .21).

The prior flu season’s severity was also shown to not influence pediatric nor adolescent vaccination rates.

Vaccine effectiveness ranged anywhere from 19% to 60% in a given year in the observed patient populations, with neither the current nor the previous year’s effectiveness showing a significant association with vaccination rates. However, investigators noted, following the 2014 vaccine effectiveness score of 14%, vaccination rates slightly declined in 2015.

Investigators noted they were limited by sparse data, particular in this last observed point—which may actually be an association which has no causal conclusion. There is also a chance that neither factor individually affects vaccination rates—but a combination of factors including them do.

“It is possible that the effects of season severity and VE may be cumulative, reducing uptake if trends continued for multiple years in succession,” Stockwell and colleagues wrote.

That said, the observed general flu vaccination rates among the younger age groups came as an alarm to the investigators. They suggest the decrease may be a reflection of the anti-vaccination movement, as well as the misperception of what reported vaccine effectiveness and flu severity trends means in relation to patient wellbeing.

“Further research understanding what factors affect vaccination decisions that could be targeted in future interventions may be warranted,” they concluded.

The research letter, “Association of Seasonal Severity and Vaccine Effectiveness With Influenza Vaccination Rates in Children,” was published online in JAMA Pediatrics.