Study Finds No Link Between Obesity and Reduced Flu Vaccine Success

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A study analyzing data over seven flu seasons found that higher body mass index (BMI) was not linked to reduced vaccine effectiveness (VE) against flu in both adults and children.

Despite previous concerns that obesity might impair how well vaccines work, the study showed that BMI categories—normal, overweight or obese—did not significantly affect how well the flu vaccine protected against lab-confirmed, outpatient flu cases for different flu strains. Vaccine effectiveness in adults ranged from 16–57%, and in children from 29–65%, across all BMI groups.

Globally, approximately 16% of adults had obesity in 2022, which had more than doubled since 1990. In the United States, more than 40% of adults and almost 20% of children had obesity, which is linked to metabolic syndrome and other comorbid conditions and might impact more severe influenza illness.

“The effects of obesity on the immune response to influenza vaccination are complex and poorly understood,” wrote lead investigator Jennifer King, MPH, epidemiologist at the Center for Clinical Epidemiology and Population Health at the Marshfield Clinic Research Institute. “Post-vaccination hemagglutinin inhibition (HI) titers to influenza antigens are largely similar in individuals with obesity and normal weight. However, antibody titers appear to decline more rapidly in people with higher BMI, and aspects of the cell-mediated immune response may be inhibited.”

The analysis included data from the Flu VE Network between 2011 and 2018. Patients who sought medical care for an acute respiratory illness with cough and illness for ≤ 7 days were recruited from facilities in Michigan, Pennsylvania, Texas, Washington and Wisconsin. Vaccination was determined using electronic medical record vaccine receipt of ≥ 1 dose of the current season influenza vaccine or those who were undocumented but had plausible self- or parent-reported vaccination ≥ 14 days prior to the onset of symptoms.

A total of 31,647 adult patients were enrolled in the Flu VE Network, of whom 28,089 were included in the analysis. Among these patients 22% had a normal weight BMI, 30% had an overweight BMI and 48% had an obese BMI. For patients with obese BMI, 25% had a BMI of ≥ 40. Most patients (80%) were non-Hispanic White, 26% had an underlying medical condition besides obesity, 54% were vaccinated and 37% of vaccinated patients received the vaccine less than 90 days prior to the onset of illness.

Compared with patients of normal weight BMI, a greater percentage of adult patients with obesity were Hispanic, non-Hispanic Black, had an underlying medical condition and had received the current influenza vaccine. More patients in the normal range BMI reported excellent or very good general health compared with patients with obesity.

Among the 16,636 children enrolled in the Flu VE Network, 12,380 were included in the analysis. Most (63%) exhibited a normal range BMI, 16% were overweight and 20% were obese. Similarly, most (64%) were non-Hispaic White, 45% were vaccinated and 41% received the vaccine less than 90 days before illness onset.

Both adults and children in the obesity or overweight categories at the time of illness had comparable VE against medically attended influenza when compared with those with normal weight BMI. Results were similar across influenza types and subtypes, indicating the vaccination is effective at preventing influenza regardless of BMI.

“Further research to assess the association between influenza vaccine effectiveness and obesity in the hospital setting may be beneficial since influenza related illness may be more severe in people with obesity,” investigators wrote. “Our findings from the outpatient setting reinforce current recommendations for universal influenza vaccination in all adults and all children greater than 6 months of age, to protect against influenza and its complications.”