New Study Ties Obesity to Increased Risk of Flu Transmission

August 13, 2018 11:04 am Michael Devitt – Although it's well-known that obesity increases a person's risk of severe complications from the flu, new research shows that obesity also may play a role in how influenza is spread by influencing how long patients with obesity who c,ontract the flu remain contagious.

In a study published online in the Journal of Infectious Diseases, researchers from the University of Michigan School of Public Health in Ann Arbor found that patients who are obese and infected with certain flu strains may remain contagious for considerably longer periods of time than patients with flu who are not obese, thereby likely increasing the risk of spreading the virus to others.

"This is the first real evidence that obesity might impact more than just disease severity," said senior study co-author Aubree Gordon, Ph.D., M.P.H., in a news release. "It might directly impact transmission, as well."

Study Details and Results

The study authors analyzed flu symptom data from two cohorts of about 1,800 people in Managua, Nicaragua, during three flu seasons (i.e., late 2015, 2016-2017, and mid-to-late 2017).

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For both cohorts, after a patient seen at the Health Center Sócrates Flores Vivas study clinic was identified as having flu symptoms, all members of that person's household were enrolled and monitored for 10 to 13 days.

To measure the duration of viral shedding, the authors kept symptom diaries of all participants and obtained nose and throat swabs every two to three days. Swabs were analyzed for the presence of flu virus RNA.

A total of 145 adults in the clinic's catchment area contracted the flu during the study period; 87 were infected with influenza A, and 58 with influenza B.

Story Highlights

The researchers found that on average, adults with obesity shed influenza A virus nearly 1.5 times longer (5.23 days) than adults without obesity (3.68 days). Adult patients who were obese and had two or more flu symptoms shed the virus 42 percent longer than those who were not obese, whereas for adult patients with obesity who were asymptomatic or had only mild infection, the shedding duration was 104 percent longer those who were not obese.

Of note, the increases in shedding duration were seen only in patients with obesity who were infected with influenza A. The researchers found no association between obesity and the duration of shedding in those infected with influenza B virus. In addition, obesity did not appear to affect the duration of viral shedding in children ages 5 to 17.

The authors were at a loss to explain why obesity was associated with prolonged viral shedding time. However, they noted that obesity is known to alter immune function and may cause chronic inflammation, which could lead to increased shedding duration.

The authors also pointed out that the swab analysis did not indicate whether the viruses detected were actually infectious -- a question that may be answered by additional research that is currently underway.

Implications for Public Health

From a public health perspective, the study results suggest that as obesity rates continue to increase, so could episodes of prolonged virus shedding. This, in turn, may create more opportunities for the virus to spread and infect certain at-risk populations.

Stacey Schultz-Cherry, Ph.D., laid out the implications of the study's findings in an accompanying editorial.

"What does this mean for public health? It could mean that, with an increasing prevalence of obesity, the number of episodes of prolonged influenza virus shedding will increase, leading to increased opportunities to infect naïve and high-risk populations," she wrote. As a result, developing better methods to prevent and control influenza, particularly among a growing population of patients who are overweight or obese -- who typically display suboptimal response to influenza vaccine -- will become paramount.

Schultz-Cherry suggested that the study's findings may apply to other pathogens that can cause respiratory problems, and she noted that more research is needed.

Family Physician Expert Perspective

Because obesity is one of the most common medical problems in the United States, it should come as no surprise that family physicians frequently care for obese patients in their practices. For them, the study's findings offer a way to talk with their patients about a variety of topics, including weight loss, diet, lifestyle modification and seasonal flu vaccination.

Jennifer Frost, M.D., medical director for the AAFP's Health of the Public and Science Division, outlined ways to lower the risk of contracting or spreading the flu.

"Influenza vaccination is the best way to prevent morbidity and mortality from the flu, so family physicians should encourage vaccination for all patients over 6 months of age, with rare exceptions," she told AAFP News. "Vaccination decreases the risk of getting the flu, but even those who become ill despite vaccination may have a milder illness. Staying home when ill, covering your nose and mouth when coughing or sneezing, and frequent hand-washing are also key in reducing transmission of the virus."

Clinical Practice Guideline: Overweight and Obesity in Adults