Scientists already know that obesity increases the risk of severe flu complications, but obesity may also increase transmission of the disease, researchers reported today.
Describing their findings in the Journal of Infectious Diseases, researchers who studied households in Managua, Nicaragua, over three flu seasons found that obese adults shed the virus about 1.5 times longer than non-obese adults.
Aubree Gordon, PhD, MPH, senior study coauthor who is at the University of Michigan School of Public Health, said in a press release from the Infectious Diseases Society of America (IDSA) that the findings are the first real evidence that obesity might affect more than just disease severity. "It might directly impact transmission as well."
Longer shedding, even with mild infections
The team monitored two cohorts that included 1,783 people from 320 households during three flu seasons from 2015 to 2017. The study was mainly supported by the National Institute of Allergy and Infectious Diseases through its Centers of Excellence for Influenza Research and Surveillance program. A team from the University of Michigan coordinated the study with their counterparts in Nicaragua and the University of California at Berkeley.
Among the group, 87 people were infected with influenza A and 58 were sickened by influenza B.
Obese adults with two or more symptoms shed the virus 42% longer than non-obese adults. And obese adults who had asymptomatic or milder infections shed the virus for 104% longer.
The team found, however, that obesity wasn't a risk factor for increased viral shedding in children ages 5 to 17 or for adults who had influenza B illness.
Gordon said the nose and throat swabs used in the study can detect flu virus RNA but don't tell if the virus is infectious. She said more research is under way to see if the flu virus shed over longer periods by obese patients is infectious and if it can spread the disease to others.
What's behind prolonged shedding?
The researchers don't know exactly how obesity could extend viral shedding in those with flu, but they know that obesity alters the immune system and can lead to chronic inflammation, which also increases with age. Also, they note that obesity can make breathing more difficult and increase the need for oxygen.
The investigators suggest that chronic inflammation triggered by obesity may be responsible for increased influenza A shedding. Reducing obesity rates might have an added benefit of limiting the spread of viral diseases, the authors said.
Key public health implications
In a commentary on the study in the same issue, Stacey Schultz-Cherry, PhD, with St. Jude Children's Research Hospital, wrote that more studies are needed to determine if the longer shedding duration in obese people correlated with increased viral load and shedding of infectious virus. She added, though, that today's report and a recent study of exhaled breath in college students—which found a link between obesity and the amount of flu virus shedding—imply that obesity may play an important role in flu transmission.
Schultz-Cherry was not involved in the study.
If confirmed, the connection has important public health implications, including a flu transmission threat that rises along with increasing obesity prevalence, Schultz-Cherry said. Developing strategies to prevent and battle flu could be a challenge, because of poor vaccine response in overweight and obese populations, she added.
Though improved protection is a focus of universal flu vaccine development, questions remain about whether they will protect this population and reduce viral shedding, Schultz-Cherry wrote. "In addition, will these findings with influenza virus extend to other respiratory pathogens? This important question remains unanswered."
Aug 2 J Infect Dis abstract
Aug 2 J Infect Dis commentary
Aug 2 IDSA press release
Aug 2 NIH press release