It’s known that children are more contagious than adults and that obesity plays a role in flu severity. New research, published in the Journal of Infectious Diseases, indicates obesity is also a factor in the length of time that flu patients transmit virus to others.

The research draws upon two studies of about 1,800 people in 320 Nicaraguan households. It examines factors like fever, sore throat, cough, and runny nose to determine how long a patient “sheds” a virus, or potentially transmits it to others. On average, researchers tracked each patient for 10 days and found as follows:

  1. Children, up to age 4, shed influenza 40 percent longer than do adults, aged 18 to 92.
  2. Young people, aged 5 to 17, shed influenza 30 percent longer than adults, 18 to 92.
  3. This holds for influenza and A and B, although B virus, across the board, had longer shedding and more variance than A.
  4. On average, patients 0 to 4 shed for 7.7 days; 5- to 17-year-olds for 7.2 days; and the older group for 5.5 days.
  5. Across the three age groups, average shedding for influenza A was 7, 6.4, and 5.1 days, while for influenza B it was 9.3, 8.8, and 6.4 days respectively.

When it came to obese adult patients, the numbers get worse. Obese adults with several symptoms shed the virus 42 percent longer than did non-obese adults, the researchers found, while those with fewer symptoms shed for more than twice as long: an 104 percent increase. The researchers didn’t find any connection between obesity among children aged 5 to 17 and shedding, and there wasn’t enough information to study obesity in the cohort under 5-years-old.

They also didn’t find an association between obesity and influenza B shedding. “It is unclear why this association is specific to influenza A virus,” they wrote, “but it is consistent with previous findings of obesity and severe influenza outcomes primarily for influenza A(H1N1) virus.” They concluded, “these results add to existing evidence linking obesity to infectious diseases, making it now even more important to work toward controlling and preventing the obesity epidemic.”

In an interview, study author Hannah Maier, an epidemiology doctoral candidate at University of Michigan, told CreakyJoints that her and colleagues’ findings make it even more important for obese people to get flu vaccine, practice good hand washing, seek medical care early, and consider taking antivirals.

“For the individual, vaccines and early antivirals are important; to help prevent flu transmission to families and community, hand washing and sneezing into your elbow, avoiding work/school/places with lots of people and vulnerable population (kids, elderly, immunocompromised, unvaccinated) are additional important practices,” she said.

It’s not understood exactly why obesity extends shedding of virus, notes a University of Minnesota news report. “Obesity alters the immune system and can lead to chronic inflammation, which also increases with age,” the university states. “Also, [the researchers] note that obesity can make breathing more difficult and increase the need for oxygen.”

Maier added that obesity alters the immune system in many ways and is thought to do so through chronic inflammation, which builds over time. That means that people who have been obese for longer will experience more serious effects.

“Some vaccines don’t have as good of responses in obese people for this reason — though any response is still better and more protective than not getting vaccinated,” she said. “We think that because obese people’s immune systems are less effective in some ways, they are less able to fight off the influenza virus as quickly, which means that they may transmit it to others for longer periods of time.”

The study drew attention in the Daily Mail (UK) and Economic Times (India), and the National Institutes of Health issued a release. Stacey Schultz-Cherry, of Memphis’ St. Jude Children’s Research Hospital who wasn’t involved in the study, commented for a MedPage article.

“It is even more important to develop effective strategies to prevent and control influenza, especially in the overweight and obese population, which could be challenging because of the poor vaccine responses in this population,” she said. “With increasing focus on the development of a universal influenza vaccine, improved protection from influenza is on the horizon. The question remains whether these approaches will not only protect this target population, but also reduce viral shedding duration.”