Chronic medical conditions can also be made worse by influenza, and vice versa. Respiratory illnesses such as asthma can worsen flu symptoms or be exacerbated by the virus.
Obesity is also a condition that increases flu severity. Some studies have shown that obesity raises the risk of complications and even death from the flu, particularly in seniors.
But, a new study suggests that obesity may also influence the period of time that it takes a person to shed the virus from their body.
Aubree Gordon, of the University of Michigan School of Public Health in Ann Arbor, is the lead author of the new research, which was published in The Journal of Infectious Diseases.
Obesity raises shedding period by 42 percent
Gordon and colleagues collected and analyzed data on about 1,783 people in 320 households from Managua in Nicaragua over the course of three flu seasons between 2015 and 2017.
To ascertain the duration of the viral shedding period, the researchers took nose and throat samples, which were tested for flu virus RNA. However, the samples did not give any information about whether the viruses were still contagious.
The study found that it took people with obesity significantly longer to shed the flu virus than it did those without. Specifically, people with obesity who had influenza shed the virus for 42 percent longer than those without obesity.
Also, people with obesity who had only mild influenza symptoms took even longer to recover. It took these individuals 104 percent longer to shed the virus, compared with people who did not have obesity.
"This is the first real evidence that obesity might impact more than just disease severity [...] It might directly impact transmission as well."
Implications for public health
There are some caveats to the study, the authors write. The results are limited to influenza A, which is one of the two types of flu viruses. The other type, influenza B, is usually less serious and less likely to cause epidemics in adults.
Also, the study did not find any effect of obesity in children. Finally, more research is now needed to establish whether the flu virus is infectious during the longer shedding period found in adults with obesity.
In a linked editorial, Stacey Schultz-Cherry — of the St. Jude Children's Research Hospital in Memphis, TN — comments on the public health implications of the study.
"It is therefore even more important to develop effective strategies to prevent and control influenza, especially in the overweight and obese population," she writes, "which could be challenging because of the poor vaccine responses in this population."
"With increasing focus on the development of a universal influenza vaccine, improved protection from influenza is on the horizon," the author adds. "The question remains whether these approaches will not only protect this target population, but also reduce viral shedding duration."