Flu sufferers could be contagious for longer if they are fat, research suggests.
The study by the University of Michigan found that obese adults tended to harbour the virus for longer, giving them more time to spread it.
Researchers said those with excess weight should be targeted for flu jabs.
It is already know that obesity increased the risk of suffering complications from flu, and is linked to higher severity of disease.
But the research is the first to suggest that fatter people are also contagious for longer.
Assistant Professor of Epidemiology Dr Aubree Gordon of the University of Michigan School of Public Health said: "This is the first real evidence that obesity might impact more than just disease severity. It might directly impact transmission as well."
The study analysed 1,800 adults and children in 320 households in Managua, Nicaragua, to investigate the effect of obesity on the duration of viral shedding over three influenza seasons from 2015 to 2017.
Nose and throat swabs determined the duration they shed the virus.
It found obese adults with flu symptoms shed influenza A virus for 42 per cent longer than adults with flu who were not obese.
And infected obese adults with mild or no symptoms shed the virus 104 per cent longer than non-obese adults with flu.
The study was published in The Journal of Infectious Diseases.
Prof Gordon said further research was underway to establish if the flu virus shed for longer periods by obese individuals continued to be infectious, spreading the illness to others.
It is suggested being overweight alters the body's immune response and lead to chronic inflammation, which increases with age, as well as making breathing harder.
These factors may help explain how obesity could affect influenza risk, severity, and transmission potential, the study authors said.