Swine flu seems to be less contagious than previous pandemic flu strains, a new study suggests.

Children are twice as susceptible as adults to catching the H1N1 virus, but flu patients under 18 are no more likely than adults to infect others, British and U.S. researchers report in Thursday's issue of the New England Journal of Medicine.

"People at all ages were just as likely to spread the virus," said study author Lyn Finelli of the U.S. Centers for Disease Control and Prevention. "That was surprising, since we always think of kids as super-spreaders ."

'People at all ages were just as likely to spread the virus.'— Lyn Finelli, study author

The researchers studied 216 flu patients — half of them children — and the 600 other people living in their households.

The team was trying to find out how H1N1 spreads within a household and whether factors such as age, severity of symptoms and the number of people in a household make a difference.

Respiratory illnesses that the researchers assumed were swine flu developed in 78 of the 600 household members, or 13 per cent.

This is less than the rate of spread seen during the 1957 and 1968 flu pandemics, when 14 per cent to 20 per cent of household members became infected. Little is known about the spread of the disease during the 1918 pandemic.

More than 90 per cent of patients in the study had a cough. (iStock)The H1N1 research found that people under 18 years of age in the same household as a flu patient were almost twice as susceptible to catching the flu as people 19 to 50 years of age.

Children younger than four were most susceptible, Finelli said.

People over 50 were less susceptible to catching the flu than people aged 19 to 50, confirming a trend observed during this year's experience with swine flu.

"It fits with what I'm seeing clinically," said James King, a family doctor in Selmer, Tenn., and chairman of the board of the American Academy of Family Physicians. "Most of the people I'm seeing are people under 20, mostly kids."

How long to stay home

An average 2.6 days elapsed between the start of symptoms in the first person to get sick in a household and the start of symptoms in the household contacts.

"Our study also suggests that people infected with swine flu might not need to stay at home as long as we previously thought," said lead author Dr. Simon Cauchemez of the MRC Centre for Outbreak Analysis and Modelling at Imperial College London.

"If they are only likely to transmit the virus to other people for the first few days of their illness, keeping people off work for a week may be unnecessary and could be detrimental to the economy," Cauchemez said in a news release.

In view of the findings, recent guidelines from the Centers for Disease Control, which advise patients to stay home for 24 hours after the fever ends without the use of fever-reducing medications, are "very sensible," Cauchemez said.

The research was funded by the Medical Research Council, the Bill and Melinda Gates Foundation, the European Union, National Institute of General Medical Sciences, the Royal Society, and Research Councils U.K.

Cauchemez reported receiving consulting fees from Sanofi Pasteur MSD.

With files from The Associated Press