How contagious is swine flu? Less than the novel viruses that have caused big world outbreaks in the past, new research suggests.
If someone in your home has swine flu, your odds of catching it are about one in eight, although children are twice as susceptible as adults, the study found. It is one of the first big scientific attempts to find out how much the illness spreads in homes versus at work or school, and who is most at risk.
The study was done by outbreak specialists from Imperial College London and from the U.S. Centers for Disease Control and Prevention. Results are in Thursday's New England Journal of Medicine.
Swine flu has sickened an estimated one-sixth of Americans since the novel virus was first identified in April. The second wave of cases now seems to have peaked, and health experts do not know if another surge lies ahead.
People with swine flu are advised to stay home for at least a day after their fever goes away by itself to avoid spreading illness. That puts family members at risk, but who is vulnerable and to what extent has not been known.
About 60 percent of swine flu cases have been in children, but researchers wondered: are they truly more likely to get swine flu, or just more likely to be taken to a doctor and tested for it? Are they more likely to spread the virus than adults are?
To find out, researchers studied infection patterns in 216 people with swine flu from around the United States (half of them children) and 600 people living with them.
Respiratory illnesses that researchers assumed were swine flu developed in 78 of the 600 household members, or 13 percent. However, 10 percent had symptoms more specific to flu.
That's less than the "spread" rate during earlier flu pandemics in 1957 and 1968, when 14 percent to 20 percent of household members were infected. Less is known about spread in the 1918 pandemic, but households and lifestyles were very different then. In an ordinary flu season, the virus spreads to 5 percent to 40 percent of household members, various studies have shown.
Children were twice as susceptible to catching swine flu as adults were, and even more so if they were younger than 4, said one of the researchers, Lyn Finelli, surveillance chief for the CDC's flu division.
"It fits with what I'm seeing clinically," said Dr. James King, chairman of the American Academy of Family Physicians' board of directors and a family medicine doctor in Selmer in western Tennessee. "Most of the people I'm seeing are people under 20, mostly kids," he said.
Nearly three-fourths of households in the study managed to avoid spreading the illness to any family members.
In homes where the germ was transmitted, researchers found something unexpected: "People at all ages were just as likely to spread the virus," Finelli said. "That was surprising, since we always think of kids as super-spreaders."
The study was funded by several public and private health-related groups in England and the United States, including the Bill & Melinda Gates Foundation.