SEATTLE — It’s known that swine flu is a fairly contagious disease. What’s been less clear is just how quickly the H1N1 virus gets transmitted in homes and schools — and how effective vaccines would be in containing a widespread outbreak.
For the first time, researchers at the Fred Hutchison Cancer Research Center and the University of Washington are filling in some of the details about the virus, whose return to the Northern Hemisphere has already touched off anxiety in some quarters.
The researchers analyzed reported data about the past spring’s swine flu pandemic and concluded that — barring mutations in the virus — a person has about a one-in-four chance of catching it from an infected family member.
The findings were published Thursday in the early online edition of the journal Science.
With the majority of schools around the country reopening soon, “we should start seeing outbreaks in the schools pretty fast,” said Ira Longini, a biostatistician at the university and the cancer center who oversaw the paper.
Since swine flu first arrived in Washington state in late April, the virus has never completely left. In fact, it’s assumed to be so prevalent in the community that testing for H1N1 is now done only in cases of hospitalization. In King County, the most recent confirmed case of swine flu was reported during the week of Aug. 8.
Yang Yang, a staff scientist at the research center and the paper’s lead author, said H1N1 appears to be slightly more contagious than initially believed. The virus has a household “secondary attack rate” of 27 percent, the probability that an ill person will infect a family member.
Seasonal flu has a typical secondary attack rate of 10 to 12 percent, though it can get as high as the rate for swine flu, Longini said.
Once infected, a person with swine flu likely will pass it on to another person in an average of three days, Longini said. “That’s pretty fast.”
Measles, which is highly contagious, takes about 2 1/2 weeks to spread, which gives people time to quarantine a patient or take other preventive steps.
Swine flu so far has caused symptoms similar to seasonal flu: Fever, aches, coughs and chills. The viruses also get transmitted in the same ways, mainly through large droplets from coughs and sneezes from within 3 feet.
If swine flu cases flare up in significant numbers in the next week or two as researchers expect, vaccines that are due to become available in October most likely “are not going to get here soon enough,” Longini said.
The paper calculated that timely vaccinations of children and an overall coverage of 70 percent would be enough to curtail a severe H1N1 outbreak. But Yang said that if the pandemic spreads more slowly than projected, the vaccines arriving next month would help to mitigate the outbreak.