Children should be first in line for H1N1 shots to stem the outbreak, U.S. researchers say. (Greg Baker/AP Photo)

Vaccinating children by mid-September and following with most of the U.S. population could delay the peak of swine flu infections, according to a new computer modelling study.

In Thursday's issue of the journal Nature, researchers estimated how transmissible the pandemic H1N1 virus is in schools.

Half of the U.S. cases so far have been among children aged five to 18. Yang Yang of the University of Washington in Seattle and colleagues estimated that each school child infects 2.4 other children within a school.

"If vaccine were available soon enough, vaccination of children, followed by adults, reaching 70 per cent overall coverage — in addition to high-risk and essential workforce groups — could mitigate a severe epidemic," the team concluded.

October peak?

Availability of an effective vaccine against H1N1 and timing of the outbreak will determine how quickly the epidemic can be slowed, the researchers said.

The current pattern of pandemic spread suggests H1N1 will be similar to the H2N2 pandemic of 1957-58, with a substantial spread expected to begin in early September and a peak in mid- to late October, the team found.

Canada wants to have an appropriate vaccine as soon as possible, Dr. David Butler-Jones, the chief public health officer, told a news conference Thursday, noting that pandemics typically occur in waves.

"Even if we see a lot of disease in September and we're not able to immunize until later in the fall, then it's still going to be important to do so," Butler-Jones said.

"In Canada, it's not just a vaccine strategy, it's not just an antiviral strategy, it's not just education. It is, in fact, a package that all these measures can do several things, one of which is to slow down the spread and to smooth out that curve, so that we will hopefully see a later peak and have more people immunized in advance of that."

He added that if officials are confident about the safety and effectiveness of Canada's pandemic vaccine, injections should begin sooner than November, should there be an earlier surge.

Ad campaigns start soon

Vaccination increases the immunity of a population and helps slow the spread of infection, reducing the number of cases at the peak of the epidemic while reducing the overall attack rate, hospitalizations and deaths, the U.S. researchers said.

Federal Health Minister Leona Aglukkaq announced Thursday that the number of deaths from the H1N1 virus rose to 78, an increase of one since last week.

The Public Health Agency of Canada also released new guidelines Thursday for people organizing or attending mass gatherings such as concerts and church services and offered more explicit advice for public health campaigns.

The advice for events planners goes as far as suggesting printing the advice to stay home if sick on ticket stubs.

But decisions to cancel events such as the Vancouver Olympics in February would be made at a local level in conjunction with local public health departments, if its thought that hospital capacity will be overstretched.

Canadians who are pregnant or have an underlying illness that puts them at risk for severe illness or complications should also consider staying away from mass gatherings, the guidelines advised.

Aglukkaq also said that a four-week campaign of radio ads promoting basic hygiene measures, including handwashing and staying home when sick, will start next week. Ads promoting vaccination will run closer to when the immunizations will roll out, Butler-Jones said.

The U.S. plan calls for vaccination to start in mid October, which could still be effective if the epidemic peaked in November or December as the Hong Kong flu of 1968-69 did, said study author Ira Longini, a biostatistician in the Center for Statistical and Quantitative Infectious Diseases at the Fred Hutchinson Cancer Research Center.

Tracking transmission

The U.S. study also looked at where transmission of the H1N1 virus has occurred:

• 30 to 40 per cent in households.
• 20 per cent in schools.
• 40 to 50 per cent in workplaces and the community.

The predicted rate of pandemic H1N1 transmissibility — how many people an infected person will infect during influenza's infectious period at the beginning of an outbreak — was estimated to be 1.3 to 1.7. A value of 1.6 could result in 2.2 billion cases worldwide over a year, or a third of a typical population, they said.

The time it takes from when one person is infected until the virus spreads to someone else is about 3.2 days on average, the researchers estimated.