Early Vaccination Best Way to Combat H1N1, Study Finds

October 13, 2009 - News Release

Early action, especially rapid rollout of vaccines, is the best way to reduce the infection rate of the H1N1 influenza virus, according to a new study involving a University of Guelph mathematics professor.

But delaying vaccination by 30 days or more from the start of an outbreak significantly erodes its effectiveness, according to the paper published today in the Canadian Medical Association Journal that involved U of G professor Chris Bauch.

Bauch is one of a handful of researchers in Ontario who were approached by the Ontario Agency for Health Protection and Promotion (OAHPP) last spring to create a mathematical model to help the province develop effective mitigation strategies against H1N1.

Their model projects how many people will be infected under different disease-control strategies. It simulated a pandemic outbreak based on demographic information from London, a mid-sized city in Ontario, as well as epidemiologic influenza pandemic data.

A broad range of scenarios were introduced into the model involving vaccination timing, school closures and antiviral drug treatment strategies as well as the effect of pre-existing immunity. The model then provided mathematical predictions for how and when that could happen under 630 different combinations of scenarios.

The researchers found that, no matter the scenario, early vaccination had a disproportionately large impact on reducing the infection rate in a “fall wave” of the pandemic. For example, vaccinating 60 per cent of the population before the outbreak was found to virtually prevent an epidemic.

In addition, “vaccination in the first 30 days reduced the peak of the outbreak as much as 50 per cent,” Bauch said. Early vaccination also helped the health-care system cope by flattening the peak, and vaccination before the outbreak further delayed the peak.

The longer vaccination is delayed, the poorer the outcome. For example, in the setting of a school or day-care closure, vaccinating 60 days after the outbreak began had little impact on the infection rate, the study found.

But Bauch said starting to vaccinate now in Ontario “would still have significant positive benefits.”

The researchers also discovered that, across all scenarios, having a pre-existing immunity of 15 per cent or more in the population kept infection rates low, even if vaccination is delayed or an entire population is not vaccinated.

In addition, they found that a pandemic can be halted almost entirely if schools are closed early on in combination with at least some vaccination. But Bauch said there are important social costs of school closures that the study did not examine.

The researchers also did not attempt to predict influenza-related deaths or assess vaccination strategies targeted to high-risk groups or specific age groups, such as schoolchildren.

Bauch said having a pre-existing model can play an important role in deciding which mitigation strategies to adopt and which ones not to, especially in future pandemics. “You won’t have to start from scratch. The hope is that next time around, the model can be taken off the shelf and you just plug in new data. It will really reduce the time required to get useful projections out of the model.”

Other researchers involved in the study are from the University of Toronto, the Ontario Agency for Health Protection and Promotion and the Research Institute of the Hospital for Sick Children.

For media questions, contact U of G Communications and Public Affairs: Lori Bona Hunt, at 519-824-4120, Ext. 53338, or lhunt@uoguelph.ca , or Barry Gunn, Ext. 56982 or bagunn@uoguelph.ca .

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