Doctors have stressed the importance of prompt medical care for people showing severe flu symptoms. (CBC)

Severe H1N1 infection is linked with delays in getting treated with antivirals, researchers in Manitoba have found.

Public health experts aim to predict disease and mitigate hazards in populations at risk, said study author Dr. Ryan Zarychanski of the University of Manitoba in Winnipeg. The researchers set out to identify factors linked with severity of disease in H1N1.

Zarychanski's study, published in Thursday's online issue of the Canadian Medical Association Journal, analyzed the 795 laboratory confirmed cases of H1N1 in Manitoba as of Sept. 5, 2009. Manitoba had the highest incidence of severe swine flu cases during the first wave of the pandemic in the spring.

The median or average time from onset of symptoms to start of antiviral treatment was two days for cases in the community, four days for patients admitted to hospital, and six days for the most severe cases admitted to an intensive care unit.

"If people, especially people with pre-existing medical conditions, have progressive symptoms, especially shortness of breath, they should seek medical attention earlier than later," Zarychanski said in an email.

"Early therapy may alter their outcome and prevent them from becoming severely ill or on life support."

The researchers also found more H1N1 cases among First Nations people as the severity of disease increased:

• 28 per cent of confirmed H1N1 cases in the community.
• 54 per cent of those admitted to hospital.
• 60 per cent of those admitted to ICU.

Similar trends were observed in aboriginal communities in Australia and New Zealand, and historical records indicate higher mortality among aboriginal communities during the 1918 Spanish flu pandemic.

The common patterns motivate thinking that First Nations people may be genetically predisposed to severe infection with pandemic flu viruses.

Researchers are investigating the Manitoba findings by looking at specific genetic differences present in First Nations, compared with other ethnic groups.

"Intriguing as this hypothesis may be, Canadian aboriginal people and Australian Aboriginal and Torres Strait Islanders [south of New Guinea] do not share a common ancestry," the study's authors noted. "What they do have in common is a history of colonization, combined with historical and continuing social inequities that have led to significant health disparities."

Other factors such as housing or living conditions, lower income, diet, underlying diseases or access to health-care resources could be behind the link, the researchers said.

"First Nations people have a higher chance of severe disease, compared to other ethnic groups," Zarychanski said. "In addition to vaccination, health-care providers and First Nations groups should have a low threshold for seeking treatment if they have symptoms of H1N1 infection."

Infections in England

A second study published in Thursday's online issue of the medical journal The Lancet suggested one child in three caught H1N1 in the first wave of infections in England last year — up to 10 times more than originally thought.

The study used blood samples to determine the extent of H1N1 infections in children, which the researchers suggested should be a key group to target for vaccination.

Earlier estimates of the number of cases from England's first wave during the summer of 2009 were based on surveys of people going to their doctor with flu symptoms, which misses those who do not seek medical attention because their case is mild.

The blood samples showed infection rates of 32 per cent in children in London under age 15, and 20 per cent for those aged 20 to 24, which is 10 times higher than the original estimates, the researchers said.

"This serological study shows the true extent of H1N1 infection in the initial wave of the pandemic in England in 2009," the team from Britain's Health Protection Agency concluded. "Its findings should be applicable to other countries that have experienced a similar first wave."

As of Jan. 10, at least 13,554 H1N1 flu deaths have been reported by almost 200 countries, according to the World Health Organization. But it could take a couple of years to better estimate H1N1's toll.

The second wave of the pandemic now seems to be waning in North America, health officials say.