The young and healthy who feel invincible from the H1N1 "swine flu" influenza pandemic may not be as bulletproof as they think, warn public health experts.
Nearly two-thirds of Canadians hospitalized due to swine flu, and half of those who have died, had no underlying health conditions.
Experts do not yet understand why the new strain affects some healthy people so severely, ravaging their lungs with an aggressive pneumonia and forcing them to spend weeks in hospital, attached to breathing machines.
"They are ending up on ventilators and it can last from weeks to months," said Michael Gardam, director of infectious diseases at the Ontario Agency for Health Protection and Promotion. "I would like people to be concerned about H1N1, without panicking. More concerned than they are about seasonal flu."
A new study tracking the epidemic in Mexico also found the flu strain hits those between the ages of 20 and 50 the hardest, with a higher death rate than other age groups.
In recent weeks, as swine flu has faded from the world's radar, infectious disease specialists worry that people have become complacent about the pandemic, which is expected to infect one third of the population, or about 10 million Canadians. (In contrast, seasonal flu affects about one in 10 people.)
"You should not be worried that your child will suddenly die of H1N1," Dr. Gardam said. "But you should be prepared that a family member will get sick."
The World Health Organization has confirmed 77,201 cases of H1N1 as of July 1, with 332 deaths.
As of June 29, 2009, a total of 7,983 laboratory-confirmed cases of H1N1 flu virus have been reported in all provinces and territories in Canada. To date, 538 people have been admitted to hospital and there have been 25 laboratory-confirmed deaths - including Rubjit "Ruby" Thindal.
The Grade 1 student from Brampton, Ont. died in her father's arms June 15 en route to the hospital, a day after first complaining of achy arms and legs, and a slight fever. Doctors await autopsy results to find out whether she had an underlying health condition that contributed to her death.
According to the Public Health Agency of Canada, two-thirds of 94 hospitalized cases where information was available showed the patients were perfectly healthy before being admitted.
Of the 25 Canadians who died with H1N1, 13 had other health problems. These can range from obesity, diabetes and mild asthma to chronic lung or heart disease. Also at risk are smokers, those who are immune-compromised, and pregnant women, who have a greater chance of developing complications.
"By the time you add up all these underlying health conditions, you end up with a large chunk of the population," Dr. Gardam observes.
Epidemiologists are studying cases of healthy people who have become severely ill after contracting the virus, to gain insight into why they are vulnerable.
"[We are]trying to understand from a medical standpoint why they would be affected this way by the virus, and whether this is a signal that the virus is changing and becoming more virulent," David Butler-Jones, Canada's chief public health officer, said in a statement. "It is because of these severe cases, although a minority, that we must remain vigilant, both at the government level by continuing to investigate these cases, and at the individual level, by taking personal action to prevent infection."
The other troubling characteristic of the swine pandemic is that, unlike other flus, it is not tapering off in the summer months.
Instead, Canada is experiencing five times the rate of flu activity it normally does at this time of year, all of it H1N1. "The other flu bugs have vanished. But not H1N1," Dr. Gardam says.
The other key difference is the age of those infected: most are aged 20 to 50. With seasonal flu, one quarter of those infected are over the age of 65, Dr. Butler-Jones says.
In Mexico, 87 per cent of the deaths, and 71 per cent of the cases of severe pneumonia due to H1N1 occurred in people between the ages of 5 and 59. With seasonal flu, usually one third of those affected are in this age group.
Doctors speculate that this could be because the H1N1 virus resembles a strain of flu that circulated before 1957, to which older people have been exposed, says Colin Lee, associate medical officer of health in Ontario's Simcoe-Muskoka region.
The first detailed study of the swine flu outbreak in Mexico, H1N1's original epicentre, was published last week in the New England Journal of Medicine.
"The features of the H1N1 epidemic are somewhat similar to past influenza pandemics in that circulation of a new influenza virus is associated with an unseasonal wave of disease affecting a younger population," wrote authors Gerardo Chowell-Puente, an epidemiologist at Arizona State University, and Stefano Bertozzi of the National Institute of Public Health in Mexico.
Dr. Butler-Jones reiterated his message that prevention is key in stopping the spread of all flu viruses, and urged people to wash their hands, cough into their sleeves, and avoid others when they're ill. People with a fever and cough who develop shortness of breath or difficulty breathing should get medical attention right away, he added.