Un prof. de Harward déclare le H1N1 de niveau 1, le plus bas niveau
With H1N1 poised to enter history as the least deadly of four global flu pandemics, some experts are calling for an end to Canada's mass vaccination program.
Nature is already achieving what we would hope to achieve by vaccinating, they say.
H1N1's "reproductive number" — the number of people each infected person passes the virus to — was above one when the epidemic began, which led to the explosive initial increase in cases.
Now it is less than one, because many people have become immune, and each old case is making less than one new case. When the reproductive number falls below one, the epidemic can't sustain itself, and fades away.
The drop in cases suggests Canada has hit the critical fraction of the population that needs to be vaccinated to control the pandemic, says Dr. David Fisman, a University of Toronto expert in infectious disease dynamics.
Fisman can't understand the rational for continuing mass vaccinations. He said that for a virus as contagious as H1N1, fewer than 30 per cent of the population needed vaccination to reach a critical level of immunity.
"I'm sure that the vaccine has prevented some deaths. I'm sure that there are people who are alive right now who would not have been alive if we hadn't vaccinated," he says. But the pandemic was already peaking, and then subsiding before the vaccination was rolling out in force.
"That's nobody's fault, that's just how long it took to make a vaccine against a brand new virus. Those were the cards we were dealt," says Fisman, an associate professor of infectious diseases epidemiology at the University of Toronto's Dalla Lana School of Public Health.
Despite that view, Canada's top doctor this week pleaded with Canadians to get vaccinated if they have not already done so. Chief public health officer David Butler-Jones said that, while 30 per cent of the population is now immune to H1N1, either because they have been vaccinated or because they have already been infected, "millions" of people are still at risk of infection.
Someone vaccinated today may be protected against infection two weeks from now, "if there is still enough of (H1N1) kicking around," Fisman says. But the benefit diminishes the further into the future we go, and he says other public health programs have suffered as staff and resources were redeployed to the H1N1 campaign. In some jurisdictions, breastfeeding support programs, sexually transmitted diseases clinics and other usual activities were cancelled or postponed as public health was forced to bear the brunt of delivering the largest immunization program in Canada's history.
"At this point, in terms of saying everybody must get vaccinated because there is a pandemic abroad, it's kind of done," Fisman says.
H1N1 will likely die out with a death rate lower than that for regular seasonal flu.
Harvard University infectious disease expert Marc Lipsitch is pegging H1N1's case fatality rate — the proportion of people infected with a disease who die of it — at less than 0.1 per cent, placing human swine flu in a category 1 pandemic, the lowest level of severity for a worldwide disease outbreak. Swine flu's case fatality rate is less than what was seen for the three worldwide flu outbreaks in the 20th century: 1918, 1957 and 1968.
Unless the virus mutates, and there is no strong sign H1N1 is becoming noticeably different, "it's almost certainly going to be the mildest of the four (pandemics) that we have good data on," says Lipsitch, a professor of epidemiology at the Harvard School of Public Health.
When the World Health Organization declared the pandemic in June, officials warned the rogue virus threatened all of humanity. The reality is so far proving strikingly different from what was expected.
The world agency, and Canada, had done their pandemic planning with a different flu virus in mind. They spent five years watching H5N1, or "bird flu," which kills up to 60 per cent of those it infects.
We got off lucky when the real pandemic hit, WHO says, but the public doesn't always understand that.
"Adjusting public perceptions to suit a far less lethal virus has been problematic," WHO said in a statement Thursday, issued in response to media reports that ties with the drug industry among expert advisers may have influenced WHO's policy decisions related to H1N1.
"Given the discrepancy between what was expected and what has happened, a search for ulterior motives on the part of WHO and its scientific advisers is understandable, though without justification.
"WHO has consistently assessed the impact of the current influenza pandemic as moderate. WHO has consistently reminded the medical community, public and the media that the overwhelming majority of patients experience mild influenza-like illness and recover fully within a week, even without any form of medical treatment."
But some critics say WHO set itself up for blame.
"They had been talking for years about the possibility of some kind of reprise of the 1918 pandemic," says Philip Alcabes, author of Dread: How Fear and Fantasy Have Fueled Epidemics from the Black Death to the Avian Flu. "A huge amount of money and person power was devoted to preparing" for the next outbreak, he says.
But, "there was never any evidence, not from Day 1, and not anytime since, that this strain of flu was going to be a particularly dangerous strain, either in terms of its capacity to make people sick or its capacity to kill people," says Alcabes, associate professor in urban public health at Hunter College's School of Health Sciences in the City University of New York.
"Yet that's the story that's always been told, almost every single day since it appeared in April. 'We don't know what's going to happen.' Essentially, 'the sky might fall.'"
Lipsitch, of Harvard, says that when the first reports were coming out of Mexico, the case fatality rate estimates were as high as four per cent, which is twice the rate for the 1918 Spanish flu.
"If you don't know how bad your enemy is, you plan as if it's bad, and you hope that you're over-planning. In this case, that's exactly what was done," he says.
"I think the response has been appropriate for what we knew at each stage, but what we know has changed, because the data gets better."