H1N1 activity waxes, wanes: WHO

It's too early to say whether H1N1 activity is peaking in the Northern Hemisphere, a flu expert with the World Health Organization said Thursday.

"It is quite possible to have a pandemic on the milder side," Keiji Fukuda, the WHO's top flu expert, told a news conference in Geneva.

"And if we are experiencing that and the number of serious cases is kept down, then it is something again for which we should all be thankful."

In the 20th century, pandemics ranged from mild to extreme, he noted.

The UN health body will probably decide when to declare an end of the H1N1 pandemic some time next year after extensive monitoring and international discussions with expert advisers, he said.

"I think it's a little bit early to begin those discussions now because we are still in a period where some countries are still increasing in terms of infections, even though in some countries, such as the United States and Canada,it looks like infections and cases are going down," Fukuda said.

WHO declared the pandemic on June 11 after the virus emerged in March. It has caused moderate symptoms in most people, but poses greater risk to pregnant woman, young people and patients with underlying health conditions.

Current estimates suggests the death rate from H1N1 is several times lower than that of seasonal influenza, butFukuda noted that seasonal flu death estimates are based on calculations of excess deaths that occur during flu season, not confirmed flu deaths.

No spread of resistant strain

Health officials won't have a firm idea of how many people were killed by H1N1 for a few years, he said.

The WHO also said that Tamiflu-resistant H1N1 doesn't appear to have spread to hospital staff beyond two clusters of patients in Britain and the U.S.

In October and November, hospital wards in Wales and North Carolina reported a resistant form of swine flu that affected a total of 12 patients. All of them hadseverely suppressed immune systems because of underlying medical conditions.

So far, 150 million doses of H1N1 vaccine have been distributed in more than 40 countries. To date,WHO said no unexpected safety issues have emerged and the safety profile is similar to seasonal flu vaccines.

H1N1 transmission

Also on Thursday, the Canadian Medical Association Journal published a modelling study that suggests the relatively low number of new cases of H1N1 may mean that mitigation strategies such as vaccination, social distancing and use of antiviral drugs such as Tamiflu may be paying off.

For the study, Dr. David Fisman of the Dalla Lana School of Public Health at the University of Toronto and his colleagues looked at laboratory-confirmed cases of H1N1 between April 13 and June 20 in Ontario.

The researchers performed 1,000 simulations of disease transmission based on the lab-confirmed cases. They found the median incubation period was four days, about double the usual for flu, Fisman said. Symptoms lasted seven days on average.

'This isn't big-time infectious.' Dr. David Fisman

People under the age of 18 recovered faster than older patients. Those over age 50 made up seven per cent of cases admitted to hospital but accounted for seven of 10 initial deaths. In general, the H1N1 virus was similar to seasonal flu.

"However, when combined with high attack rates in younger groups, there may be greater absolute numbers of hospital admissions and deaths than are observed in a typical influenza season," the study's authors wrote, noting this may also be attributed to better surveillance during a pandemic.

The relatively low estimate of the basic reproductive number or Ro the number of new cases created by a single infected person in a susceptible population, such as at the beginning of an epidemic suggests that "effective use of mitigation strategies may substantially reduce the final size of the pandemic," the study's authorsconcluded.

Fisman said the low Ro suggests the attack rate may have been driven downthrough:

• Public health messages on covering coughs and sneezes andpractising hand hygiene.
• Getting antivirals when appropriate.
• Taking precautions at summer camps.

The team's low Ro estimate for H1N1, 1.3, is similar to regular flu and lower than estimates for the SARS virus of 2.0. It's alsomuch lower than other infectious diseases, such as 12 to 15 for measles or 10 for chicken pox.

"This isn't big-time infectious," Fisman said in an email. "We don't know [why], but part of the mystery with flu may relate to the fact that many or even most infections are transmitted by people before they get sick, or when they have very mild symptoms, so it's hard to know who to isolate, treat, etc."

The mild nature of the pandemic relates to the fact that young people, who are most susceptible to H1N1, mostly didn't get that sick. Andolder people, who could become very ill,usually weren't susceptible in the first place thanks to past immune experience developed when they hada similar H1 virus.

"I think that dissociation between susceptibility and severity really helped save us," Fisman said.