WHO: H1N1 flu more contagious than seasonal virus

May 11, 2009 (CIDRAP News) – The World Health Organization (WHO) today said the novel H1N1 influenza (swine flu) virus seems to be more contagious than seasonal flu, but it generally causes "very mild illness" in otherwise healthy people.

"H1N1 appears to be more contagious than seasonal influenza," the WHO said in an online statement released today. "The secondary attack rate of seasonal influenza ranges from 5% to 15%. Current estimates of the secondary attack rate of H1N1 range from 22% to 33%." (The secondary attack rate is defined as the frequency of new cases of a disease among the contacts of known cases.)

The agency noted that, because the virus is new, scientists expect that few people are likely to have any immunity to it. In that context, the statement that the new virus is more contagious than seasonal flu is not surprising, but it appears to be the first time the WHO has offered any specific figures comparing the contagiousness of the novel virus and seasonal flu.

The WHO further stated, "With the exception of the outbreak in Mexico, which is still not fully understood, the H1N1 virus tends to cause very mild illness in otherwise healthy people. Outside Mexico, nearly all cases of illness, and all deaths, have been detected in people with underlying chronic conditions."

The statement also noted that the outbreaks in Mexico and the United States have affected younger people more than seasonal flu typically does: "Though cases have been confirmed in all age groups, from infants to the elderly, the youth of patients with severe or lethal infections is a striking feature of these early outbreaks."

Pandemic phases versus severity
Much of today's WHO statement, titled "Assessing the severity of an influenza pandemic," explained the numerous variables that affect the severity of a pandemic. It was released the same day that Dr. Keiji Fukuda, speaking at a press briefing, took pains to explain that the WHO's pandemic alert phases do not describe the severity of an outbreak but refer only to how widely the disease has spread.

"In the past few weeks we've been asked, is this a mild event? The response is that we are not sure right now. The situation is evolving," said Fukuda, the WHO's assistant director-general for health security and environment.

Phase 5, which the WHO declared on Apr 29, means that sustained community transmission is occurring in more than one country in one global region. Phase 6 means a full-scale pandemic, with community transmission going on in more than one region. The WHO says that has not happened yet: While countries such as Spain and the United Kingdom have dozens of cases, they have been limited to school and institutional settings and have not escaped into the wider community.

"Now severity is a different characteristic," Fukuda said. The severity of an epidemic can refer to the incidence of mild, moderate, or severe illness, and it can also refer to the overall social and economic impact of an outbreak on a country, he said.

The WHO statement goes into more detail. It says the virulence of the virus largely determines the number of severe illnesses and deaths, but many other factors influence the overall severity, including the contagiousness of the virus, the age distribution of cases, the prevalence of chronic health problems and malnutrition in a population, viral mutations, the number of waves of illness, and the quality of health services.

Although WHO officials have been careful not to characterize the severity of the H1N1 situation, the agency is working on a system to help provide that kind of information, Fukuda said.

A reporter asked him if the WHO could come up with something like the US government's "Pandemic Severity Index," which was inspired by hurricane classifications. The reporter said the public is confused because the world is in phase 5 with a "mild" virus, in the context of pandemic preparations triggered by the often-lethal H5N1 avian flu virus.

Fukuda replied, "WHO, with the same group of people who have been working on phases and on pandemic preparedness plans, has been working on developing a way to grade severity. We have refrained at this point as to posting whether we think it's a mild stage or medium or severe. I think we will be trying to provide this guidance as soon as we can."

He said that providing severity information has been "an active part of the pandemic preparedness thinking" in recent years, but he gave no details about what kind of system the WHO might come up with or when it would be unveiled.

The US Department of Health and Human Services (HHS) announced its Pandemic Severity Index in February 2007 as part of its guidance on community interventions for combating a pandemic. The index is based on case-fatality rates (CFRs), with a CFR of 2% or greater signaling the most severe pandemic: category 5. The pandemics of 1957 and 1968 qualify as category 2 events, with CFRs between 0.1% and 0.5%, HHS officials said.

Defining community spread
In other comments today, Fukuda said the criterion for "community spread" of a disease is "when you begin to see people who are getting infected and you're just not clear where they're getting infected from." He added that many US cases can't be traced anywhere, unlike the cases in school and institutional outbreaks.

But in response to questions, he said there is no specific number of cases that signals community spread. "What you're really looking for is something that's convincing. . . not something that's just a quirk or an oddity," he said. "We're very mindful that going from phase 5 to phase 6 is a very important step and it really would be interpreted that way. I can't tell you whether that's 10 people or 100 people or so on."

See also:

May 11 WHO statement

Feb 1, 2007, CIDRAP News story "HHS ties pandemic mitigation advice to severity"

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