Why some adults are more protected against avian flu

Main Category: Infectious Diseases / Bacteria / Viruses
Also Included In: Flu / Cold / SARS
Article Date: 02 Feb 2004 - 0:00 PDT

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The pattern of human illness in the avian influenza outbreak sweeping Asia may offer a glimmer of hope for what might evolve if the H5N1 strain triggers a human flu pandemic, some Canadian researchers are suggesting.

They have posed the theory the strain may not be hitting adults as hard as children because adults have had years of exposure to human influenza viruses containing an N1 component.

The scientists are not suggesting there is anything reassuring about the concept of mass numbers of children falling prey to a killer flu.

But they do argue that if it can be determined adults are at a much reduced risk, all available resources - limited stocks of anti-viral drugs, the first doses of an eventual vaccine - could be directed at protecting the world's children.

It's a theory that needs to be explored, given the implications it would have on governmental responses to a pandemic, says influenza expert Dr. Danuta Skowronski, one of the researchers behind the hypothesis.

'If it (vaccine) is limited - and it will be in the beginning - who should we be protecting first? And should children be at the top of that (list)?' she asks.

If the theory has merit, it might even be worthwhile to inoculate children with vaccines developed to protect against human H1N1 strains, as they might offer some protection for those who become infected with the pandemic virus, she and her colleagues argue.

'I just think that this adds a glimmer of hope. I can tell you I feel a glimmer of hope,' Skowronski said in an interview from Vancouver (Canada).

Of the 13 confirmed human cases so far, 10 have died. Most are children. No one is sure why that is.

'It's still speculative right now but the hypothesis is that most of these (cases) were in rural areas of Vietnam, where it was backyard farms,' says Maria Cheng, a spokeswoman for the World Health Organization.

'So these kids were probably playing with the chickens or in the same environment, inhaling dust, inhaling the virus.'

Skowronski doesn't buy that theory. 'I don't believe that can entirely explain it.'

There would be clear benefits if a pandemic strain largely spared adults. It could minimize the social disruption expected to go hand-in-hand with a pandemic.

When a pandemic strikes, mass numbers of people fall ill at the same time. Food and drug production and distribution would be affected; essential services like power and water purification could be threatened.

Hospitals could collapse under the combined weight of sick staff and too many patients.

'What it does say is the support systems may be able to continue . . . and other essential elements of society - and that we might be able to target then our limited control measures in that way,' Skowronski explains.

She and three colleagues from the British Columbia Centre for Disease Control (Canada) posed the theory last week on ProMed, a global electronic bulletin board for tracking emerging diseases operated by the International Society for Infectious Diseases.

They asked for feedback. They're getting lots.

'Whereas there is conceivably some protection, it would be surprising and is not supported by previous work in the area,' Earl Brown, a University of Ottawa virologist who specializes in the evolution of influenza virulence, responded in an e-mail.

Others think the idea has merit.

'It's a very possible, plausible hypothesis,' says Richard Webby, a virologist at St. Jude Children's Hospital in Memphis, Tenn., who is part of a team working with the WHO to create a prototype vaccine to the H5N1 virus.

Webby suggests the theory may explain the last flu pandemic, the Hong Kong flu of 1968-69, which was moderate by historical standards.

That pandemic was caused by an H3N2 strain. The pandemic before, the Asian flu of 1957-58, was caused by an H2N2 virus. The N2 components of the two viruses were identical, Webby says.

Influenza A viruses are subtyped based on two proteins on their surface, hemagglutinin (the H) and neuraminidase (the N). There are 15 known hemagglutinins and nine neuraminidases and they can combine in any fashion.

So far, only viruses of the H1, H2 and H3 subtypes have acquired the ability to infect people and spread from human to human.

It's generally been assumed the hemagglutinin portion of the viral equation is far more significant in terms the immune system's response to a flu virus. That's one of the reasons authorities have been so worried about the possibility an H5 virus might spark a pandemic.

H5 viruses have never circulated in the human population before, so there is no innate immunity. And since H5N1 seems to be highly virulent as well, it could be a viral nightmare.

But some experts believe the neuraminidase component has been underestimated.

'Antibodies to neuraminidase don't necessarily neutralize the virus. But it does reduce spread and it does reduce severity,' Webby says.

In fact, some believe the best hope for producing a universal vaccine for influenza - one that would offer lifetime protection against whatever strain is circulating - lies with targeting the neuraminidase. Currently, flu vaccines target the hemagglutinin of circulating viruses.

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