Swine flu on path to match 1957 flu pandemic

The new flu, whether you call it swine or H1N1, is spreading fast and spreading widely.

And while in its current form its symptoms appear to be comparatively mild, the flu is expected to have a significant impact because it will hit so many people across the globe.

"Our early analysis would suggest this is going to be an outbreak comparable to that of 20th century pandemics regarding the extent of its spread - it’s very difficult to quantify the human health impact at this stage, however," said Prof. Neil Ferguson of Imperial College London.

He is a co-author of a paper published in the journal Science, entitled "Pandemic Potential of a Novel Strain of Influenza A H1N1: Early Findings," by the World Health Organization's Rapid Pandemic Assessment Collaboration.

Science normally keeps a news embargo on its reports until the publication is actually released late in the week, but determined that this information is so new and so important, that it lifted the embargo this morning (May 11, 2009).

The paper notes that it's early to be drawing assessments from this flu outbreak, but that preliminary data do provide some clues. It compares this flu to two earlier pandemics, the one in 1918 and the one in 1957.

“While substantial uncertainty remains, clinical severity (of this flu) appears less than that seen in 1918 but comparable with that seen in 1957,” the paper says.

The 1957 flu killed about 70,000 Americans at a time when the nation's population was 172 million. The 1918 flu killed about 675,000 when the national count was about 105 million.

The preliminary data suggest the death rate from this flu (the term used is CFR, for case fatality ratio) is in the range of 3 to 6 per thousand people infected. That's based on deaths suspected to be from H1N1. If only confirmed cases are counted, it's 10 times less than that.

Investigative techniques suggest the first case appeared in a human about February 15, 2009. Genetic techniques suggest the virus evolved to use humans as a host between November 2008 and March 2009.

In early cases in the Mexican community of La Gloria, in Veracruz, young people caught the flu much more readily than older folks. Those under 15 were twice as likely to become affected as those older than that. Does that mean that older folks have some level of resistance, from exposure to a previous flu strain? That's not known, yet.

“The strong age dependence in clinical attack rates seen in La Gloria is intriguing,” the paper said.

There remain lots of questions. Among them, whether the flu will move aggressively to the Southern Hemisphere during its winter flu season, and whether it will then sweep back into the Northern Hemisphere late this year. Also, whether the flu will hit in waves, as previous pandemics have.

“The future evolution of the transmissibility, antigenicity, virulence and antiviral resistance profile of this or any influenza virus is difficult to predict,” the authors write.

The paper's authors, all members of the World Health Organization's Rapid Pandemic Assessment Collaboration, are from a range of research institutions in England, Mexico and Switzerland. They include Christophe Fraser, Christl A. Donnelly, Simon Cauchemez, William P. Hanage, Maria D. Van Kerkhove, T. Déirdre Hollingsworth, Jamie Griffin, Rebecca F. Baggaley, Helen E. Jenkins, Emily J. Lyons, Thibaut Jombart, Wes R. Hinsley, Nicholas C. Grassly, Francois Balloux, Azra C. Ghani, Neil M. Ferguson, Andrew Rambaut, Oliver G. Pybus, Hugo Lopez-Gatell, Celia M Apluche-Aranda, Ietza Bojorquez Chapela, Ethel Palacios Zavala, Dulce Ma. Espejo Guevara, Francesco Checchi, Erika Garcia, Stephane Hugonnet, Cathy Roth.

© Jan TenBruggencate 2009