Epidemiology

For general planning purposes, it is usually assumed that approximately 25–30% of the population will be infected by pandemic influenza. However, there are very large differences in the odds of being infected depending on the age of an individual. Some models predict that over 50% of all children will be infected by a pandemic strain of influenza. The elderly are the least likely to be infected. The percent infected is also known as the “attack rate” of a virus. Equally important is the mortality rate, that is, the percent of infected people that die.

H1N1 Epidemiology

Thus far, the vast majority of the people infected with pandemic H1N1 have been children who have been infected in schools. However, the case fatality rate appears to be higher in adults than in children. Reports of large scale outbreaks among adults are rare, at this time. Pregnant women appear to be at a much higher risk from death from pandemic flu. Others who appear to be at increased risk from the new H1N1 virus include people with:

Asthma
Down's Syndrome
Cerebral Palsy
Neurogenetic Disorders
Muscular dystrophy
Morbid Obesity
Cancer
Diabetes

The CDC provides very limited information regarding pandemic H1N1 cases. The online community has put together a variety of resources which are useful in tracking the epidemiology of the pandemic:

Reported Deaths in the US
Database prepared by EMTim

Details on US Deaths
Spreadsheet prepared by Aurora

US Child H1N1 Deaths
List prepared by Homebody and contributors at PFI_Forum

Details on deaths among Health Care Workers and Educators
Spreadsheet prepared by Influentia

H5N1 Epidemiology

Most of the people who have died from H5N1 infections have been children and young adults. This is a very different pattern of mortality from that observed with “normal” flu, which is more likely to kill the elderly than children, although some children do die of "normal" flu. The ablility of H5N1 to kill otherwise healthy young people is similar to the very lethal 1918 pandemic strain of influenza but quite different from the milder pandemics of 1957 and 1968. This is one reason experts are so concerned about H5N1. The mortality rate for H5N1 is currently 50%. Although no-one knows what the mortality rate for a pandemic strain of H5N1 would be, flu experts have pointed out that H5N1 may cause a pandemic without decreasing its ability to kill. This would result in a massive loss of life, particularly among children.

One of the signs that a novel virus is adapting to humans is the presence of clusters of illness that indicate that the virus has the ability to pass from human to human. Such clusters have been observed with H5N1 and are documented here: H5N1 Clusters.

References

Russell et al. (2008) The Global Circulation of Seasonal Influenza A (H3N2) Viruses. Science.

Rambaut et al. (2008) The genomic and epidemiological dynamics of human influenza A virus. Nature.

Writing Committee of the Second World Health Organization Consultation on Clinical Aspects of Human Infection with Avian Influenza A (H5N1) Virus (2008) Update on Avian Influenza A (H5N1) Virus Infection in Human, New England Journal of Medicine, 358:261-273.

Uyeki (2008) Global epidemiology of human infections with highly pathogenic avian influenza A (H5N1) viruses. Respirology. Suppl 1:S2-9.

Wu et al. (2007) Antigenic profile of avian H5N1 viruses in Asia from 2002-2007 J. Virol.

Epidemiology of WHO-confirmed human cases of avian A(H5N1) infection. Weekly Epidemiological Record, 81: 249–260.

Blogs

Down’s Syndrome – A Risk Factor for the New Flu?
July 23, 2009

Spread and incidence of pandemic flu
August 3, 2009

High incidence, low case fatality rate countries
August 4, 2009

Age Structure and Case Fatality Rate
August 10, 2009

Total Number of Expected Deaths
August 11, 2009

The Myth of the Missing Mild Cases
August 12, 2009

Incidence of Death
August 29, 2009

Incidence of Death in US States
September 3, 2009