Nearly 90 per cent of the people who have been diagnosed so far with H5N1 strain of the avian flu virus were under age 40, a new analysis from the World Health Organization shows.
 
And two British scientists suggest that as yet unexplained phenomenon could be a clue that widespread immunity to infection with the virus may exist in people aged 35 and older.
 
The age distribution of H5N1 human cases is "consistent with a biological model of geographically widespread immunity to avian influenza A (H5N1) in persons born before 1969," Matthew Smallman-Raynor of the University of Nottingham and Andrew Cliff of the University of Cambridge note in a letter to the March issue of the journal Emerging Infectious Diseases.

They said such a model would account for:

• The similar rates of disease in younger people.
• The "sudden and pronounced reduction" of cases in patients over age 30-35.
• The age skew that "transcends the sociocultural and demographic contexts of countries and continents."

Different theories

It is not known why H5N1 seems to prefer the young and rarely infects the elderly, the age group hit hardest by seasonal flu.

Suggested theories have included that children and young people may have closer exposure to poultry in countries where outbreaks are occurring.
 
Another possibility could be that older people are actually being infected but suffer such mild illness that they don't come to the attention of health authorities.

The few studies that have looked for mildly symptomatic or asymptomatic cases don't support this idea, but experts believe larger studies need to be done before it could be ruled out.
 
Smallman-Raynor and Cliff, who teach analytical and theoretical geography respectively, looked at ages of reported cases and compared them to population figures for countries reporting human infections, looking to see if the demographic compositions of those countries provide some clues.
 
Their analysis showed that the imbalanced distribution of cases is seen in both genders, throughout the duration of the ongoing H5N1 outbreak (which began in late 2003) and across all countries which have had enough human cases so that statistics could be crunched.

That suggests that the trend probably isn't due to local cultural or geographic factors, argued Smallman-Raynor and Cliff, who said the idea needs further study.
 
"If an element of immunity to avian influenza A (H5N1) does exist in older populations, its possible association with geographically widespread (intercontinental) influenza A events before the late 1960s merits further investigation," they said.

52% of cases were younger than 20: WHO study

The WHO analysis, published in the agency's online journal the Weekly Epidemiological Review, also argues that the higher proportion of cases in younger age groups probably isn't just due to the fact that young people make up a big part of the age structures of affected countries.
 
The report, which covers the 256 laboratory confirmed cases that occurred between Nov. 25, 2003 and Nov. 24, 2006, showed that the median age of cases was 18 years old.
 
Fifty-two per cent of cases were younger than 20 years old and 89 per cent were under age 40. Men and women made up virtually an equal number of cases.
 
The death rate was highest among cases aged 10 to 19; 76 per cent of cases in that group died. Cases aged 50 and over had the lowest death rate (40 per cent) followed by children under age five (44 per cent) and children aged five to nine (49 per cent).
 
The total case fatality rate was 60 per cent.
 
Cases have increased over time, the unnamed authors reported, with the cases in the second year of the three-year period twice as high as those recorded in the first year. From Year 2 to Year 3, the number of cases rose by about 25 per cent.
 
A graph of cases showed that while there are definitely seasonal peaks and troughs in human infections, there have been cases recorded every month since November 2004.