90 percent of human bird flu cases under 40 years of age
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Nearly 90 per cent of the people who've been diagnosed so far with
H5N1 avian influenza virus infection were under age 40, a new
analysis from the World Health Organization shows. And 2 British
scientists suggest that as-yet-unexplained phenomenon could be a clue
that widespread immunity to infection with this virus may exist in
people aged 35 and older.
In a letter to the March [to be published 2007] issue of the journal
Emerging Infectious Diseases, Matthew Smallman-Raynor of the
University of Nottingham and Andrew Cliff of the University of
Cambridge note that 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."
"Such a model would account for the similar rates of disease activity
in younger age categories, the sudden and pronounced reduction of
cases in patients over 30-35 years of age, and the age skew that
transcends the socio-cultural and demographic contexts of countries
and continents," wrote Smallman-Raylor and Cliff, who teach
analytical and theoretical geography respectively.
It is not known why H5N1 seems to prefer the young and rarely infects
the elderly, the age group hardest hit 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.
The 2 British scientists 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.
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 25 Nov 2003 and 24 Nov 2006, showed that
the median age of cases was 18 years old. 52 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 5 (44 per cent) and
children aged 5-9 (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 2nd year of the 3-year period twice as high as those
recorded in the 1st 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.
[Byline: Helen Branswewell]
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ProMED-mail
<promed@promedmail.org>
[The reference for the letter cited in the above report is the
following: Smallman-Raynor M, Cliff AD. Avian influenza A (H5N1) age
distribution in humans [letter]. Emerg Infect Dis [serial on the
Internet]. March 2007 [date cited]. Available from
<http://www.cdc.gov/EID/content/13/3/06-0849.htm>. The authors
conclude that: "Although anecdotal, reports of completed surveys
point to a lack of widespread human infection with the virus. Current
evidence indicates that pandemic influenza of humans since 1918 has
been restricted to 3 influenza A virus subtypes: H1 (1918-57 and
1977-present); H2 (1957-68); and H3 (1968-present). 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." - Mod.CP] |
