14 February=20 2005

WHO Pre-Publication: 88% of H5N1 cases occurred in persons 0-30 = years old=20

The World=20 Health Organization (WHO) posted on their website in January 2005 a = valuable=20 =93pre-publication=94 document titled: =93Avian Influenza: Assessing = the pandemic=20 threat=94. This 62-page document is composed of four sections, = followed by=20 tables of non-medical interventions at national and international = levels, and a=20 list of WHO recommendations and official reports on H5N1 avian = influenza.=20

The four=20 sections are: (1) =93The H5N1 outbreak in 2004: a pandemic in = waiting?=94 (2)=20 =93Lessons from past pandemics=94 that includes 12 lessons from the = 1918-19,=20 1957-58, and 1968-69 pandemics; (3) =93Understanding the outbreaks in = poultry=94=20 that emphasizes that the 2004 outbreaks were unprecedented, being the = largest=20 and most ominous ever; and (4) =93Action in the face of an uncertain = threat=94 that=20 gives updates on vaccine issues and the different roles of antiviral = drugs in=20 different phases of the epidemic.

The focus of=20 this newsletter is on the data from Vietnam and Thailand provided in the = first=20 section (p. 21-22) of this document concerning the initial 50 patients = with=20 laboratory-confirmed H5N1 infection. Two tables include the = gender, age,=20 and clinical outcome in the initial 33 patients in Vietnam, and 17 = patients in=20 Thailand. There is no difference by gender (25 females and 25 = males). A=20 striking effect by age is evident, however, with 44/50 (88%) being = between 1-30=20 years of age. An additional 4/50 (8%) are 31-40. None are >60 years = of age.=20

19/50 (38%) are 1-10 years of age and 15/50 (30%) = are 11-20=20 years old. Thus, over two-thirds (34/50, or 68%) are 1-20 years of age. = Combined=20 with the lack of any laboratory-confirmed cases over 60 years of age, = there is a=20 clear contrast between the age distribution of SARS in 2003 and H5N1 = infection=20 in 2004.

The overall=20 case fatality rate for the 50 patients was 76% (38/50). This compares = with the=20 33% (6/18) case fatality rate of the 18 symptomatic, H5N1 = laboratory-confirmed=20 patients in Hong Kong in 1997. Although the numbers are small, = only 11%=20 (1/9) of children 10 years of age or less in 1997 died of lab-confirmed = H5N1=20 infection (J Infect Dis 1999;180:1763-70), whereas 84% (16/19) of = lab-confirmed H5N1 infections in children 10 years of age or less in = Vietnam and=20 Thailand have died.

Whether=20 there are more symptomatic, or asymptomatic, cases of H5N1 infection = that have=20 not yet been determined, especially in older age groups requires more = widespread=20 laboratory studies. It would be surprising if both symptomatic and = asymptomatic cases of H5N1 infection were not being missed in the = ongoing=20 epidemic due to lack of testing. In the 1997 H5N1 outbreak in Hong = Kong=20 evidence of asymptomatic infection and limited, non-sustained = person-to-person=20 transmission, was published (J Infect Dis 2000;181:344-48). = One of=20 the first studies looking for H5N1 antibody in health care workers = exposed to=20 four lab-confirmed H5N1 patients, or their clinical specimens, in 2004 = in a=20 hospital in Hanoi found that 0/83 (0%) had H5N1 antibodies = (Emerg=20 Infect Dis J 2005 (Feb); 11:210-15). More studies of health = care=20 workers, family members, and social contacts, as well as asymptomatic = persons=20 exposed to the same poultry as lab-confirmed H5N1 infected patients are = urgently=20 needed to better assess the case-fatality rate and age distribution. =

Daniel R.=20 Lucey, MD, MPH