Epidemiology and etiology of childhood pneumonia
Igor Rudan, Cynthia Boschi-Pinto, Zrinka Biloglav, Kim Mulholland, Harry Campbell
Volume 86, Number 5, May 2008, 408-416
|Step||Methods and models|
|Step 1||The Child Health Epidemiology Reference Group (CHERG) working group on pneumonia did an extensive review of the research on childhood pneumonia that was subsequently synthesized in a database including more than 2200 sources of information. Further details on search strategies, inclusion criteria and methods are published elsewhere.
|Step 2||A review of the database with 2200 CHERG studies identified risk factors for pneumonia at the community level. Only studies that investigated the role of several risk factors at the same time at the community level using a multivariate design and that included more than 500 children were initially used to establish definite, likely and possible risk factors. This step was needed to avoid confounding issues and publication bias typical of studies that use univariate design, study a single risk factor or are simply based on a too-small sample and lack power.|
|Step 3||Four studies were identified
|Step 4||We decided to use only the prevalence of exposure to definite risk factors for distributing global number of pneumonia cases by individual countries. However, we decided to exclude measles immunization coverage on two grounds: because the coverage has approached high levels in recent years (while the studies that identified it as an important risk factor were done mainly in the 1980s and 1990s), so this factor is less discriminative than in was several years ago; and because there is no theoretical justification for including it apart from historically serving as a proxy for health system functioning (but this cannot justify its inclusion).|
|Step 5||After defining five risk factors that will be used to distribute all cases of childhood pneumonia that occur in 1 year globally, this total number was computed. It was derived by:(i) multiplying the number of all children aged less than 5 years living in developing countries (this includes WHO regions AFR D, AFR E, AMR B, AMR D, EMR B, EMR D, SEAR B, SEAR D and WPR B)a in the year 2003 with incidence of 0.28 episodes per child-year, as estimated by Rudan et al.;
|Step 6||Three parameters were then used to distribute the global number of episodes into regional and national estimates:(i) national under-5 population;
|Step 7||Data on the prevalence of children underweight (weight-for-age z-score < –2), low birth weight (≤ 2500 g), and non-exclusive breastfeeding (during the first 4 months of life) were obtained from the Demographic and Health Surveys (DHS) or from the Multiple Indicators Cluster Surveys (MICS).
|Malnutrition (weight-for-age z-score < –2)||98||27||141|
|Low birth weight (≤ 2500 g)||107||16||83|
|Non-exclusive breastfeeding (for first 4 months)||98||64||337|
|Indoor air pollution||117||66||342|
|Crowding (≥ 5 people per household)||52||53||278|
|Step 8||Estimates of relative risks for each of the five definite risk factors for malnutrition, low birth weight, non-exclusive breastfeeding and crowding were obtained from available studies
|Step 9||The global number of new episodes of clinical pneumonia was calculated for each developing country with a model based on the epidemiological concept of potential impact fraction
|Step 10||Cautionary notes on limitations of this approach:(i) In our calculations, we used the child population estimates for the year 2000 and the prevalence of exposures to risk factors relevant to the years 2001–2003; however, the global childhood pneumonia incidence estimate is based mostly on studies conducted in the 1980s and 1990s, and so are relative risks associated with different risk factors.(ii) Prevalence of malnutrition, low birth weight and lack of exclusive breastfeeding mostly comes for MICS and DHS data that were made available in 2003–2004, but relevant to the years 2000–2001; indoor air pollution information comes from the World Bank’s source and refers to 2002–2003, while the search of the information for crowding was also done during 2002; we decided that it is most appropriate to present national-level estimates for the year 2000, as these then ensure consistency and complement the papers on global incidence of childhood pneumonia
AFR, African Region; AMR, Americas Region; EMR, Eastern Mediterranean Region; EUR, European Region; SEAR, South-East Asia Region; WPR, Western Pacific Region.a WHO regions are subdivided based on child and adult mortality strata: A, very low child and very low adult mortality; B, low child and low adult mortality; C, low child and high adult mortality; D, high child and high adult mortality; E, high child and very high adult mortality.