Divergent age patterns of under-5 mortality in south Asia and sub-Saharan Africa: a modelling study

Abstract

Background: Understanding the age pattern of under-5 mortality is essential for identifying the most vulnerable ages and underlying causes of death, and for assessing why the decline in child mortality is slower in some countries and subnational areas than others. The aim of this study is to detect age patterns of under-5 mortality that are specific to low-income and middle-income countries (LMICs).

Methods: In this modelling study, we used data from 277 Demographic and Health Surveys (DHSs), 58 Health and Demographic Surveillance Systems (HDSSs), two cohort studies, and two sample-registration systems. From these sources, we collected child date of birth and date of death (or age at death) from LMICs between 1966 and 2020. We computed 22 deaths rates from each survey with the following age breakdowns: 0, 7, 14, 21, and 28 days; 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 18, and 21 months; and 2, 3, 4, and 5 years. We assessed how probabilities of dying estimated for the 22 age groups deviated from predictions generated by a vital registration model that reflects the historical mortality of 25 high-income countries.

Findings: We calculated mortality rates of 81 LMICs between 1966 and 2020. In contrast with the other regions of the world, we found that under-5 mortality in south Asia and sub-Saharan Africa was characterised by increased mortality at both ends of the age range (ie, younger than 28 days and older than 6 months) at a given level of mortality. Observed mortality in these regions was up to 2 times higher than predicted by the vital registration model for the younger-than-28 days age bracket, and up to 10 times higher than predicted for the older-than-6 months age bracket. This age pattern of under-5 mortality is significant in 17 countries in south Asia and sub-Saharan Africa. Excess mortality in children older than 6 months without excess mortality in children younger than 28 days was found in 38 countries. In south Asia, results were consistent across data sources. In sub-Saharan Africa, excess mortality in children younger than 28 days was found mostly in DHSs; the majority of HDSSs did not show this excess mortality. We have attributed this difference in data sources mainly to omissions of early deaths in HDSSs.

Interpretation: In countries with age patterns of under-5 mortality that diverge from predictions, evidence-based public health interventions should focus on the causes of excess of mortality; notably, the effect of fetal growth restriction and infectious diseases. The age pattern of under-5 mortality will be instrumental in assessing progress towards the decline of under-5 mortality and the Sustainable Development Goals.

Funding: Eunice Kennedy Shriver National Institute of Child Health and Human Development of the National Institutes of Health.

Figure 1:. Ratio of observed to predicted cumulative probabilities of dying from birth to age x (q[x]), controlling for the level of the under-5 mortality rate, including DHS age patterns that are within the range predicted by the vital registration model (A) and DHS age patterns that diverge from the vital registration model (B)

Ratios were computed by dividing observed by predicted probabilities of dying from birth to age x values for 22 age groups between age 0 years and age 5 years. Ratios are presented on a log scale. The blue area covers the range of DHS estimates. The blue line represents the mean ratios across DHS. The red area represents the range of predictions of the vital registration model. Curves were smoothed with splines. The unsmoothed curves and spread of the raw data are shown in the appendix (p 4). DHS=Demographic and Health Surveys.

Figure 2:. Clustering of categories of age patterns of under-5 mortality by region, from the first collected DHS in a given country (average year of collection 1993; A) to the last collected DHS in a given country (average year of collection 2014; B)

For countries with only one survey, the result was shown in A if the data were collected before 2000 and in B if after. As an exception, the data for China come from the National Maternal and Child Health Surveillance System (2001–15; appendix p 5). DHS=Demographic and Health Surveys.

Figure 3:. Ratio of observed versus predicted mortality using DHS data with divergent age patterns and alternative sources of data, by age group and controlling for the level of mortality between 4 and 6 months

Ratios were computed by dividing observed by predicted probabilities of dying between ages z and x for six age groups between age 0 years and age 5 years. Ratios are presented on a log scale. The shaded areas cover the range of estimates. The lines represent the mean ratios across estimates. The spread of the raw data is shown in the appendix (p 7). Alternative sources of data include HDSS and sample registration system. DHS=Demographic and Health Surveys. HDSS=Health and Demographic Surveillance Systems.