6. Incorporating Deaths Near the Time of Birth into Estimates of the Global Burden of Disease

The Burden of Disease Resulting from Events Near the Time of Birth

This section explains the use of DALYs as a measure of the disease burden and identifies a number of problems associated with the traditional DALY formulation when dealing with events around the time of birth. It proposes a generalized formulation (which annex 6A describes more fully). The chapter then calculates the disease burden using two approaches to explore the sensitivity of GBD estimates to alternative formulations as follows:

• the current DALY formulation extended so as to value the DALY loss from a stillbirth the same as the DALY loss from a death at age 0,

• a generalized DALY formulation allowing the acquisition of life potential (ALP) to be gradual rather than instantaneous.

Defining and Redefining DALYs

The DALY family of indicators measures the disease burden from the age of onset of a condition by summing an indicator of YLL due to the condition and an indicator of disability-adjusted YLD resulting from the condition. While, in principle, the disability weights used in this adjustment could arise from any of the procedures typically used to construct quality- adjusted life years, obtaining disability weights for a large number of causes using any procedure other than the judgments of selected reference groups is currently impractical. Chapter 3 describes the methods currently used.

DALYs generate a measure of the disease burden resulting from premature mortality by integrating a discounted, age-weighted, disability-adjusted stream of life years from the age of death (see equation 6A.2 in annex 6A). The formulation within the family of DALYs previously used to empirically assess the global burden of disease specifies a constant discount rate of 3 percent per year and an age-weighting function that gives low weight to early childhood and older ages and greater weight to middle ages. This volume reports global burden of disease estimates generated using uniform age weights. Chapter 5 provides an extensive exploration of the uncertainty and sensitivity inherent in disease burden assessment, including the results of differing assumptions about age weighting and discount rates.

To be clear about the particular form of DALY being used, the following terminology is employed throughout this volume. DALYs(r,K) are DALYs constructed using a discount rate of r percent per year and an amount of age weighting indexed by a parameter K. Two versions of the DALY are discussed at some length in chapter 5, both using a discount rate of 3 percent per year. DALYs(3,1) are DALYs generated with a discount rate of 3 percent per year and with full age weighting, that is, K = 1. DALYs(3,0) are DALYs generated with a discount rate of 3 percent per year and with no age weighting, that is, K = 0. This volume's results concerning the burden of disease (chapter 3) and of risk (chapter 4) are based on DALYs(3,0). Annex 6B contains tables summarizing alternative calculations of the global burden of disease, and table 6B.4 presents the chapter 3 GBD results based on DALYs(3,0), using this chapter's aggregation of causes, for age groups under five and over five as an aggregate.

This chapter extends the DALY family by modeling a concept of ALP. The intuition behind the ALP concept is that an infant (or fetus) only gradually acquires the full life potential reflected in a stream of life years beginning at birth, that is, ALP can be gradual. The ethical understanding of the concept is based on two judgments: (a) an individual life acquires value only as it acquires self-awareness, and (b) an individual life acquires additional value as it develops bonds with others. (See the discussion in Steinbock 1992, who argues that what we label as life potential is probably acquired some time in the second trimester of pregnancy. Her position is, implicitly, that whenever it occurs, ALP is instantaneous.) To some extent, the age-weighting function of the current DALY formulation attempts to capture these judgments, and in this chapter, gradual acquisition of ALP is modeled as an alternative to age weighting.³ Mathematically, however, ALP and age weighting are independent and can be introduced simultaneously.

Our objective in this chapter is not to provide a detailed philosophical, economic, or medical rationale for gradual ALP, but to generate and apply a straightforward mechanism that allows for it. Annex 6A describes this mechanism, which essentially consists of multiplying the DALYs conventionally generated by a factor that is less than one for younger ages. This factor is zero for an age of -13 weeks (or -0.25 years), rises to a factor value of f ⁰ at birth, then rises to 1 at time T. Figure 6.2 graphs both the ALP function used later in this chapter and the special case of ALP that jumps from 0 to 1 at age 0 (instantaneous ALP). The ALP implicit in traditional DALYs is instantaneous.
[Figure 6.2]

Annex 6A introduces a parameter, A, that indicates the speed of ALP (see equations 6A.3 through 6A.5 for a precise definition of A). A is constructed so that for the fastest possible speed of ALP, namely, instantaneous ALP, A = 1. A is bounded below by 0. This chapter extends the notation DALYs(r,K) used elsewhere in the book in two ways. First, it explicitly indicates the level of A by extending the DALY nomenclature to DALYs(r,K,A). Thus using this nomenclature, DALYs(3,0) become DALYs(3,0,1), because the standard DALY is the special case with instantaneous ALP. Second, when stillbirths are included in the range of events to be measured in the global burden of disease, this is explicitly noted in the DALY nomenclature as DALYs_SB(r,K,A). Notation around YLL is similarly extended.

Explicit modeling of ALP allows not only the reflection of the ethical judgments just indicated, but also permits three instrumentally useful improvements to the current family of DALYs:

• The DALY loss from a death seconds before birth is, in the current formulation, 0; it jumps to more than 30 years at birth. The ALP formulation allows, but does not require, this discontinuity to be avoided. See column (a) of table 6.5 for values at different ages of the ALP function associated with traditional DALYs and columns (c), (d), and (e) for values of three ALP functions defined in annex 6A.

  
  [Table .]
  

• The ALP formulation allows, but does not require, a positive DALY loss associated with stillbirths.

• The ratio of the DALY loss from a death at age 20, say, to that at birth is close to 1 for any reasonable set of parameter values in the current DALY formulation. Many people's ethical judgments would give this ratio a value substantially greater than 1. The ALP formulation allows, but does not require, these judgments. Figure 6.3 shows how this ratio varies as a function of the age-weighting parameter (K) for values of r equal to 3 percent and 10 percent. The ratio rises only to 1.7 with full age weighting and an implausibly high discount rate of 10 percent.
  [Figure 6.3]
  

Alternative Calculations of the Burden of Disease

As previously indicated, annex table 6B.4 (based on annex tables 6B.1 to 6B.3) presents the chapter 3 GBD estimates in terms of DALYs(3,0)—or DALYs(3,0,1)—for the under and over five age groups. The DALY(3,0) is the sum of the YLL(3,0,1) and YLD. Annex tables 6B.1, 6B.2, and 6B.3 report deaths by cause, YLL(3,0,1) by cause, and YLD by cause from chapter 3. The numbers in table 6B.4 are the sum of the corresponding numbers in tables 6B.2 and 6B.3.

We generate two alternative assessments of the global burden of disease. Both incorporate stillbirths and the second permits gradual ALP. The YLD numbers that we use come from annex table 6B.3. The YLL differ from YLL(3,0,1) for ages under age five, but are the same for over age five.

Our first alternative is probably the simplest way to incorporate stillbirths. It does so by having an instantaneous ALP function, as with traditional DALYs, but by having that function jump from 0 to 1 at age -13 weeks (-0.25 years) instead of at age 0. Stillbirths are then given the same DALY loss as a death at birth in generating YLL. Column (b) of table 6.5 shows values for this ALP function, which is uniformly 1. We label the YLL generated from this ALP function and a 3 percent discount rate the YLL_SB(3,0,1). We label the DALYs based on this YLL as DALYs_SB(3,0,1). Table 6.6 shows values of YLL_SB(3,0,1) compared with YLL(3,1) and YLL(3,0) for different ages. Annex table 6B.5 shows values for YLL_SB(3,0,1) and annex table 6B.6 shows the resulting burden of disease estimates in terms of DALYs_SB(3,0,1).

Our second alternative burden of disease assessment is based on gradual ALP. Equation 6A.1 in annex 6A provides our general ALP function and the text describes the meaning of its four parameters. One of the parameters, f⁰, is the value of the function at age 0. The intuitive interpretation of f⁰ is that it is approximately the ratio of the YLL loss associated with a death at age 0 to that from a death at age 20. Another parameter is T, the age at which the function becomes 1. Annex 6A characterizes three alternative gradual ALPs: f₁, f₂, and f₃. Figure 6.4 shows YLL at different ages for these functions and for YLL(3,0) and YLL(3,1). Table 6.5 shows values for the functions at different ages in columns (c), (d), and (e). We use f₂ (with A = .54) to construct the disease burden estimates reported in this chapter and label the resulting YLL and DALYs as YLL_SB(3,0,.54) and DALYs_SB(3,0,.54). Table 6.6 shows YLL_SB(3,0,.54), which are, as intended, markedly lower than YLL_SB(3,0,1) for very young ages. That is, YLL_SB(3,0,.54) gives less weight to deaths near the time of birth or to deaths immediately after birth than YLL_SB(3,0,1).
[Figure 6.4]

Only a limited number of empirical studies have attempted to assess directly the views of individuals concerning deaths at different ages. In an important early study, Crawford, Salter, and Jang (1989) relate grief from a death to the concept of reproductive potential in population biology. They conclude that for several diverse human groups the relationship shows grief to be closely related to prehistoric reproductive value. Cropper, Aydede, and Portney (1994) and Johannesson and Johansson (1997) survey members of populations of high-income countries for trade-offs between deaths in middle and older ages. All three of these studies find that people judge deaths at older middle age as much less important than deaths at younger middle age, but provide no information concerning the trade-off for deaths near the time of birth.

An Institute of Medicine (1985) review of vaccine development priorities uses infant mortality equivalence in cost-effectiveness calculations. The committee members preparing the report collectively judged that the loss from a death at age 20 should be about two times that from an infant death, well above the numbers shown in figure 6.3 for any standard DALY. However, some preliminary trade-off studies by one of the authors of this chapter suggest a value closer to three or four times. What is clear is that no defensible estimate (or even range) is currently available, and hence the numbers we report should be viewed only as perhaps reasonable but only suggestive and as indicating the sensitivity of global burden of disease results from younger ages to better estimates of this parameter.

Annex tables 6B.7 and 6B.8 show YLL_SB(3,0,.54) and DALYs_SB(3,0,.54). While table 6B.7 only shows the total of DALYs for ages under five, the calculations underlying those totals reflect the age distribution of deaths under age five shown in table 6.4 and the YLL_SB(3,0,.54) for deaths at different ages as shown in table 6.6.

Annex tables 6B.1, 6B.6, and 6B.8 provide three alternative assessments of the global burden of disease based on deaths by cause, on DALYs(3,0), DALYs_SB(3,0,1), and DALYs_SB(3,0,.54). Table 6.4 provides estimates of deaths by cause that include stillbirths (table 6.4, column [k]). We thus have five alternative indicators of the importance of disease at different ages and from different causes. Table 6.7 provides a summary for low- and middle-income countries of the distribution of the disease burden at different ages as assessed by these different measures. DALYs_SB(3,0,1) and DALYs_SB(3,0,.54) both point to the significance of stillbirths relative to DALYs(3,0), which exclude them altogether, but the gradual ALP approach of DALYs_SB(3,0,.54) gives much less importance to stillbirths than DALYs_SB(3,0,1) and substantially less importance to the under five burden than DALYs(3,0).

Table 6.8 provides a similar summary of how the assessed burden across groups varies with the measure used. DALYs_SB(3,0,.54) give more weight to Group II (noncommunicable diseases) and Group III (injuries) causes than do DALYs(3,0), while DALYs_SB(3,0,1) give less weight to these groups than DALYs(3,0). For example, DALYs_SB(3,0,.54) give about a 10 percent greater weight to cardiovascular disease than does the DALY (3,0), that is, 14.2 percent versus 12.9 percent.