Bulletin of the World Health Organization

Potential lost productivity resulting from the global burden of uncorrected refractive error

TST Smith a, KD Frick a, BA Holden b, TR Fricke b & KS Naidoo c

a. Bloomberg School of Public Health, Johns Hopkins University, 624 North Broadway, Baltimore, MD, United States of America.
b. International Centre for Eyecare Education, Australia.
c. Africa Vision Research Institute, University of KwaZulu-Natal, Durban, South Africa.

Correspondence to TST Smith (e-mail: tasaneesmith@doctors.org.uk).

(Submitted: 10 June 2008 – Revised version received: 20 August 2008 – Accepted: 25 August 2008 – Published online: 04 March 2009.)

Bulletin of the World Health Organization 2009;87:431-437. doi: 10.2471/BLT.08.055673

Introduction

Uncorrected refractive error (URE) for distance vision, including undercorrected refractive error in more economically developed countries, has recently been highlighted as the main cause of low vision globally and the second leading cause of blindness after cataract.1 An estimated 153 million people had visual impairment (VI) from URE in 2004, and 8 million of them were blind.1 The magnitude of this correctable burden of VI has been overlooked because epidemiological studies have tended to focus on “best corrected” sight rather than presenting visual acuity (VA).2 Present estimates of the magnitude of URE include myopia (short-sightedness) and hypermetropia (far-sightedness),3 but they do not include presbyopia (an age-related impairment of near vision), for which there are few data on prevalence or visual function.1,4,5

Refractive error is correctable with eyeglasses, contact lenses or laser surgery. In the absence of correction, distance vision impairment may limit function. There are many possible approaches to estimating the global economic productivity loss associated with URE. In this paper, conservative assumptions and national data are used to estimate the purchasing power parity-adjusted gross domestic product (PPP-adjusted GDP) loss for all individuals with impaired vision and blindness, and for individuals with normal sight who provide them with informal care. No such estimate has been reported previously.

Definitions

The term “visual impairment” previously encompassed “low vision” and “blindness”, as defined in the International statistical classification of diseases, injuries and causes of death, tenth revision (ICD-10).6 However, the ICD-10 definition focuses on “best corrected” VA and has historically overlooked people who have “presenting” VI as a result of URE.4 Definitions are being revised for ICD-11, but publication is not anticipated until 2014.7 Therefore, this paper adheres to current WHO definitions.8 Specifically, “visual impairment” is defined as a presenting VA of less than 6/18; “moderate or severe visual impairment” (moderate/severe VI), as a presenting VA of less than 6/18 and equal to or better than 3/60 in the better eye; and “blindness”, as a presenting VA of less than 3/60 in the better eye.

Methods

We combined the most recent population and economic data on the prevalence of URE in 2004 by WHO subregion, for all countries with epidemiological data previously reported by Resnikoff et al.1,9 The WHO Global burden of disease in 2002 report defines 14 subregions based on strata of child and adult mortality in the six WHO regions.9 We made conservative assumptions about the economic impact of URE to derive estimates of economic productivity loss.

The reported age-specific prevalence for VI resulting from URE, by WHO subregion, has previously been estimated using meta-analytic techniques that are briefly reviewed here.1 Sixty-eight population-based studies from 31 countries met the appropriate inclusion criteria, namely, having an adequate sample size, appearing in the published or unpublished literature between 1995 and April 2006, reporting the prevalence of both a presenting and a best-corrected VA of worse than 6/18 in the healthier eye, and including both subjective and objective refraction under cycloplegia where subjects included children 15 years of age and under. “Presenting VA” was defined as the VA in the better eye, tested with the subject’s usual refractive correction. “Best-corrected VA” was also based on the better eye, with correction achieved by pinhole or refraction. “VI resulting from URE” was defined as a VA worse than 6/18 in the better eye that could be improved to 6/18 or better with refraction or a pinhole. The prevalence of URE was estimated as the difference between the prevalences of a presenting and a best-corrected VA worse than 6/18. Available age-specific data were used to derive prevalence estimates for the age groups 5–15, 16–39, 40–49 and ≥ 50 years. The prevalence of URE was assumed to be the same for subjects aged 5–15 and 16–39 years. Where data were not specifically available for those aged 40–49 years, a linear interpolation between the prevalence at the ages of 39 and 55 years was used. The age-specific prevalence data for each WHO subregion was based on a population and rural–urban distribution-weighted estimate from the available data. Resnikoff et al. found no evidence of VI resulting from URE in children younger than 5 years. The authors also reported the regional prevalence of blindness from URE in the population aged ≥ 50 years and 40–49 years, assuming that the number of cases was 5.13 times lower in the latter. They found no blindness from URE in people under 40 years of age and none in populations from the America’s A subregion, subregions of the European Region or the Western Pacific’s A subregion.9 Table 1 displays the prevalence of VI in people aged 5–50+ years, and the prevalence of blindness in those aged ≥ 50 years and in the total population, by WHO subregion.

Age-specific population estimates in 5-year groupings for each country, from the mid-2007 United States Census Bureau’s International Database (http://www.census.gov/ipc/www/idb/),10 were used to estimate the total population in the age categories of 0–4, 5–15, 16–39, 40–49 and ≥ 50 years. Population data for Niue, Tokelau and the Cook Islands were not available and were taken instead from the United States’ Central Intelligence Agency’s The world factbook for Niue and Tokelau,11 and from the national statistics office web site for the Cook Islands. Age-specific data for these places were imputed from the regional population structure (percentage in each age group) and the total population.

For each country, total cases of VI were calculated as the product of the number of people in each age group and the subregional age-specific prevalence, summed across age groups. In the WHO classification system, 36 countries or areas, with a total population of 52.3 million (< 1% of the global population), are officially “unassigned”.9 In this study, we assigned these countries or areas to a subregion based on geographic location and economic status. (A full list of the country assignments is available from the authors on request.) Additional blindness prevalence data were used to estimate cases of blindness within the total cases of VI in the groups aged 40–49 and ≥ 50 years.1 These were subtracted from the total cases of VI in each country, and the remaining cases were assumed to have moderate/severe VI. For regions where blindness prevalence was reported separately for urban and rural areas, the population living in urban or rural areas in each country was used to calculate the estimated cases separately.11 As no data were available on the prevalence of URE in children under 5 years of age, this group was excluded from the calculations.

Within each country, we assumed that every person aged 16 years or over with normal VA would produce the PPP-adjusted GDP per capita,11 and that the product of this and a disability weight,9 for either blindness or moderate or severe VI, would equal the individual productivity loss for cases 16 years of age or over. The most recent available regional disability weights for “vision disorders, age associated and other” were 0.600 for blindness across all WHO regions, and they ranged from 0.244 to 0.282 by region for moderate/severe VI (previously termed “low vision” in the Global burden of disease in 2002 report).9 We assumed no personal economic productivity for cases 15 years of age or under. Additionally, we assumed that each person with VI would require some care from an adult with normal sight, who would thus lose productive time to that individual. We assumed a 10% productivity loss for the care of each person with blindness,12 and a 5% productivity loss for the care of each person with moderate/severe VI. This produced an estimate of the “unadjusted GDP loss” within each country.

The unadjusted GDP loss was then adjusted by the labour force participation rate (LFPR) and the employment rate (ER) for each country.13 Data were obtained from the most recent statistics in The world factbook.11 Countries for which the LFPR was not available were assigned a population-weighted subregional average (total labour force divided by the total population aged > 15 years). Countries for which the unemployment rate was not available were assigned a population-weighted subregional average [1 – (sum of unemployed persons divided by the total regional labour force)]. A second, more conservative estimate, for the ER and LFPR-adjusted GDP loss was derived by assuming that individuals aged ≥ 50 years contribute no productivity to the economy, and by applying the disability weights to 16–49-year-olds only.

Results

Table 2 summarizes, by WHO subregion, the total number of cases of VI and the subset of cases of blindness, and presents the disability and care-associated productivity loss estimates, before and after adjustment for LFPR and ER. There were an estimated 158.1 million people with VI from URE in the 2007 population; of these, 8.7 million were blind. The Western Pacific’s B subregion had the greatest case load (62.0 million cases), while South-East Asia’s D subregion had 48.7 million cases. The total global productivity loss, in international dollars (I$), associated with this burden was I$ 427.7 billion (one thousand million) before, and I$ 268.8 billion after, adjustment by LFPR and ER. Almost half of the potential productivity loss was in the Western Pacific Region (I$ 111.2 billion). By comparison, the WHO African and the Eastern Mediterranean regions had fewer cases (6.7 and 3.4 million, respectively) and a resulting estimated productivity loss of only I$ 3.5 billion each. With a more conservative approach, in which individuals aged ≥ 50 years of age were assumed to have no economic productivity (i.e. disability weight × GDP per capita = 0), the total productivity loss was I$ 191.1 billion before and I$ 121.4 billion after adjustment. Table 2 also shows the ratio of the adjusted to unadjusted GDP loss, by WHO subregion. This reveals lower ratios in the Eastern Mediterranean’s D subregion (0.39) and the African Region’s E subregion (0.45), as a result of higher unemployment and a lower LFPR.

Table 3 shows the number of cases of VI from URE in each WHO subregion as a percentage of the total subregional population. The highest prevalence was in the Western Pacific’s B subregion (3.98%) and South-East Asia’s D subregion (3.23%), while the lowest was in the African Region (0.82–0.86%). The table also displays the regional adjusted GDP loss associated with VI from URE, under the assumption of economic productivity for those aged > 15 years as a percentage of the total GDP. This ranged from 0.86% of total PPP-adjusted GDP in the Western Pacific’s B subregion to 0.14% in the Eastern Mediterranean’s D subregion.

Discussion

By applying previously presented prevalence data to the 2007 world population by country, we estimated that VI resulting from URE, including blindness, affected 0.8 to 4.0% of the world’s population in 2007, with an estimated cost to the global economy of I$ 268.8 billion after PPP adjustment.

The true cost of providing eyeglasses to meet this global burden is unknown. Eyeglasses are a low-cost intervention and, if we assume they are replaced at the rate of one pair every 3 years, an additional 53 million eyeglasses would be needed annually to address the current burden of URE – a figure comparable to previous estimates.14 The direct cost of a vision test and pair of eyeglasses in the United States was estimated to be 139 United States dollars (US$) in 2000.15 We have no reason to suppose that direct costs would be as high elsewhere, but when this rate is inflated to 2007 prices for reference, we calculate a global direct cost of approximately US$ 26 billion.16 This is an order of magnitude less than the estimated cost to the global economy associated with URE. However, many less economically developed countries still lack basic infrastructure for distribution and training and have insufficient equipment and personnel to provide eyeglasses to those in need. Cost estimates that focus only on the direct cost of providing services conditional on existing infrastructure are thus likely to be a substantial underestimate of the true cost.

WHO’s Western Pacific and South-East Asia regions have the greatest number of cases, both in absolute terms and as a percentage of the total population and the population in each age group. The African and Eastern Mediterranean regions have the least number of cases. One explanation is that the incidence of refractive error is lower for genetic and environmental reasons; heritability is an established risk factor for refractive error, and cross-sectional data reveal an association with higher educational attainment, near work and less outdoor activity.1720 Another explanation is that a higher case load occurs in regions with ageing populations and longer life expectancies because the prevalence of URE is greatest in those aged ≥ 50 years.

The distribution of cases by WHO region correlates with the distribution of cost in terms of lost productivity. The Western Pacific Region has the highest cost by a considerable margin, at I$ 111 billion dollars, equivalent to 0.86% of the regional GDP. In contrast, both before and after adjustment for LFPR and employment rate, the productivity cost in the African and Eastern Mediterranean regions is very small compared to that of other regions, at I$ 3.5 billion in both, equivalent to 0.18% and 0.15% of the GDP, respectively. This reflects both the lower case load in the African and Eastern Mediterranean regions, and the generally lower income per capita in these regions.

Some of the assumptions and the disability adjustments we used seriously limit the estimates presented. The most recent disability weights were derived in 1990, and refractive error was not included as an independent disease state.9 Thus, we had to use generic disability weights for “vision disorders, age related and other”.9 In addition, we applied the disability weights to two estimates of the GDP per capita, assuming that all individuals older than 16 years were either economically productive until death or until 49 years only, as there was a paucity of regional information on retirement ages. We also assumed, in the absence of published data, that all individuals with VI required some care from a person with normal vision, leading to a 5–10% loss of their productivity. Furthermore, the adjustment of GDP productivity by LFPR and ER assumed no direct cost to individuals over 15 years who were not in the formal labour force or who were unemployed. These individuals, especially in more agrarian societies, make important contributions to the production of goods and services in the household or in informal markets, which are not captured in monetized terms by the PPP-adjusted GDP. Therefore, the “true” cost of URE may be underestimated for countries with a higher unemployment rate or a lower LFPR. The regional population-weighted LFPR ranged from 0.764 in the Western Pacific B subregion to 0.446 in the Eastern Mediterranean D subregion, and the population-weighted regional ER ranged from 0.944 in the Western Pacific B subregion to 0.666 in the Eastern Mediterranean D subregion, further polarizing the estimated productivity loss difference between these regions. The accuracy of the adjustment was slightly limited by a lack of up-to-date information on ER and LFPR for some countries, mainly in Africa, although regional population-weighted averages were used to provide a reasonable approximation.

In recognition that our approach did not accurately value the cost to all people equally, we determined an upper estimate, of I$ 427.7 billion for the GDP productivity loss without adjustment for LFPR and ER, and low estimates of I$ 191.1 billion and I$ 121.4 billion (without and with adjustment for LFPR and ER, respectively), by assuming that individuals aged ≥ 50 years contribute no productivity to the economy. The range in cost estimates under these varying assumptions is likely to have captured the “true” productivity loss. The lowest adjusted estimate, which amounts to I$ 121.4 billion, is approximately one thousand times greater than the number of cases of VI resulting from URE in the world. This suggests that even under the most conservative assumptions, the global provision of eyeglasses would result in considerable savings per case treated and in a net benefit to the global economy.

The approach of using GDP per capita as a proxy for income to estimate the cost of a disease burden has several limitations. In focusing on the cost of URE to the national economy, it ignores costs to the individual in terms of functionality, quality of life and opportunities lost, and it assigns greater value to providing eyeglasses to individuals in higher income countries. Furthermore, it does not incorporate income inequality within a country. The aggregated GDP may thus mask inequity in access to health and vision services, and the reality that many consumer goods, including eyeglasses, may not be affordable to poorer sectors of any society. In support of this, blindness has been found to disproportionately affect individuals of lower income and educational status, and women.21 However, the GDP income approach remains a valuable tool for contextualizing the magnitude of a disease burden, its potential impact on economic growth relative to other diseases and the potential value of intervention.

Conclusion

URE is a preventable cause of VI and a priority in the WHO Vision 2020 initiative to eliminate avoidable blindness.22 Based on existing disability weights, the productivity loss estimates presented here suggest that the current burden of URE has a potentially greater impact on the global economy than all other preventable vision disorders.13 However, the VI associated with URE is distance-dependent, rather than absolute, so the association between URE and disability may be more complex than for other causes of VI. Updated disability weights specific to URE in different WHO regions and subregions would provide valuable assistance in determining more accurate estimates of lost productivity. Furthermore, estimates of the global direct and indirect costs of scaling up existing refractive services to address URE are urgently needed. Although policy-makers will need to consider the potential economic productivity loss associated with URE within the broader framework of individual and societal costs, the estimates calculated in our study highlight the potential economic significance of this global burden. ■


Funding: Tasanee Smith was supported by an award from the US-UK Fulbright Commission; Brien Holden and Tim Fricke were both supported by a Blindness Prevention Grant from the Institute for Eye Research, Australia.

Competing interests: None declared.

References

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