THE NATIONAL GONORRHEA RATE declined by 75% from 1975 to 2003 after implementation of the federally funded gonorrhea control program in the mid-1970s.1 A wide range of sexually transmitted disease (STD) prevention activities is supported by federal funds, including disease surveillance and data management, clinical services for the diagnosis and treatment of STDs, provision of condoms, primary prevention counseling, partner services, and the implementation of community and individual behavior change interventions.2 Compelling evidence suggests that STD prevention programs reduce STD incidence, and the decline in gonorrhea incidence since the 1970s is likely attributable in large part to STD prevention efforts.1,3-7 In fact, a recent study found that greater amounts of federal STD and human immunodeficiency virus (HIV) prevention funding in a given year were associated with decreases in gonorrhea rates in subsequent years.3 The purpose of the current study was to apply the previously published estimates3 of the impact of prevention funding in order to (1) estimate national gonorrhea rates that might have occurred from 1971 to 2003 had there been no federally funded STD prevention activities and (2) calculate crude estimates of the cost-effectiveness of these STD prevention activities.
Methods
Hypothetical gonorrhea rates in the absence of federally funded STD prevention efforts were estimated using 3 main steps. First, for each year from 1971 to 2003, the annual percentage change (Δt, where t denotes year) in the observed national gonorrhea rate was calculated as Δt = (Rt - Rt-1)/Rt-1, where Rt is the national reported gonorrhea rate in year t. Second, the annual percentage change (Δ̄t) in the gonorrhea rate that would have occurred in each year had there been no federal STD prevention efforts was estimated as Δ̄t = Δt + ΩFt, where Ft is the average amount of funding (in 2003 US dollars) per capita dispersed nationwide by the Centers for Disease Control and Prevention (CDC) in years t-3, t-2, and t-1, and Ω is the estimated percentage decrease in gonorrhea rates associated with 1 dollar of prevention funding (Ft). (For example, the average funding per capita from 1984 to 1986 was about $0.30 [F1987 = 0.30]. For the base-case value of Ω [13.3%], it was estimated that the percentage change in gonorrhea rates in 1987 would have been about 4% points greater [ΩF1987 = 13.3% × 0.30 = 3.99%] had there been no prevention compared to the observed percentage change.) Third, hypothetical gonorrhea rates in the absence of prevention (Δ̄t) were estimated based on the estimated percentage change in rates in the absence of prevention efforts (Δ̄t). For example,¯R1971 equals; R1970 × (1 + Δ̄1971); ¯R1972 = ¯R1971 × (1 + Δ̄1972); ¯R1973 = ¯R1972 × (1 + Δ̄1973), and so on.
To examine the cost-effectiveness of the STD prevention efforts, the number and cost of gonorrhea cases averted were estimated by comparing the number of hypothetical cases of gonorrhea in the absence of prevention (N̄) to the actual number gonorrhea cases (N).8 The actual number of gonorrhea cases was estimated by multiplying the reported number of gonorrhea cases by an adjustment factor to account for underreporting. The hypothetical number of cases of gonorrhea in the absence of prevention (N̄t) was calculated for each year based on the estimated rates (Δ̄t) described above, adjusted for underreporting.
The cost-effectiveness ratio (CER) was calculated as CER = (P - A)/C, where P = prevention expenditures, A = averted medical costs, and C = number of cases averted.9 The analysis was conducted using the societal perspective, meaning that all costs were included regardless of who pays the costs.10,11 Prevention expenditures and averted medical costs were discounted forward to the base year of 2003 using a 3% annual discount rate. For example, expenditures and averted costs in 2002 were multiplied by 1.03; expenditures and averted costs in 2001 were multiplied by the squared value of 1.03; expenditures and averted costs in 2000 were multiplied by the cubed value of 1.03, and so on.
Only 6 main inputs were needed for the calculations (Table 1). The reported gonorrhea case numbers and rates were obtained from CDC's annual surveillance report.1 The estimated impact (Ω) per dollar of prevention funding on gonorrhea incidence rates was based on a recent study of the effect of HIV and STD prevention funding allocated by CDC to state and local health departments on gonorrhea incidence rates in the United States from 1981 to 1998.3 Federal prevention expenditures were obtained from CDC records as described in more detail elsewhere.3 Federal STD prevention expenditures were limited to general STD prevention funding awards and direct assistance from CDC to state and local health departments. Funding allocated by the CDC or other federal agencies specifically for HIV prevention, syphilis elimination activities, infertility prevention, and other special projects were not included. In the base-case analysis, $170 was applied as the direct medical cost per case of gonorrhea (an average of $57 and $284 cost per case in men and women, respectively), which includes the average costs of diagnosis and treatment of acute infections and sequelae associated with untreated or inadequately treated infections.12
Sensitivity analyses were conducted to see how the results changed when the input values were varied. In the 1-way sensitivity analyses, each input was varied one at a time while holding other inputs at their base-case values. In the 2-way sensitivity analyses, 2 inputs (the estimated impact [Ω] per dollar of prevention funding and 1 additional input) were varied simultaneously while holding all other inputs at their base-case values. In addition, a Monte Carlo simulation was conducted to examine how the results changed when 4 inputs (adjusting for underreporting, the discount rate, the estimated impact [Ω] per dollar of prevention funding, and the cost per case of gonorrhea) were varied simultaneously.13 Specifically, the CER was calculated after assigning to each input a random value between its lower and upper bound values (assuming a uniform distribution). This procedure was performed 10,000 times to obtain a distribution of the estimated cost-per-case of gonorrhea prevented.
Results
Hypothetical gonorrhea rates without federally funded prevention efforts varied, depending on assumptions about the estimated impact per dollar of prevention funding (Fig. 1). Without prevention efforts from 1971 to 2003, the base-case analysis indicated that hypothetical reported gonorrhea rates in 2003 would have been 476 per 100,000 people, about 4 times higher than the reported rate 116 per 100,000 people that was actually observed in 2003. If the estimates in Figure 1 were adjusted for underreporting, the absolute difference between the hypothetical no federal prevention gonorrhea rates and actual rates would be increased, but the relative difference would be unchanged.
Results under base-case assumptions indicated that prevention efforts averted an estimated 32 million cases of gonorrhea from 1971 to 2003 (Table 2), after adjusting for underreporting. This estimated reduction in gonorrhea incidence saved $8.1 billion in direct medical costs for the treatment of gonorrhea and its sequelae. In the base-case analysis, these federally funded prevention efforts were cost saving, meaning that the estimated averted costs of gonorrhea ($8.1 billion) exceeded the cost of the prevention efforts ($4.3 billion).
In the sensitivity analyses, the CER (cost per case of gonorrhea averted) ranged from <$0 (cost saving) to $405 (Table 3). When varying only the estimated impact (Ω) of prevention expenditures, the CER was <$0 for the base case and upper-bound values of Ω and $185 for the lower bound value of Ω. When varying only the discount rate, the cost of gonorrhea, or the adjustment for underreporting, the CER ranged from <$0 to $9. When varying the impact (Ω) of prevention expenditures and 1 other variable simultaneously, the cost-effectiveness ratio was <$0 when the upper-bound value of Ω was applied and ranged from $57 to $405 when the lower-bound value of Ω was applied. When varying all inputs simultaneously (Monte Carlo simulation), the CER was <$0 in 73% of the estimations, <$50 in 83% of the estimations, and <$174 in 95% of the estimations, with a maximum value of $633 per case averted.
Discussion
Gonorrhea rates would likely be considerably higher than observed today had there been no federally funded STD prevention efforts from 1971 to 2003, under the assumptions about the estimated impact (Ω) of prevention expenditures considered in this analysis. The base case results suggest that gonorrhea rates in 2003 in the absence of prevention would be quite similar to the peak reported rates actually observed in 1975. Under several plausible scenarios, the federally funded prevention efforts were found to be cost saving, meaning that the averted costs of gonorrhea were greater than the federal expenditures used to fund the prevention efforts.
Although there is no way to know what gonorrhea rates might truly have been had there been no federally funded prevention efforts, the feasibility of the model results can be examined. For example, the base-case results suggested that gonorrhea rates would have increased by about 65% from 1971 to 1975 had there been no federally funded STD prevention. This increase is similar to the 63% increase in reported gonorrhea rates from 1966 to 1970 before the implementation of the national gonorrhea control program in the early 1970s. In addition, the model predicted a decline in gonorrhea rates from the mid-1980s to the mid-1990s under all scenarios, although these declines were relatively more pronounced when the estimated impact of federally funded STD prevention efforts were included. Such declines might be expected as a result of HIV prevention efforts, behavioral changes in response to HIV/AIDS, and AIDS mortality among persons at increased risk for STDs.8,14-18
This analysis used a simple model to calculate what gonorrhea rates might have been over a range of assumptions about the impact of prevention funding and other factors. The model applied is not an STD transmission model. Models of transmission typically incorporate many epidemiologic and behavioral factors such as the probability of STD transmission per partnership, the number of sexual partnerships per year, and the duration of infectiousness.19 The use of transmission models to estimate the nationwide impact of federal STD prevention efforts over time would require a substantial amount of information, such as estimates of the decrease in duration of infectiousness as a result of screening and treatment and partner services, the increase in condom usage or decrease in the number of sex partners as a result of education and counseling, and so on. Because the approach used here does not simulate the actual epidemiology of gonorrhea transmission, it requires much less information to generate estimates of the cost-effectiveness of federally funded prevention efforts at the macro level. The viability of this approach is perhaps its main advantage. Another advantage of this approach is that it is based on the observed relationship between prevention expenditures and gonorrhea rates in the United States over an extended period of time. The disadvantages of this approach, and the efforts made to address these limitations, are discussed in more detail below.
In order to estimate hypothetical gonorrhea rates in the absence of federally funded prevention efforts, this analysis used the estimated impact of federal STD and HIV prevention expenditures on gonorrhea rates in the United States obtained from a previously published study.3 Limitations of the previous study (and other studies on which the values of the other inputs were based) therefore are limitations in the current analysis. For example, the original study of the impact of prevention funding included only CDC-allocated funds for HIV and STD prevention; non-CDC federal funds, state and local resources, and private donations were not included.3 For this and other reasons, the estimated impact per dollar of prevention funding that was applied in this study is subject to considerable uncertainty.
To address the uncertainty associated with the inputs, particularly the estimated impact (Ω) of prevention expenditures on gonorrhea rates, wide ranges of values for the inputs were applied in the sensitivity analyses. It is possible, however, that the true value of the estimated impact (Ω) of prevention expenditures on gonorrhea rates could be below the lower bound value or above the upper bound value applied in the sensitivity analyses, which would result in an overestimation or underestimation, respectively, of the impact and cost-effectiveness of prevention. For example, if federally funded prevention activities in fact have no effect on gonorrhea rates, the true value of Ω would be zero, no cases of gonorrhea would have been prevented, and the cost per case averted would be infinite.
Although the original study3 that estimated the impact (Ω) of prevention funding included federal HIV prevention funds, federal HIV prevention funds were not included in this analysis. The estimated impact of prevention efforts on gonorrhea rates would have been greater if federally funded HIV prevention activities were included. This study focused on the impact and cost-effectiveness of prevention activities supported by general STD prevention funding awards and direct assistance from CDC to state and local health departments. If funding for targeted STD prevention efforts (such as infertility prevention and syphilis elimination) had been included, the estimated impact of prevention on gonorrhea rates would have been even greater. Similarly, the estimated impact of prevention on gonorrhea rates in the United States would have been even greater if state and locally funded STD prevention activities were included.
In the cost-effectiveness analysis, gonorrhea cases were adjusted for underreporting. This adjustment for underreporting may have overstated the cost-effectiveness of federal prevention efforts if these prevention efforts have less impact on gonorrhea in populations less likely to be reported (such as patients with private insurance) than on gonorrhea in populations more likely to be reported (such as patients at public STD clinics).1 Furthermore, it is possible that the degree of underreporting may have changed over time. To help mitigate potential biases associated with underreporting, a more conservative assumption was applied in base-case analysis that the proportion of gonorrhea cases not reported is one-third rather than one-half, as suggested in the literature.20,21 Although assumptions about the degree of underreporting did influence the estimated impact and cost-effectiveness of prevention efforts, prevention efforts were estimated to be cost saving even when no underreporting was assumed.
Despite its limitations, this analysis provides useful, empirically based estimates of the effect and efficiency of federally funded STD prevention efforts in the United States. These prevention efforts appeared to be cost saving when considering only the averted costs associated with gonorrhea. STD prevention activities (such as screening and treatment, primary prevention counseling, and behavior change interventions) affect other STDs, such as syphilis and chlamydia. If the benefits of preventing syphilis, chlamydia, and other STDs were considered, the estimated effectiveness and cost-effectiveness of federally funded STD prevention efforts in the United States would be even greater.
ACKNOWLEDGMENT
The author thanks Kathleen Irwin for helpful comments.
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