Sexually Transmitted Diseases:
Decreasing Age Disparities in Syphilis and Gonorrhea Incidence Rates in the United States, 1981–2005
Chesson, Harrell W. PhD; Zaidi, Akbar A. PhD; Aral, Sevgi O. PhD
From the Division of STD Prevention, National Center for HIV, STD, and TB Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia
The authors thank the Division of STD Prevention seminar participants for helpful comments and suggestions.
The findings and conclusions in this report are those of the authors and do not necessarily represent the views of the Centers for Disease Control and Prevention.
Correspondence: Harrell W. Chesson, PhD, Division of STD Prevention, National Center for HIV, STD, and TB Prevention, Centers for Disease Control and Prevention, Atlanta, GA. E-mail: firstname.lastname@example.org.
Received for publication July 9, 2007, and accepted October 17, 2007.
Background: Compared to older age groups, teenagers and young adults in the United States are at high risk of acquiring sexually transmitted diseases (STDs). Although the disparity in STD rates across age groups is well documented, changes in the degree of disparity in STD rates across age groups over time have not been examined in detail.
Methods: We examined age-, sex-, and race-specific incidence rates of syphilis and gonorrhea in the United States (excluding New York owing to incomplete age- and race-specific data) from 1981 to 2005. STD rates in younger age groups (ages 15–29 years) were compared to STD rates in older age groups (ages 40–54 years) for each year over the 25-year period. We used regression analyses to examine the trend in the age rate ratio (STD rate in the younger age group divided by STD rate in the older age group) over time, adjusting for autocorrelation.
Results: The age disparity in syphilis and gonorrhea declined from 1981 to 2005. The estimated annual decline in the age rate ratio was 5.3% for syphilis and 2.0% for gonorrhea for all races overall (P <0.01). Overall, the age disparity in STD rates was more pronounced for females than males.
Conclusions: Future research is needed to clarify the main determinants of the relative decline in STD rates in younger persons and to inform programmatic responses to the changing age disparity in STD rates.
BETWEEN 1981 AND 2005, THE REPORTED incidence rates of primary and secondary (P&S) syphilis and gonorrhea decreased by about 78% and 73%, respectively, in the United States.1 Over this time, however, changes in sexually transmitted disease (STD) incidence rates were more pronounced in certain subpopulations than others. For example, the racial disparity in syphilis incidence, as measured by the black-to-white incidence ratio, increased in the late 1980s and declined substantially beginning in the early 1990s.2,3 Conversely, the male-to-female syphilis incidence ratio decreased steadily in the mid to late 1980s but increased substantially from 1997 to 2003.4,5
The purpose of this article is to examine changes in the age disparity in syphilis and gonorrhea rates from 1981 to 2005. Compared with older age groups, teenagers and young adults in the United States are at high risk of acquiring STDs.1 Rates of reportable STDs such as primary and secondary (P&S) syphilis and gonorrhea are consistently higher in younger age groups (such as ages 15–29 years) than in older age groups (such as ages 40–54 years).1 Although the disparity in STD rates across age groups is well-documented,1,6–10 changes in the degree of disparity in STD rates across age groups over time have not been examined in detail.
We examined national-level, age-specific incidence rates of syphilis and gonorrhea in the United States from 1981 to 2005, obtained from state surveillance reports maintained by the Centers for Disease Control and Prevention.1 STD rates in younger age groups (ages 15–29 years) were compared to STD rates in older age groups (ages 40–54 years) for each year over the 25-year period. Specifically, we calculated the age rate ratio (RR) in each year as R15–29/R40–54, where R15–29 and R40–54 are STD rates (new cases per 100,000 population) in persons 15 to 29 years old and 40 to 54 years old, respectively. We examined the ratio of the rates (rather than the ratio of cases) to avoid undue influence of changes in the age distribution of the population over time. We examined gonorrhea and syphilis rates for the entire United States, excluding the state of New York. New York was excluded owing to incomplete STD data by age for 1983 and 1984.
We used autoregressive models of time series analyses to examine the trend in the age RR (STD rate in the younger age group divided by STD rate in the older age group) over time. Specifically, we estimated the following model: Log(RRt) = Constant + TRENDt + Et, where the subscript t denotes year, RRt is the age rate ratio in year t, TRENDt was set to 1 in 1981, 2 in 1982, and so on, and E is the error term, assumed to be an autoregressive process of order p. All significant autoregressive terms up to order 2 (p = 2) were included in the model. All analyses were performed using SAS 9.1 (SAS Institute, Cary, NC), and we used the SAS “autoreg” procedure to estimate the parameters.
We examined the age RRs (of syphilis and gonorrhea) for 9 population groups based on race and age: 3 race categories (total, white, black) × 3 sex categories (total, male, female). To address the impact of the exclusion of New York on our results, we repeated the analysis for overall gonorrhea and syphilis rates for years 1985 through 2005 using national STD rates (including New York).
For both age groups, syphilis and gonorrhea rates were lower in 2005 than in 1981 (Figs. 1, 2). The age disparity in syphilis and gonorrhea rates was also lower in 2005 than in 1981 (Figs. 1, 2, right axis). This decline in age disparity held for all population groups we examined for both syphilis (Figs. 3A–C) and gonorrhea (Figs. 4A–C). The disparity in syphilis and gonorrhea between the younger and older age groups was more pronounced for females than it was for males for all population groups we examined (Figs. 3A–C, 4A–C).
In the regression analyses, the coefficient of the TREND variable was negative for all 9 population groups examined for both syphilis and gonorrhea (Table 1), indicating a decrease in age disparity in syphilis and gonorrhea incidence over time. The estimated annual decline in the age RR was 5.3% for syphilis and 2.0% for gonorrhea for all races overall, and was most pronounced in white males (Table 1).
Our exclusion of New York did not impact our results. When we repeated the analysis from 1985 to 2005 including New York, we found similar, significant declines in the overall syphilis and gonorrhea age RRs (results not shown).
Our analysis yielded 2 main findings. First, for all populations examined, the age disparity in STD rates was more pronounced for females than for males. Second, for all populations examined, the age disparity in STD rates decreased from 1981 to 2005. These findings are consistent with recent reports of the changes in age groups at highest risk of syphilis. For example, the highest P&S syphilis rates in men in 1990 were observed in 20- to 24-year-olds, whereas the highest rates in men in 2003 were observed in 35- to 39-year-olds.5
It is likely that numerous factors contributed to the declining age disparity in STD rates. For example, potential contributing factors might include: Increased resources for STD prevention activities,11–14 which are typically targeted towards higher-risk groups (such as younger adults); behavioral responses to the acquired immunodeficiency syndrome epidemic,15–17 which may have been more pronounced in younger persons18; improved diagnostic tests7,19,20 and their use in nontraditional venues21; changing social and sexual norms, such as views on premarital sex and divorce22–25; the impact of sexual enhancement aids such as Viagra (sildenafil citrate; Pfizer, NY, NY) on sexual behavior and STD risk, particularly among older adults26–28; more restrictive alcohol policies, particularly towards youth alcohol consumption (such as higher alcohol taxes, increased minimum legal drinking ages, and zero-tolerance driving laws)29–31; incarceration and its impact on the sex ratio of nonincarcerated persons32–36; and changes in acquired immunity in the population.37
This list of possible factors that may have influenced the distribution of STDs across ages is speculative, as the assessment of the potential impact of these factors on STD age RRs is beyond the scope of this report. Furthermore, this list of possible factors is not exhaustive, as many other factors not addressed may have contributed to the decline in the age disparity in STD rates over time.
Limitations and Summary
Our analysis is subject to the usual limitations associated with historical STD surveillance data.1 For example, the degree to which STDs are underreported and the number of reported STD cases with missing, unknown, or invalid data for age may change over time.1 It is unlikely, however, that underreporting would influence our results unless there were substantial age-specific changes in the degree of underreporting or incomplete reporting during the years we examined.
In summary, the age disparity in syphilis and gonorrhea rates declined from 1981 to 2005, due to declining STD rates in younger persons as well as the lack of a comparable decline in STD rates in older persons. Future research is needed to clarify the main determinants of the relative decline in STD rates in younger persons and to inform programmatic responses to the changing age disparity in STD rates.
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