Staras, Stephanie A.S. PhD*†‡; Flanders, W Dana MD, ScD†; Dollard, Sheila C. PhD‡; Pass, Robert F. MD§; McGowan, John E. Jr MD†; Cannon, Michael J. PhD†‡
WORLDWIDE APPROXIMATELY 0.7% OF INFANTS are born with congenital cytomegalovirus (CMV) infections, potentially leading to permanent neurologic and sensory impairment.1,2 During the twentieth century, congenital CMV incidence rates remained high while prevention efforts reduced incidence rates of other congenital infections.3 Although CMV is a leading cause of congenital infection, development of vaccines, educational prevention campaigns, and treatment options for CMV-infected infants continue to progress slowly.4–6
Reduction of congenital CMV infections requires reduction in the number of pregnant women acquiring primary CMV infections. Experts agree that educational efforts such as highlighting the importance of washing hands after contact with young children should reduce CMV infection because CMV is transmitted through contact with infected body fluids (e.g., saliva, urine).7–9 Approximately 40% of US children acquire CMV before the age of 11 years from nonsexual sources including their mother (transmitted in utero,10 during birth,11 or during breast-feeding12) and close contact with others, especially children.13–16 For adolescents and adults, in addition to transmission from close-nonsexual contact, CMV can be transmitted during activities associated with sex including kissing.15,17,18 It is unknown whether a substantial proportion of individuals acquire CMV infections during sexual activities, thus it is unclear if prevention messages should include sexual activities.
We previously reported demographic differences in CMV seroprevalence and the force of infection in the United States using data from the Third National Health and Nutritional Examination Survey (NHANES III).16,20 CMV seroprevalence increased by age (36% among 6- to 10-year olds to 65% among 40- to 49-year olds), was higher in women than men and it was lower among non-Hispanic whites than non-Hispanic blacks and Mexican Americans.16 This article continues this work by assessing the association between CMV seroprevalence and sexual activity markers among 15- to 44-year-old NHANES III participants.
Design and Sample
Methodology and response rates have been published for NHANES III, a cross-sectional survey of the US population.21,22 Among individuals selected to participate in NHANES III, 73% agreed to the examination. We tested sera from NHANES III participants for CMV-specific-IgG, using an enzyme linked immunosorbent assay (ELISA) procedure, at the Centers for Disease Control and Prevention, Atlanta, GA.16 We used the Triturus robot (Grifols USA, Inc., Miami, FL) with SeraQuest ELISA reagents (Quest International, Inc., Miami, FL) and validated the performance of the SeraQuest test with VIDAS ELISA (bioMérieux, Durham, NC).
To assess the relationship between these serologic results and sexual activity markers, we included 15- to 44-year-old NHANES III participants who had serum available for CMV testing (95% of examined participants). Data were also available for NHANES III participants 6 to 14 years old (CMV seroprevalence) and 12 to 14 years old [herpes simplex virus type 2 (HSV-2) seroprevalence] (http://www.cdc.gov/nchs/about/major/nhanes/nh3data.htm). The study protocol was approved by ethics review boards at the Centers for Disease Control and Prevention, Emory University, and University of Florida.
Sexual Activity Markers
We obtained information on participant's sexual activities from the questionnaire and laboratory examinations. We defined categories of sexual activity markers differently by age and gender because (a) the NHANES III questionnaire content differed by age and gender and (b) strong associations existed between sexual activity and age and gender in our preliminary analysis and in previous studies.23,24
Participants ≥15 years old were asked about their sexual activities. For participants 15 to 44 years old, the NHANES III dataset contained serologic results for a sexually transmitted infection (STI) (HSV-2 glycoprotein gG-2 antibody)25 and 3 self-reported sexual activity markers (ever had sexual intercourse, age at first sexual intercourse, and potential years of sexual activity). We restricted analysis of ever had sexual intercourse to 15- to 19-year olds because few older participants reported never having sexual intercourse. We categorized age at first intercourse, collected as a continuous marker, into approximate quartiles. We calculated potential years of sexual activity by subtracting age at first intercourse (years) from age at interview (years).
Additional markers (number of lifetime and past-year sexual partners) were available for individuals 17 to 44 years old; for these continuously collected markers we created quartile categories. Also women reported whether they ever used oral contraceptives.
We created a composite sexual activity marker using the sexual activity markers described above except HSV-2. We restricted analysis of the composite marker to 17- to 44-year olds because several sexual activity markers were not assessed among 15- to 16-year olds. For each sexual activity marker, we assigned a value of 1 to the lowest category of sexual activity and a value of 0 to the other categories. We summed these values across the sexual activity markers, yielding a score of 0 to 4 for men and 0 to 5 for women, and grouped the composite marker by number of variables in the lowest risk category: high risk = 0, intermediate risk = 1, and low risk >1 variable in the lowest category of sexual activity. We calculated the composite variable in 2 additional ways for sensitivity analyses: without the use of oral contraceptives for women (i.e., identical to men) and with HSV-2 for both genders.
Based on a priori information, we identified demographic variables (race/ethnicity, gender, age, family size, census region, country of origin, and education level) and children in the home as potential confounders.15,16 We categorized demographic variables similar to our previous analysis,16 with the following modifications: we assessed age in 1-year categories and we combined 2 categories of family size (1 person and 2–4 people).
The presence of children in participant's homes was not directly provided in the NHANES III dataset. For 15- to 44-year-old participants (n = 9705), we assessed (72%) and inferred (9%) whether children were present. In some families, all members participated in NHANES III; among these 4180 participants we observed family members' ages. An additional 2784 participants responded to a version of the questionnaire where the presence of children younger than 17 years in the home was clear. For the remaining 2741 participants, we inferred that those reporting a family size ≥6 had children (n = 832), those reporting a family size of 2 and a spouse did not have children (n = 87), and made no inferences for the other 1822 participants. Characteristics were similar for participants for whom child status was and was not inferred. Because we could not control for the presence of children in the entire NHANES III population, we restricted analyses to the 81% of participants for whom child status was recorded or inferred (n = 7883).
We used SUDAAN software version 9.0 (RTI International)27 to account for the sample design and incorporate sampling weights, allowing estimates to represent the US population.22,28 Early in the analysis, we detected effect measure modification of sexual activity and CMV seroprevalence between age, race/ethnicity, and gender strata. Thus, we stratified analyses by 10-year-age groups, race/ethnicity, and gender.
For each marker of sexual activity, we used both age-adjusted (1-year-age categories) and multivariate (a priori selected covariates) logistic regression to estimate the association between sexual activity and CMV seroprevalence. In multivariate logistic regression, we combined categories of covariates to avoid small cells in 7 of the 18 age-race/ethnicity-gender strata.
Using predicted marginals,27,29 we calculated prevalence estimates, standardized prevalence differences (PDs) (the standardized prevalence among higher sexual risk groups minus the prevalence for low sexual risk30), and 95% confidence intervals. We chose standardized PDs instead of prevalence ratios (PR) to facilitate comparisons between strata with diverse seroprevalence estimates. For example, comparing seroprevalence estimates between high and low sexual risk groups (e.g., 80% and 60% among non-Hispanic blacks and 60% and 40% among non-Hispanic whites) represents a 20% PD. Using PR, however, would inadvertently show a greater influence of sexual activity in non-Hispanic whites (PR = 1.5) than non-Hispanic blacks (PR = 1.3).
Additionally, we analyzed the association between HSV-2 seroprevalence and sexual activity (ever had sex and composite sexual activity markers). We used this analysis to compare the association between sexual activity and CMV seroprevalence with the association between sexual activity and a virus predominately transmitted sexually.
Among 16-year olds, approximately 50% reported ever having sex increasing to almost 100% of 24-year olds. Among 17- to 44-year olds, the median reported number of lifetime sex partners was 7 among men and 3 among women. In age-adjusted and multivariate analyses, we found similar results for the association between CMV seroprevalence and sexual activity markers, thus only multivariate results are shown. Similar results were also found in the subset of participants who did not have children in the home.
Individual Sexual Activity Markers
The association between CMV seroprevalence and reporting ever having sex differed by gender and race/ethnicity (Table 1). We found a significant PD comparing women who reported “ever had sex” to women who reported “never had sex” among non-Hispanic blacks. CMV seroprevalence was higher among non-Hispanic white and Mexican American women who reported having sex compared with those who did not report having sex.
Among 15- to 44-year olds, the association between CMV seroprevalence and the sexual activity markers differed by age-race/ethnicity-gender (Fig. 1). Among non-Hispanic black women, all PDs were positive (Fig. 1A). Among non-Hispanic white women, we found positive PDs for most of the markers studied. Among Mexican American women, we found fewer positive PDs and many estimates close to 0. We compared the number of positive PD estimates for the 3 age groups and 6 sexual activity markers (a total of 18 PD estimates for each race/ethnicity category) and found a higher number of positive PD estimates among non-Hispanic black women compared with women of other race/ethnicities (18 of 18 vs. 28 of 36, P = 0.017).
Among men, nearly all PDs were positive among non-Hispanic blacks, but many confidence intervals included 0 (Fig. 1B). We found little or no association between CMV seroprevalence and sexual activity among non-Hispanic white or Mexican American men. We compared the number of positive PD estimates for the 3 age groups and 5 sexual activity markers (a total of 15 PD estimates for each race/ethnicity category) and found a higher number of positive PD estimates among non-Hispanic black men compared with men of the other race/ethnicities (14 of 15 vs. 19 of 30, P = 0.031).
Composite Sexual Activity Marker
Among 17- to 44-year olds, each category of the composite sexual risk marker separated the individual sexual activity markers well. For example, the median reported number of lifetime sex partners increased across categories of composite sexual risk (low, intermediate, high): 1, 3, 5 among women and 2, 6, 14 among men. Similarly, HSV-2 seroprevalence increased by category of composite sexual risk: among women (17.6%, 25.9%, 37.8%) and among men (8.6%, 16.0, 21.9%).
We found similar results for the association between CMV seroprevalence and the composite sexual activity marker regardless of the inclusion of HSV-2 or use of oral contraceptives (women only). Thus results are shown for our primary analysis: composite marker calculated without HSV-2 and with the use of oral contraceptives among women. Within each age stratum of non-Hispanic black women, CMV seroprevalence increased by category of sexual risk (Table 2). PD estimates, the weighted average of high and intermediate sexual risk minus low sexual risk, were similar across age categories. The association was statistically significant when all ages were included.
Within each age stratum of non-Hispanic white women, we found the lowest CMV seroprevalence estimate in the low sexual risk category and the highest CMV seroprevalence estimate in the high sexual risk category (Table 2). The magnitude of the PD varied by age, but the association between sexual activity and CMV seroprevalence was consistently positive (Table 2). The association was statistically significant among 25- to 34-year olds and when all ages were included.
Among Mexican American women, we observed little difference in CMV seroprevalence with increasing category of sexual risk (Table 2). We found a negative seroprevalence difference in the 17- to 24-year stratum, but we observed increasingly positive seroprevalence differences with increasing age (Table 2). Among ages 17 to 44 years, we found a small, borderline significant PD. Among men, we found little or no association between CMV seroprevalence and the composite sexual activity marker (Table 2).
When compared with a predominately sexually transmitted virus among women, prevalence estimates for CMV were generally 10% to 60% higher than prevalence estimates for HSV-2 (Fig. 2). HSV-2 seroprevalence among 15- to 16-year-old women who reported they never had sex was ≤5% and 4% to 16% among those who reported having sex (Fig. 2).
Even after controlling for several potential confounders, we found sexual activity was associated with CMV seroprevalence in the general population of the United States. This suggests that sexual activities were an important source of CMV infections for women of childbearing age and should be a focus of prevention efforts. Because women acquiring primary CMV infections from sexual activities are also at risk of becoming pregnant, CMV infections acquired during sexual activity may lead to congenital infections.15
Our finding that CMV seroprevalence and sexual activities were most clearly associated among non-Hispanic blacks was consistent with higher rates of several STIs (e.g., HSV-2, chlamydia, gonorrhea, and human immunodeficiency virus) among non-Hispanic blacks than among non-Hispanic whites with similar sexual profiles.23,25,31 Reasons for the clearer association between CMV and sexual activity among non-Hispanic blacks could not be investigated in this study. For some STIs (e.g., chlamydia and gonorrhea), data support several potential explanations, such as racial/ethnic differences in sexual partner selection (including partner sexual risk and race/ethnicity) and differences in vaginal flora.32–35 These factors may also influence CMV transmission. From the limited information available in NHANES III, household income group did not appear to be an important confounder (data not shown).
Consistent with findings for other STIs,25,36,37 we found stronger associations between CMV seroprevalence and sexual activity among women than among men. Possible explanations for the disproportionate burden of STIs among women include biologic differences and partner selection. For example, some STIs are more efficiently transmitted from male-to-female than from female-to-male, perhaps from higher viral concentrations in semen than in cervical secretions.36,38 Additionally, women may be more likely than men to have a sexual partner with a STI.37
Our estimation of the association between CMV seroprevalence and sexual activity was likely an underestimate for several reasons. First, there is risk of sexual transmission of organisms within the groups we defined as “low sexual risk,” illustrated by HSV-2 seroprevalence estimates above zero among the low sexual risk groups (Fig. 2). Second, there was likely risk of sexual transmission among individuals <17 years old (whose risk could not be assessed with the composite marker), illustrated by HSV-2 seroprevalence estimates above zero among 15- to 16-year olds who reported having sex (Fig. 2). Third, many sexual activity markers were self-reported and probably influenced by recall, societal pressures, and interpretation of the questions. This likely nondifferential misclassification of sexual risk would bias our estimates toward the null. Fourth, sexual activities with no or reduced risk of CMV transmission (uninfected partner or condom use39–41) could still place a participant in a category of higher sexual risk because data were not available on condom use or partner's CMV excretion.
This study was not designed to assess nonsexual modes of CMV transmission. The cross-sectional design prohibited assessment of whether individuals were susceptible to CMV during reported sexual activities. NHANES III did not collect information about the participants' childhood exposures to CMV risk factors. Evidence of CMV transmission via nonsexual modes10,11,13 and our reported CMV seroprevalence of 36% among 6- to 11-year olds16 suggest that many individuals acquire CMV through nonsexual modes during childhood and therefore are not susceptible to CMV during sexual activities. For a rough estimate of the percent of infections acquired from nonsexual activity, we assumed CMV seroprevalence among 15- to 16-year olds who reported never having sex was attributable to nonsexual activities. Assuming only small changes in CMV seroprevalence occurred over time (i.e., little cohort effect), nonsexual activities would appear to account for at least half of the CMV infections acquired from birth to ages 35 to 44 years old among women (Fig. 2). For example, among non-Hispanic black women, we assumed the 57% CMV seropositive 15- to 16-year olds who reported never having sex acquired CMV from nonsexual sources. Therefore, at least half (57% of 95%) of the CMV seropositive 35- to 44-year-old women acquired CMV from nonsexual sources. To control for close, nonsexual contact with children during adulthood we adjusted analyses for a rough proxy, the presence of children in the home.
This study had several important strengths. First, we demonstrated an association between CMV and sexual activity in 2 unstudied populations: the US general population as a whole and Mexican Americans in particular. In general, our findings complement similar findings regarding CMV seroprevalence among convenience samples of women attending contraceptive and STD clinics.17,18 Second, the over 300 participants per age-race/ethnicity-gender strata allowed evaluation of sexual activity within strata while controlling for several potential confounders, including children in the home. Third, we considered multiple sexual activity markers, including a laboratory marker (antibody to HSV-2). Finally, inclusion of individuals who reported never experiencing sexual intercourse allowed partial assessment of the role of nonsexual transmission during childhood.
The association between sexual activities and CMV seroprevalence in the US population is notable because it was detected with limited exposure measurements in the presence of competing exposures to nonsexual sources. When considered with the increased risk of delivering a newborn with congenital CMV infection among women with STIs during pregnancy,15 our results suggest congenital CMV infections could be prevented by reducing sexual acquisition of CMV among pregnant women, especially among non-Hispanic black women. Further study is needed to estimate the number of preventable congenital infections and expand upon limited studies aimed at preventing sexual transmission of CMV. In a study among human immunodeficiency virus-infected women, condom use reduced CMV acquisition.40 Thus, behavioral modifications (e.g., practicing monogamy, condom use with new partners, avoiding pregnancy in new sexual relationships) may reduce CMV infections among pregnant women. To eliminate congenital CMV infections, however, a vaccine may be required. Nevertheless, the large number of CMV-susceptible women,16 who have little understanding of CMV infection or its potential consequences,42 should be informed about CMV acquisition from sexual and nonsexual sources.
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