Among the 1110 women, serologic evidence of 793 STIs were found in 581 women. Two hundred fifty‐seven, 49, and 4 women were seropositive for a minimum of two, three, and four STIs, respectively, whereas 178.8, 14.1, and 0.25 women were expected to be infected by chance, respectively. Finally, we compared the estimated expected numbers of women with concomitant seropositivity with observed numbers. The proportions of excess concomitant seropositivity for STIs among women with concomitant seropositivity for two, three, and four STIs were 30% (257‐178.8/257) (Table 6), 71% (49‐14.1/49), and 94% (4‐0.25/4), respectively. For the four youngest age groups the corresponding figures were 32%, 63%, and 95%.
Our results indicate that a majority of pregnant Estonian women younger than 30 years have been exposed to one or more STIs. The relative proportions and age distributions of the seroprevalences were different between the three most common agents: HPV‐16, HSV‐2, and C trachomatis. The figures for HSV‐2 seroprevalence are comparable with those in Finland and Sweden, where the seroprevalences in the similar age groups ranged from 13.9% to 17.4% and 17% to 32% in the late 1980s and early 1990s.19,20 Seroprevalence of C trachomatis among a comparable Swedish population was 24.7%,21 which also comes close to our figures. Seroprevalence of HPV‐16 was 1.5‐ to 2‐fold higher among pregnant women in Tallinn, Estonia than in Stockholm (20%) or Helsinki (24%).22,23 Differences in time, study materials, and primiparous women versus all pregnant women may not totally explain the difference, and other factors such as rapidly changing behavioral and socioeconomic factors may be involved.9
We expected to identify subjects with coinfections of multiple STIs if seropositive persons with different STIs were markers of sexual behavior. Therefore, we assessed the observed numbers of women concomitantly seropositive for two, three, or four STIs and compared the observed numbers with expected numbers on basis of the prevalences of any STIs occurring alone and assuming total independence of spread. Given the seroprevalences, 193 subjects concomitantly seropositive for the different STIs were expected by chance whereas 310 were observed, yielding 38% additional STIs among those with concomitant seropositivity. This is a much smaller figure than 71% and 94% of additional that STIs we found among seropositive persons for three and four STIs, respectively. However, only 5%37 of all the women were seropositive to three or more STIs. In a population with a prevalence of 20% for C trachomatis and 36% for HPV‐16, the presence (i.e., prevalence of women with risk‐taking sexual behavior) is probably much higher than the 5%. When only the most prevalent STIs‐HPV‐16, HSV‐2, and C trachomatis‐were considered, 32% additional infections (250‐170,5/250) were observed. If seropositivity to T pallidum (2.4%) was accepted as a proxy of an STI core group (i.e., group with high risk‐taking sexual behavior24,25), the Tallinn core group had a relatively small effect on concomitant seropositivity to multiple STIs.
Various large‐scale studies have evaluated by means of relative risks or odds ratios the possibility of using STI antibodies as a surrogate of sexual behavior. Seropositivity for C trachomatis was independently associated with early sexual experience and number of lifetime partners.12,21 Also, seropositivity for HPV‐16 capsids, measured by odds ratios, was a strong marker of number of lifetime partners.1,13 Seropositivity for HSV‐2 has been launched as an objective marker of sexual behavior.14,26 We used chi‐square statistics as an indicator of the strength of the association between two STIs. P values were highly significant, indicating an association. By replacing C trachomatis with C pneumoniae or infections with low rates of sexual transmission,27 we observed no such associations. However, antibodies to a minimum of two STIs did not identify the same women. Therefore, measures of the strength of the association, relative risks or odds ratios28 and chi‐square statistics may not be considered as conclusive evidence that seropositivity can be regarded as a surrogate for risk‐taking behavior. In fact, traditionally recognized behavioral correlates of sexual behavior (e.g., number of life‐time partners) or serology have been reported not to identify all cases in communities with high prevalences of HSV‐2 or HPV infections.12,29 We arrived at the same conclusions. Most likely, the interplay between sexual behavior and coinfection of different STIs at the individual or population level is more complex (e.g., because of different transmission probabilities of the STIs or different levels of assortative spread30–32) than number of lifetime partners, which is well reflected by STI seropositivity. This cannot be studied in detail without a questionnaire about sexual behavior.
Transmission of an STI within a population is influenced by behavioral and sociocultural factors and changing environmental reservation.30–32 Sexual behavior is without doubt the most important single factor; however, the reverse seems not to be true. The observed relatively low proportion of additional infections among women seropositive for two STIs (30%) suggests that seropositivity to these STIs is not an accurate measure of sexual behavior. The use of three or more STI antibodies has a low sensitivity (i.e., underestimates the size of female population with risk‐taking behavior).
The current study was first attempt to estimate the seroprevalences of several STIs in Estonia at the population level. We found that seropositivity to multiple STIs was not common, and low rates of nonsexual transmission4,5,17,33 make alternative modes of transmission an unlikely explanation. Within the sexual transmission there are determinants increasing or decreasing the probability of infection. Furthermore, multiple exposures to closely related STIs might cause perturbation of the measurable specific immune responses. As a practical application of our findings, it is questionable whether any of the STI antibodies, which are commonly referred as surrogates of sexual behavior, can alone adequately reflect such behavior in women.
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