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Background Cluster-randomized trials (CRTs) involve randomizing groups of individuals (e.g. hospitals, schools or villages) to different interventions. Various approaches exist for analysing CRTs but there has been little discussion around the treatment effects (estimands) targeted by each. Methods We describe the different estimands that can be addressed through CRTs and demonstrate how choices between different analytic approaches can impact the interpretation of results by fundamentally changing the question being asked, or, equivalently, the target estimand. Results CRTs can address either the participant-average treatment effect (the average treatment effect across participants) or the cluster-average treatment effect (the average treatment effect across clusters). These two estimands can differ when participant outcomes or the treatment effect depends on the cluster size (referred to as ‘informative cluster size’), which can occur for reasons such as differences in staffing levels or types of participants between small and large clusters. Furthermore, common estimators, such as mixed-effects models or generalized estimating equations with an exchangeable working correlation structure, can produce biased estimates for both the participant-average and cluster-average treatment effects when cluster size is informative. We describe alternative estimators (independence estimating equations and cluster-level analyses) that are unbiased for CRTs even when informative cluster size is present. Conclusion We conclude that careful specification of the estimand at the outset can ensure that the study question being addressed is clear and relevant, and, in turn, that the selected estimator provides an unbiased estimate of the desired quantity.

BACKGROUND: The COVID-19 pandemic has affected sexual and reproductive health (SRH) service use and unmet need, but the impact is unknown. We aimed to determine the proportion of participants reporting sexual risk behaviours, SRH service use and unmet need, and to assess remote sexually transmitted infection (STI) testing service use after the first national lockdown in Britain. METHODS: We used data from the National Surveys of Sexual Attitudes and Lifestyles (Natsal)-COVID cross-sectional, quasi-representative web survey (Natsal-COVID Wave 1). Adults aged 18-59 years who resided in England, Scotland, or Wales completed the survey between July 29 and Aug 10, 2020, which included questions about the approximate 4-month period after announcement of the initial lockdown in Britain (March 23, 2020). Quota-based sampling and weighting were used to achieve a quasi-representative population sample. Participants aged 45-59 years were excluded from services analysis due to low rates of SRH service use. Among individuals aged 18-44 years, we estimated reported SRH service use and inability to access, and calculated age-adjusted odds ratios (aORs) among sexually experienced individuals (those reporting any sexual partner in their lifetime) and sexually active individuals (those reporting any sexual partner in the past year). Unweighted denominators and weighted estimates are presented hereafter. FINDINGS: 6654 individuals had complete interviews and were included in the analysis. Among 3758 participants aged 18-44 years, 82·0% reported being sexually experienced, and 73·7% reported being sexually active. 20·8% of sexually experienced participants aged 18-44 years reported using SRH services in the 4-month period. Overall, 9·7% of 3108 participants (9·5% of men; 9·9% of women) reported being unable to use a service they needed, although of the participants who reported trying but not being able to use a SRH service at least once, 76·4% of participants also reported an instance of successful use. 5·9% of 1221 sexually active men and 3·6% of 1560 sexually active women reported use of STI-related services and 14·8% of 1728 sexually experienced women reported use of contraceptive services, with SRH service use highest among individuals aged 18-24 years. Sexually active participants reporting condomless sex with new partners since lockdown were much more likely to report using STI-related services than those who did not report condomless sex (aOR 23·8 [95% CI 11·6-48·9]) for men, 10·5 [3·9-28·2] for women) and, among men, were also more likely to have an unsuccessful attempt at STI-service use (aOR 13·3 [5·3-32·9]). Among 106 individuals who reported using STI testing services, 64·4% accessed services remotely (telephone, video, or online). Among 2581 women aged 25-59 years, 2·4% reported cervical screening compared with an estimated 6% in a comparable 4-month period before the pandemic. INTERPRETATION: Many people accessed SRH care during the initial lockdown; however, young people and those reporting sexual risk behaviours reported difficulties in accessing services and thus such services might need to address a backlog of need. FUNDING: Wellcome Trust, The Economic and Social Research Council, The National Institute for Health Research, Medical Research Council/Chief Scientist Office and Public Health Sciences Unit, and UCL Coronavirus Response Fund.

ObjectivesTo investigate how differential access to key interventions to reduce sexually transmitted infections (STI), HIV, and their sequelae changed during the COVID-19 pandemic.MethodsBritish participants (18-59y) completed a cross-sectional web survey one year (March to April 2021) after the initial lockdown in Britain. Quota-based sampling and weighting resulted in a quasi-representative population sample. We compared Natsal-COVID data with Natsal-3, a household-based probability sample cross-sectional survey (16-74y) conducted in 2010-12. Reported unmet need for condoms because of the pandemic and uptake of chlamydia testing/HIV testing/cervical cancer screening were analysed among sexually-experienced participants (18-44y) (n=2869, Natsal-COVID; n=8551, Natsal-3). Odds ratios adjusted for age (aOR) and other potential confounders (AOR) describe associations with demographic and behavioural factors.ResultsIn 2021, 6.9% of women and 16.2% of men reported unmet need for condoms because of the pandemic. This was more likely among participants: aged 18-24 years, of Black or Black British ethnicity, and reporting same-sex sex (past five years) or one or more new relationships (past year). Chlamydia and HIV testing were more commonly reported by younger participants, those reporting condomless sex with new sexual partners, and men reporting same-sex partners; a very similar distribution to 10 years previously (Natsal-3). However, there were differences during the pandemic, including stronger associations with chlamydia testing for men reporting same-sex partners; with HIV testing for women reporting new sexual partners; and with cervical screening among smokers.ConclusionsOur study suggests differential access to key primary and secondary STI/HIV prevention interventions continued during the first year of the COVID-19 pandemic. However, the available evidence does not suggest substantial changes in inequalities in since 2010–12. While the pandemic might not have exacerbated inequalities in access to primary and secondary prevention, it is clear that large inequalities persisted, typically among those at greatest STI/HIV risk.Key MessagesMany MSM, people of Black ethnicity and young people (i.e. groups most impacted by STIs) reported unmet need for condoms because of the pandemicWe compared inequalities in access to key interventions using Natsal-COVID (2021) and Natsal-3 (2010-12).During the pandemic (Natsal-COVID), there were stronger associations with chlamydia testing for MSM and with HIV testing for women reporting new sexual partners.There were stronger associations with cervical screening among smokers during the pandemic compared to 2010-12 (Natsal-3).However, we did not find strong evidence that vulnerable groups were at additional risk during the pandemic when compared to 2010-12.

International audience; To explore challenges in recruitment and intervention implementation in recent stepped-wedge cluster randomized trials (SW-CRTs). Study design and setting: We searched PubMed to identify primary reports of SW-CRTs (2019-2020). Two reviewers independently screened studies and extracted data from each report. Recruitment challenge was defined as planned number of clusters or participants not achieved, or any reported changes made to the design to address recruitment difficulties. Implementation challenge was defined as early, late or no implementation of the intervention in at least one cluster. Results: Of 55 SW-CRTs, 18 (33%) had a recruitment challenge, 23 (42%) had none, and for 14 (26%) it was impossible to judge. At least one implementation challenge was present in 24 (44%), 8 (15%) had none, and for 23 (42%) it was impossible to judge. Of the 35 (64%) trials with recruitment or implementation challenges, 18 (72%) had one or more modifications of their design, most often a modification of the trial duration. Conclusion: Investigators must be aware of the risks of recruitment or implementation challenges when considering use of a SW-CRT design. Mitigating strategies should be adopted when planning the trial. More transparent reporting of planned and actual design features is required.

Background: Intimate relationships are ubiquitous and exert strong influence on health. Widespread disruption to them may impact wellbeing at a population level. We investigated the extent to which the first COVID-19 lockdown (March 2020) affected steady relationships in Britain. Methods: 6,654 participants aged 18-59 years completed a web-panel survey between 29/7-10/8/2020. Quasi-representativeness was achieved via quota sampling and weighting. We explored changes in sex life (5 dimensions) and relationship quality (6 dimensions) among participants in steady relationships (n=4,271) by age, gender, and cohabitation status, and examined factors associated with deterioration to a lower quality relationship. Results: 64.2% of participants were in a steady relationship, mostly cohabiting (88.8%). 22.1% perceived no change in their sex-life quality, and 59.5% no change in their relationship quality. Among those perceiving change, sex-life quality was more commonly reported to decrease and relationship quality to improve. There was significant variation by age, but less often by gender or cohabitation. Overall, 10.6% reported sexual difficulties that started or worsened during lockdown. 6.9% reported deterioration to a ‘lower quality’ relationship, more commonly those: aged 18-24 (women only OR 2.38;1·39-4·08) and aged 35-44 (women OR1.63;1.03-2.56, men OR 2.31;1.45-3.66) compared with those aged 45-59; not living with partner (women only aOR 2.01;1.28-3.16); and reporting depression/anxiety (e.g., depression (women aOR 2.56;1.79-3.64, men aOR 3.06;2.02–4.63). Interpretation: Intimate relationship quality is yet another way in which COVID-19 has led to divergence in experience, with age a key determinant of impact. Funding Information: Natsal (Wellcome Trust/ESRC/NIHR), MRC/CSO, UCL COVID-19 Rapid Response Fund. Declaration of Interests: The authors have no interests to declare. Ethics Approval Statement: Ethical approval was obtained from the University of Glasgow MVLS College (20019174) and LSHTM research ethics committees (22565).

Background: The National Surveys of Sexual Attitudes and Lifestyles COVID study (Natsal-COVID) was designed to understand the impact of COVID-19 on Britain’s sexual and reproductive health (SRH). Natsal-COVID Wave 1 survey and qualitative follow-up interviews were conducted in 2020. The Wave 2 survey was designed to capture one-year prevalence estimates for key SRH outcomes and measure changes over the first year of the pandemic. We describe the Wave 2 survey methodology and assess the sample representativeness. Methods: Natsal-COVID Wave 2 was conducted March-April 2021; approximately one year after the start of Britain’s first national lockdown. Data were collected using an online web-panel survey administered by Ipsos. The sample comprised a longitudinal sample of Wave 1 participants who had agreed to re-contact plus a sample of participants residing in Britain, aged 18-59, including a boost sample comprising people aged 18-29. Questions covered reproductive health, relationships, sexual behaviour and SRH service use. Quotas and weighting were used to achieve a quasi-representative sample of the British population. Comparisons were made with recent national probability surveys, Natsal-3 (2010-12) and Natsal-COVID Wave 1 to understand bias. Results: A total of 6,658 individuals completed the survey. In terms of gender, age, ethnicity, and rurality, the weighted Natsal-COVID Wave 2 sample was like the general population. Participants were less likely to be married or to report being in good health than the general population. The longitudinal sample (n=2,098) were broadly like participants who only took part in Wave 1 but were older. Among the sexually active, longitudinal participants were less likely to report multiple sexual partners or a new sexual partner in the past year compared to those who only took part in Wave 1. Conclusions: Natsal-COVID collected longitudinal, quasi-representative population data to enable evaluation of the population-level impact of COVID-19 on SRH and to inform policy.

Sexual health entails the absence of disease and the ability to lead a pleasurable and safe sex life. In Britain, ethnic inequalities in diagnoses of sexually transmitted infections (STI) persist; however, the reasons for these inequalities, and ethnic variations in other markers of sexual health, remain poorly understood. We investigated ethnic differences in hypothesised explanatory factors such as socioeconomic factors, substance use, depression, and sexual behaviours, and whether they explained ethnic variations in sexual health markers (reported STI diagnoses, attendance at sexual health clinics, use of emergency contraception, and sexual function).We analysed probability survey data from Britain's third National Survey of Sexual Attitudes and Lifestyles (Natsal-3; n=15 162, conducted in 2010-12). Reflecting Britain's current ethnic composition, we included in our analysis participants who identified in 2011 as belonging to one of the following seven largest ethnic groups: white British, black Caribbean, black African, Indian, Pakistani, white other, and mixed ethnicity. We calculated age-standardised estimates and age-adjusted odds ratios for all explanatory factors and sexual health markers for all these ethnic groups with white British as the reference category. We used multivariable regression to examine the extent to which adjusting for explanatory factors explained ethnic variations in sexual health markers.We included 14 563 (96·0%) of the 15 162 participants surveyed in Natsal-3. Greater proportions of black Caribbean, black African, and Pakistani people lived in deprived areas than those of other ethnic groups (36·9-55·3%Ethnic inequalities in sexual health markers exist, and they were not fully explained by differences in their broader determinants. Holistic interventions addressing modifiable risk factors and targeting ethnic groups at risk of poor sexual health are needed.Medical Research Council, the Wellcome Trust, the Economic and Social Research Council, UK Department of Health, and The National Institute for Health Research.

Background There has been some debate in the literature as to whether baseline values of a measurement of interest at treatment initiation should be treated as an outcome variable as part of a model for longitudinal change or instead used as a predictive variable with respect to the response to treatment. We develop a new approach that involves a combined statistical model for all pre- and post-treatment observations of the biomarker of interest, in which the characteristics of response to treatment are treated as a function of the ‘true’ value of the biomarker at treatment initiation. Methods The modelling strategy developed is applied to a dataset of CD4 counts from patients in the UK Register of HIV Seroconverters (UKR) cohort who initiated highly active antiretroviral therapy (HAART). The post-HAART recovery in CD4 counts for each individual is modelled as following an asymptotic curve in which the speed of response to treatment and long-term maximum are functions of the ‘true’ underlying CD4 count at initiation of HAART and the time elapsed since seroconversion. Following previous research in this field, the models developed incorporate non-stationary stochastic process components, and the possibility of between-patient differences in variability over time was also considered. Results A variety of novel models were successfully fitted to the UKR dataset. These provide reinforcing evidence for findings that have previously been reported in the literature, in particular that there is a strong positive relationship between CD4 count at initiation of HAART and the long-term maximum in each patient, but also reveal potentially important features of the data that would not have been easily identified by other methods of analysis. Conclusion Our proposed methodology provides a unified framework for the analysis of pre- and post-treatment longitudinal biomarker data that will be useful for epidemiological investigations and simulations in this context. The approach developed allows use of all relevant data from observational cohorts in which many patients are missing pre-treatment measurements and in which the timing and number of observations vary widely between patients. Electronic supplementary material The online version of this article (doi:10.1186/s12874-016-0187-2) contains supplementary material, which is available to authorized users.

Establishing the clinical significance of symptoms of sexual dysfunction is challenging. To address this, the fifth edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) introduced two new morbidity criteria (duration and symptom severity) to the existing criteria of distress. This study sought to establish the impact of these three criteria on the population prevalence of sexual function problems. The data come from a national probability survey (Natsal-3) and are based on 11,509 male and female participants aged 16–74, reporting at least one sexual partner in the past year. The key outcomes were: proportion of individuals reporting proxy measures of DSM-5 problems, and the proportion of those meeting morbidity criteria. We found that among sexually active men, the prevalence of reporting one or more of four specific sexual problems was 38.2%, but 4.2% after applying the three morbidity criteria; corresponding figures for women reporting one or more of three specific sexual problems, were 22.8% and 3.6%. Just over a third of men and women reporting a problem meeting all three morbidity criteria had sought help in the last year. We conclude that the DSM-5 morbidity criteria impose a focus on clinically significant symptoms.

Objectives To describe the prevalence of medication use to assist sexual performance in Britain and to identify associated factors. Methods Cross-sectional probability sample, undertaken in 2010–2012, of 15 162 people aged 16–74 years, resident in Britain, of whom, 5617 men and 8095 women reported sexual experience (ever) and 4817 men were sexually-active (reported sex in the last year). Results Ever use of medication to assist sexual performance (medicated sex) was more commonly reported by men than women (12.9% (95% CI 11.9% to 13.9%) vs 1.9% (95% CI 1.7% to 2.3%)) and associated with older age in men and younger age in women. It was associated with reporting smoking, and use of alcohol and recreational drugs, as well as unsafe sex (≥2 partners and no condom use in the last year) in both men and women. Among men, the proportion reporting medicated sex in the last year was higher among those reporting erectile difficulties (ED) than those not doing so (28.4% (95% CI 24.4% to 32.8%) vs 4.1% (95% CI 3.4% to 4.9%)). In all men, medicated sex was associated with more frequent sexual activity, meeting a partner on the internet, unsafe sex and recent sexually transmitted infections diagnosis; associations that persisted after adjusting for same-sex behaviour and ED. However, there were significant interactions with reporting ED, indicating that among men with ED, medicated sex is not associated with same-sex behaviour and ever use of recreational drugs. Conclusions A substantial minority of people in Britain report medicated sex, and the association between medicated sex and risky sexual behaviour is not confined to high-risk groups.

Background: There are currently no large general population epidemiological studies of Mycoplasma genitalium (MG), which include prevalence, risk factors, symptoms and co-infection in men and women across a broad age range. Methods: In 2010-–12, we conducted the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), a probability sample survey in Britain. Urine from 4507 sexually-experienced participants, aged 16–44 years, was tested for MG. Results: MG prevalence was 1.2% [95% confidence interval (CI): 0.7–1.8%] in men and 1.3% (0.9–1.9%) in women. There were no positive MG tests in men aged 16–19, and prevalence peaked at 2.1% (1.2–3.7%) in men aged 25–34 years. In women, prevalence was highest in 16–19 year olds, at 2.4% (1.2–4.8%), and decreased with age. Men of Black ethnicity were more likely to test positive for MG [adjusted odds ratio (AOR) 12.1; 95% CI: 3.7–39.4). For both men and women, MG was strongly associated with reporting sexual risk behaviours (increasing number of total and new partners, and unsafe sex, in the past year). Women with MG were more likely to report post-coital bleeding (AOR 5.8; 95%CI 1.4–23.3). However, the majority of men (94.4%), and over half of women (56.2%) with MG did not report any sexually transmitted infection (STI) symptoms. Men with MG were more likely to report previously diagnosed gonorrhoea, syphilis or non-specific urethritis, and women previous trichomoniasis. Conclusions: This study strengthens evidence that MG is an STI. MG was identified in over 1% of the population, including in men with high-risk behaviours in older age groups that are often not included in STI prevention measures.

Objectives: Our objective was to compare data on contraceptive use in relation to reported sexual activity in women from different minority ethnic groups.Design: We analyzed the National Survey of Sexual Attitudes and Lifestyles 2000.Subjects: Women aged 16-44 years, numbering 6932 and residing in Britain, participated in this study.Main Outcome Measures: Our main outcome measures are as follows: percentage of women reporting sexual activity, use of contraception and type of contraception (hormonal, barrier or permanent methods).Results: Overall, fewer women from the LTK's four main ethnic minority groups reported recent sexual activity, compared with white women. Among sexually active women, contraceptive use was significantly lower in all ethnic minority groups than in white women, but this pattern differed according to marital status. In ever-married or cohabiting women, lower contraceptive use was reported by Indian (78%) and Pakistani women (74%) than by other groups. Among single women, black Caribbean (88%) and black African (82%) women reported using less contraception compared with white (95%) and Indian (100%) women. Women from all ethnic minority groups were less likely than white women to report using hormonal contraception and permanent methods and were more likely to use barrier methods. The differences between ethnic groups remained significant after adjusting for educational achievement and parity. Deprivation and acculturation did not account for the use of contraception or the type of contraceptive method used.Conclusion: Sexually active married Pakistani and Indian women reported the lowest overall use of contraception. Among sexually active single women, black African and black Caribbean women reported levels of contraceptive use that were lower than those reported by white women. Sexually active women from all four minority ethnic groups were less likely than white women to use reliable methods of contraception. (c) 2006 Elsevier Inc. All rights reserved.

Background In Britain, young people continue to bear the burden of sexually transmitted infections (STIs) so efforts are required, especially among men, to encourage STI testing. The SPORTSMART study trialled an intervention that sought to achieve this by offering chlamydia and gonorrhoea test-kits to men attending amateur football clubs between October and December 2012. With football the highest participation team sport among men in England, this paper examines the potential public health benefit of offering STI testing to men in this setting by assessing their sociodemographic characteristics, sexual behaviours, and healthcare behaviour and comparing them to men in the general population. Methods Data were collected from 192 (male) members of 6 football clubs in London, United Kingdom, aged 18–44 years via a 20-item pen-and-paper self-completion questionnaire administered 2 weeks after the intervention. These were compared to data collected from 409 men of a similar age who were resident in London when interviewed during 2010–2012 for the third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), a national probability survey that used computer-assisted-personal-interviewing with computer-assisted-self-interview. Age standardisation and multivariable regression were used to account for sociodemographic differences between the surveys. Results Relative to men in the general population, SPORTSMART men were younger (32.8 % vs. 21.7 % aged under 25 y), and more likely to report (all past year) at least 2 sexual partners (adjusted odds ratio, AOR: 3.25, 95 % CI: 2.15–4.92), concurrent partners (AOR: 2.05, 95 % CI: 1.39–3.02), and non-use of condoms (AOR: 2.17, 95 % CI: 1.39–3.41). No difference was observed in STI/HIV risk perception (AOR for reporting “not at all at risk” of STIs: 1.25, 95 % CI: 0.76–2.04; of HIV: AOR: 1.54, 95 % CI: 0.93–2.55), nor in reporting STI testing in the past year (AOR: 0.83, 95 % CI: 0.44–1.54), which was reported by only one in six men. Conclusions Relative to young men in the general population, football club members who completed the SPORTSMART survey reported greater sexual risk behaviour but similar STI/HIV risk perception and STI testing history. Offering STI testing in amateur football clubs may therefore widen access to STI testing and health promotion messages for men at higher STI risk, which, given the minority currently testing and the popularity of football in England, should yield both individual and public health benefit.

Background In the context of widespread opportunistic chlamydia screening among young adults, we aimed to quantify chlamydia testing and diagnosis among 16–24 year olds in Britain in relation to risk factors for prevalent chlamydia infection. Methods Using data from sexually experienced (≥1 lifetime sexual partner) 16-year-old to 24-year-old participants in Britain's third National Survey of Sexual Attitudes and Lifestyles (conducted 2010–2012), we explored socio-demographic and behavioural factors associated with prevalent chlamydia infection (detected in urine; n=1832), self-reported testing and self-reported diagnosis in the last year (both n=3115). Results Chlamydia prevalence was 3.1% (95% CI 2.2% to 4.3%) in women and 2.3% (1.5% to 3.4%) in men. A total of 12.3% of women and 5.3% men had a previous chlamydia diagnosis. Factors associated with prevalent infection were also associated with testing and diagnosis (eg, increasing numbers of sexual partners), with some exceptions. For example, chlamydia prevalence was higher in women living in more deprived areas, whereas testing was not. In men, prevalence was higher in 20–24 than 16–19 year olds but testing was lower. Thirty per cent of women and 53.7% of men with ≥2 new sexual partners in the last year had not recently tested. Conclusions In 2010–2012 in Britain, the proportion of young adults reporting chlamydia testing was generally higher in those reporting factors associated with chlamydia. However, many of those with risk factors had not been recently tested, leaving potential for undiagnosed infections. Greater screening and prevention efforts among individuals in deprived areas and those reporting risk factors for chlamydia may reduce undiagnosed prevalence and transmission.

Background Nonprobability Web surveys using volunteer panels can provide a relatively cheap and quick alternative to traditional health and epidemiological surveys. However, concerns have been raised about their representativeness. Objective The aim was to compare results from different Web panels with a population-based probability sample survey (n=8969 aged 18-44 years) that used computer-assisted self-interview (CASI) for sensitive behaviors, the third British National Survey of Sexual Attitudes and Lifestyles (Natsal-3). Methods Natsal-3 questions were included on 4 nonprobability Web panel surveys (n=2000 to 2099), 2 using basic quotas based on age and sex, and 2 using modified quotas based on additional variables related to key estimates. Results for sociodemographic characteristics were compared with external benchmarks and for sexual behaviors and opinions with Natsal-3. Odds ratios (ORs) were used to express differences between the benchmark data and each survey for each variable of interest. A summary measure of survey performance was the average absolute OR across variables. Another summary measure was the number of key estimates for which the survey differed significantly (at the 5% level) from the benchmarks. Results For sociodemographic variables, the Web surveys were less representative of the general population than Natsal-3. For example, for men, the average absolute OR for Natsal-3 was 1.14, whereas for the Web surveys the average absolute ORs ranged from 1.86 to 2.30. For all Web surveys, approximately two-thirds of the key estimates of sexual behaviors were different from Natsal-3 and the average absolute ORs ranged from 1.32 to 1.98. Differences were appreciable even for questions asked by CASI in Natsal-3. No single Web survey performed consistently better than any other did. Modified quotas slightly improved results for men, but not for women. Conclusions Consistent with studies from other countries on less sensitive topics, volunteer Web panels provided appreciably biased estimates. The differences seen with Natsal-3 CASI questions, where mode effects may be similar, suggest a selection bias in the Web surveys. The use of more complex quotas may lead to some improvement, but many estimates are still likely to differ. Volunteer Web panels are not recommended if accurate prevalence estimates for the general population are a key objective.

Background Data from the first two National Surveys of Sexual Attitudes and Lifestyles, carried out in 1990–1991 (Natsal-1) and 1999–2001 (Natsal-2), have been extensively used to inform sexual health policy in Britain over the past two decades. Natsal-3 was carried out from September 2010 to August 2012 in order to provide up-to-date measures of sexual lifestyles and to extend the scope of the previous studies by including an older age group (up to 74 years), an extended range of topics and biological measures. Methods We describe the methods used in Natsal-3, which surveyed the general population in Britain aged 16–74 years (with oversampling of younger adults aged 16–34 years). Results Overall, 15 162 interviews were completed, with a response rate of 57.7% and a cooperation rate of 65.8%. The response rate for the boost sample of ages 16–34 years was 64.8%, only marginally lower than the 65.4% achieved for Natsal-2, which surveyed a similar age range (16–44). The data were weighted by age, gender and region to reduce possible bias. Comparisons with census data show the weighted sample to provide good representation on a range of respondent characteristics. The interview involved a combination of face-to-face and self-completion components, both carried out on computer. Urine samples from 4550 sexually-experienced participants aged 16–44 years were tested for a range of STIs. Saliva samples from 4128 participants aged 18–74 years were tested for testosterone. Conclusions Natsal-3 provides a high quality dataset that can be used to examine trends in sexual attitudes and behaviours over the past 20 years.

To describe variations in age differences (ADs) between partners at first heterosexual intercourse and to examine associations between this AD, sociodemographic characteristics, and circumstances of first heterosexual intercourse, by gender.A total of 3277 men and 4734 women aged 25-44 years who reported ever having heterosexual intercourse in a probability survey of the British population to obtain an unbiased sample of ADs at first intercourse. Face-to-face interviews collected sociodemographic data and information on first intercourse. Gender-specific cumulative distributions of ADs categorized respondents reporting relatively younger first partners (below fifth percentile) and those reporting relatively older first partners (above 95th percentile).Relatively older first partners were six years older than men and 10 years older than women. Relatively younger first partners were three years younger than men and one year younger than women. Men and women with relatively older first partners were more likely to have just met their partner (odds ratios adjusted for sociodemographic characteristics [AOR] 1.8, 95% confidence interval [CI] 1.2-2.7, and 3.1, 95% CI 2.1-4.6, respectively); had a more willing first partner (AOR 2.1, 95% CI 1.2-3.8, and 1.6, 95% CI 1.2-2.2, respectively); not used condoms at first sex (AOR 1.9, 95% CI 1.2-3.0, and 1.5, 95% CI 1.1-2.0, respectively); and reported a non-autonomous reason for first sex (AOR 1.6, 95% CI 1.1-2.4, women only). Men with relatively younger first partners were more likely to regret the timing of the first occasion (AOR 2.1, 95% CI 1.0-4.5).First partnerships involving relatively older or younger partners are associated with adverse circumstances of first intercourse. Condoms and other reliable contraception are less likely to be used, and age differences may reflect unequal power relations. Promoting effective communication and negotiation skills may be particularly important for those with relatively older or younger first partners.

Sexually transmitted infections may enhance the sexual transmission of HIV-1. It is possible that loss of virological control in patients with such infections receiving antiretroviral therapy (ART) may even facilitate the transmission of drug-resistant HIV. We have recently demonstrated that in those on maximally suppressive ART this effect is reduced. We have examined the virus obtained from the blood plasma and seminal plasma of six HIV-1-infected men receiving poorly suppressive ART with acute urethritis for the presence of drug resistance-associated mutations. In four men with gonorrhoea the blood and seminal plasma HIV-1 had mutations conferring reduced susceptibility to one or more available drugs. In one of these men the viral load of drug-resistant virus in seminal plasma was 20-fold higher during gonorrhoea than following antibiotic treatment, with no change in blood plasma viral load. We conclude that in the presence of suboptimal ART, sexually transmitted infections may enhance the spread of drug-resistant HIV-1.