Internet samples are biased towards a lower median age because younger men are over-represented on MSM dating websites and therefore may be more likely to be recruited into surveys.
Men diagnosed with HIV were over-represented in the internet survey, and increasingly so in the older age groups.
This study seeks to measure the differences in rates of age-specific survey participation, new HIV diagnoses, and HIV prevalence between MSM who participate in internet surveys and the general MSM population.
A similar effect was observed in the age groups younger than 25 years.
Self-reported peak prevalence and peak HIV diagnoses rates are often shifted to higher age groups in internet samples compared to surveillance data.
Further requirements were a sufficient size of the EMIS sample, and availability of relatively reliable HIV surveillance data regarding MSM.
We set the lower and upper age limits of both the EMIS sample and the surveillance data to be 15 years and 65 years.
For the Czech Republic, as the only country from an eastern European sub-region, we also analysed the data for the narrower age range of 15 to 49 years, because the HIV epidemic among MSM in the eastern parts of Europe started about 10–15 years later than in the western parts, leading to a different age distribution of HIV infections in the MSM population.
Data on new HIV diagnoses in 2009 were taken from national infectious disease surveillance systems.
In 2010, the European MSM Internet Survey (EMIS) demonstrated the feasibility and utility of collecting data from MSM from 38 European countries with the same questionnaire – simultaneously available online in 25 languages – and using the same recruitment methods .
However, estimation and comparability of MSM HIV prevalence between countries (and between consecutive surveys) is limited by unknown sizes of MSM populations, differences in household internet access across countries and time , Marcus U, Hickson F, Weatherburn P, Schmidt AJ, et al.: Estimating the size of the MSM populations for 38 European countries by calculating the survey-surveillance discrepancies (SSD) between self-reported new HIV diagnoses from the European MSM Internet Survey (EMIS) and surveillance-reported HIV diagnoses among MSM in 2009 (as yet unpublished observations)], and possibly other unknown selection effects.
Survey-surveillance discrepancies (SSD) for survey participation, diagnosed HIV prevalence and new HIV diagnoses were determined as ratios of proportions.
Results are calculated and presented by 5-year age groups for MSM aged 15–64.
Internet samples of MSM were skewed towards younger age groups when compared to an age distribution of the general adult male population.