Estimating the Size of Key Populations in Nairobi, Kenya

A team of researchers, including three from the Population Council, have developed estimates of the sizes of three key populations at risk for HIV infection in Nairobi, Kenya: men who have sex with men, female sex workers, and people who inject drugs. These estimates are among the first solid data on the size of these populations in Nairobi, providing practitioners with evidence to inform the development of programs that meet the needs of these vulnerable groups.

HIV disproportionately affects men who have sex with men, female sex workers, and people who inject drugs. All three groups are stigmatized, discriminated against, and face legal, social, and economic barriers to accessing health services. Because of these circumstances, people in these populations often remain hidden and they may not seek the prevention and treatment services they need to safeguard their own health and the health of their sexual and drug-injecting partners.

Dearth of data

Before this study, policymakers and program planners lacked realistic estimates of the size of these groups, and such limitations restricted their ability to understand the scope of the HIV epidemic and allocate scarce resources appropriately. This evidence about the size of marginalized populations will allow policymakers to respond to the HIV epidemic more effectively.

A gold-standard population data source, such as a census, does not exist for men who have sex with men, female sex workers, or people who inject drugs. So the researchers used three other methods to calculate the size of these populations. Because these approaches are not randomized, the data they produce might be biased in one direction or another. Employing multiple approaches reduces the chance of bias. The approaches were incorporated within a behavioral surveillance study of key populations funded by the U.S. Centers for Disease Control and Prevention (CDC) in Nairobi. The three approaches were:

  • the multiplier method, which gathers data on use from various services, projects, and studies and compares them to reported service use in the surveillance survey;
  • the “Wisdom of the Crowds” method, which asks respondents from specific groups to estimate how many members of their population exist, resulting in an average or median estimate of the total population size; and
  • a literature review to identify published estimates of the size of the three populations in contexts and regions similar to Nairobi.

“We were able to access service data from a number of sources in this study, which in turn provided a range of estimates,” said Scott Geibel, a Council researcher on the study. “We then convened stakeholders to review these data and decide on the most plausible estimate and ranges.”

The behavioral surveillance study, which took place in 2010–2011, employed respondent-driven sampling (RDS) to recruit participants. RDS allows researchers to gain access to stigmatized populations through their social networks. Respondents referred a limited number of friends or acquaintances (who are also members of the key population) to participate in the study. The study involved 563 men who have sex with men, 593 female sex workers, and 263 people who inject drugs.

The study team validated the resulting estimates with experts from the Kenyan government, investigators, program managers, and advocates in Nairobi to determine whether the estimates seemed reasonable and to establish plausible upper and lower bounds for the size of the populations. Combining the three methods resulted in the most well-supported estimate of the sizes of key populations available. The researchers arrived at the following estimates for the size of the three populations in Nairobi:

  • 11,042 men who have sex with men (with low and high ranges of 10,000 and 22,222);
  • 29,494 female sex workers (with low and high ranges of 10,000 and 54,467); and
  • 6,216 people who inject drugs (with low and high ranges of 5,031 and 10,937)

The authors acknowledged some basic limitations of their study, most of which related to varying definitions of group membership. For example, how often and recently should a man have had sexual relations with another man to be considered a member of the men who have sex with men group? And should a woman be considered a female sex worker if she has sex for money only occasionally, or only if this is her primary source of income?

Nevertheless, the study was one of the first to incorporate such a wide range of data sources and research methods within a broader HIV surveillance survey. National government agencies, community-based organizations, and donor organizations can use these population size estimates to develop and launch HIV prevention, treatment, and care services. Further refinement of methods and estimates could also be undertaken. For the time being, this study clearly showed that significant numbers of men who have sex with men, female sex workers, and people who inject drugs live in Nairobi. They and their advocates now have evidence to help them campaign for greater resources to meet the health needs of these key populations.

“These data offer HIV prevention program planners some much-needed precision in key population size estimates,” commented Population Council researcher Jerry Okal.


Okal, Jerry , Scott Geibel, Nicolas Muraguri, Helgar Musyoki, Waimar Tun, Dita Broz, David Kuria, Andrea Kim, Tom Oluoch, and H. Fisher Raymond. 2013. “Estimates of the size of key populations at risk for HIV infection: men who have sex with men, female sex workers and injecting drug users in Nairobi, Kenya.” Sexually Transmitted Infections 89(5): 366–371. doi:10.1136/sextrans-2013-051071

Outside Funding

U.S. Agency for International Development (USAID)/U.S. President’s Emergency Plan for AIDS Relief (PEPFAR) and the U.S. Centers for Disease Control and Prevention (CDC)


This study was a collaboration among the following organizations:

  • Casino STI Clinic
  • U.S. Centers for Disease Control and Prevention
  • Gay and Lesbian Coalition of Kenya
  • Kenya AIDS Vaccine Initiative
  • Liverpool Voluntary Counseling and Testing
  • Nairobi Outreach Services Trust (NOSET)
  • National AIDS and STD Control Programme (Kenya)
  • National AIDS Control Council
  • Population Council
  • San Francisco Department of Public Health
  • United Nations Office on Drugs and Crime (UNODC)
  • University of California, San Francisco
  • University of Manitoba’s Sex Worker Outreach Programme (SWOP)