HIV in Rwanda: Incidence and Prevalence by Age and Gender

1Two numbers from one survey

Between October 2018 and March 2019, survey teams from the Rwanda Biomedical Centre (RBC) and the National Institute of Statistics of Rwanda (NISR) visited 11,219 households in every district and offered HIV testing to the people who lived there. The survey is called RPHIA, the Rwanda Population-based HIV Impact Assessment. Two of its results seem to disagree:

• about 5,400 adults aged 15–64 acquire HIV in a year;
• about 210,200 adults aged 15–64 are living with HIV.

The first number is less than 3% of the second. So is HIV in Rwanda a small problem or a large one? Both numbers are correct, because they measure different things. A health planner who confuses them will either buy far too few medicines or wrongly decide that prevention has failed. This lesson gives you the two measures, shows you how HIV differs by age and gender, and asks you to explain why.

2Measuring an epidemic: the two formulas

Both measures are fractions of a population, but the top and the bottom of each fraction are different.

$$\text{Prevalence}=\frac{\text{number of people living with HIV}}{\text{number of people in the population}}\times 100\mathrm{\%}\backslash text\{Prevalence\}\; =\; \backslash frac\{\backslash text\{number\; of\; people\; living\; with\; HIV\}\}\{\backslash text\{number\; of\; people\; in\; the\; population\}\}\; \backslash times\; 100\backslash \%$$ $$\text{Annual incidence}=\frac{\text{number of new infections in one year}}{\text{number of people without HIV at the start of the year}}\times 100\mathrm{\%}\backslash text\{Annual\; incidence\}\; =\; \backslash frac\{\backslash text\{number\; of\; new\; infections\; in\; one\; year\}\}\{\backslash text\{number\; of\; people\; without\; HIV\; at\; the\; start\; of\; the\; year\}\}\; \backslash times\; 100\backslash \%$$

Prevalence is a stock: everyone who has HIV today, whether they acquired it last month or twenty years ago. Incidence is a flow: only the new cases. Think of a water tank. Incidence is the tap filling it; deaths and moving away are the drain; prevalence is the water in the tank. When the drain is slowed, the tank can stay full even if the tap is turned down.

For a disease that lasts a long time and changes slowly, the three quantities are linked by an approximate rule:

$$\text{Prevalence}\approx \text{Incidence}\times \text{average duration of the condition}\backslash text\{Prevalence\}\; \backslash approx\; \backslash text\{Incidence\}\; \backslash times\; \backslash text\{average\; duration\; of\; the\; condition\}$$

ART lets people with HIV live for decades, so the average duration has grown. That is why a country can cut new infections and still see prevalence stay level.

3Who is living with HIV? Age and gender

RPHIA reported prevalence for every five-year age group, separately for males and females. Figure 1 draws its table.

Three patterns stand out.

1. The gender gap is widest among the young. At 20–24, a young woman was three times as likely as a young man to be living with HIV. Among all adults, women's prevalence (3.7%) was 1.7 times men's (2.2%).
2. Prevalence rises with age to a peak in the fifties. It reaches 7.4% among women aged 50–54 and 6.5% among men aged 55–59, then falls at 60–64. Most people in these groups were young adults in the 1980s and 1990s, when infection was much more common; ART has kept many of them alive. Prevalence counts all of them. This is a cohort effect: the peak follows a generation as it ages.
3. Prevalence is lowest among children. At ages 10–14 it was 0.4%. In children this young, HIV is usually passed from mother to child during pregnancy, birth or breastfeeding, so the testing and treatment of pregnant women (Lesson 4) also protects children from HIV.

Place matters too: prevalence was 4.8% in urban areas and 2.5% in rural areas, and 4.3% in the City of Kigali.

4Explaining the gender gap

No single cause explains why young women are more affected. Analysts group the causes into four kinds.

The most recent survey, the Rwanda Demographic and Health Survey 2025, shows the same testing gap: 77% of women aged 15–49 had ever been tested and received their result, against 71% of men. It also found that only 57% of young women and 53% of young men aged 15–24 had comprehensive knowledge of HIV prevention, down from 65% and 64% in 2014–15. (Its HIV prevalence results will appear in the final DHS report.)