When people realize that I work within an epidemiology and biostatistics research team, while completing my Masters’ degree in bioethics, a common question arises. What’s the difference between incidence and prevalence?
Even if you don’t recognize these specific terms, you’ll likely recall having seen medical or public health information presented as “1 in 100,000” people, “1 in 1,000 people”, “an estimated 5,000 people”, and so on. These numbers may represent either incidence or prevalence, which is one of the reasons for which there tends to be confusion between the two.
Another reason for confusion is that both incidence and prevalence are often presented for a specific geographical area. The rate given is generally for a country, province/state, region, or city. Some statistics might be provided for individual hospitals, groups of provinces, etc. The World Health Organization (WHO) provides global incidence rates of certain diseases, such as cancer.(1)
Now let’s look at how to distinguish between the two. Disease incidence represents the number of people who come into the world with a particular medical condition – during any given year. For a hereditary genetic disease, for example, the incidence might be shown as the number of infants born – live births – with that genetic mutation in 2006. In short, incidence will tell you how many babies were born with a disease in a one-year period.
If you see an incidence rate of 1 in 100 that would indicate that only one out of every hundred babies born would have the condition in question. You could also show this as 1% of births per year. Either way, the geographic area is specified.
Let’s look at a fun example. The worldwide incidence of red hair is only about 1 in 100. This also shows why these rates are often presented with geographical specification; in Scotland, where the genes for red hair are more frequently expressed, the incidence of red hair is as high as 13 in 100!
This would mean that, at a global level, 1 in 100 babies (or 1%) would have been born with red hair last year – although I don’t have any references to back that up! In Scotland, though, the incidence rate of 13 in 100 (13%) would translate to 13% of babies born with red hair.
Now for a serious example. If we were to modify the incidence parameter from red hair to neural tube deformities, and those same differences held true (1% versus 6%), we’d expect immediate action to be taken. This type of congenital disorder, or birth defect, is often preventable by ensuring that pregnant women consume sufficient amounts of folate – particularly in the first four weeks of gestation.
Because most women don’t realize so quickly that they’re pregnant, many countries have opted to supplement certain foods with folic acid (folate). To further reduce risks of neural tube deformities (NTDs), folate supplementation is also recommended for pregnant women. So a higher incidence of a preventable condition would likely trigger a public health initiative; perhaps to increase folate supplementation, or to create an awareness campaign on the important of folate for women in their child-bearing years to ensure that they ingest sufficient folate.
There could even be a targeted information campaign in schools, for adolescents as they attain puberty. Or this information could be integrated into existing sexual education or teen health classes. As you can see from this hypothetical situation, familiarity with incidence rates can allow for the implementation of targeted preventive measures.
As for prevalence, it’s also disease-specific. Rather than representing the number of people in a given year who are born with a particular disease within an area, prevalence provides data on how many individuals have the disease overall. Prevalence includes the newly-diagnosed, along with patients who may have been living with the condition for decades – even lifetimes.
Why did I choose NTDs for this discussion? A study was published this past summer, on this topic:
In 1998, folic acid fortification of a large variety of cereal products became mandatory in Canada, a country where the prevalence of neural-tube defects was historically higher in the eastern provinces than in the western provinces. We assessed changes in the prevalence of neural-tube defects in Canada before and after food fortification with folic acid was implemented.”(2)
This research project, using observational data, shines a light on how prevalence is used in reporting study results. It also provides solid evidence on the impact of this preventive measure:
The prevalence of neural-tube defects decreased from 1.58 per 1000 births before fortification to 0.86 per 1000 births during the full-fortification period, a 46% reduction (95% confidence interval, 40 to 51). The magnitude of the decrease was proportional to the prefortification baseline rate in each province, and geographical differences almost disappeared after fortification began…
Food fortification with folic acid was associated with a significant reduction in the rate of neural-tube defects in Canada.”(2)
If you’d like to see other relatively recent examples of both incidence and prevalence, flip through the WHO’s global cancer publication GLOBOCAN 2000: Cancer Incidence, Mortality and Prevalence Worldwide. The GLOBOCAN publication provides:
access to the most recent estimates of the cancer incidence, mortality and prevalence for twenty-eight cancers worldwide.”(1)
References
(1) Cancer: Incidence, mortality & survival databases. World Health Organization (WHO). 1996 and 2000. Online:
https://www.who.int/cancer/resources/incidences/en/
(2) Reduction in Neural-Tube Defects after Folic Acid Fortification in Canada. Philippe De Wals, Ph.D., Fassiatou Tairou, M.Sc., Margot I. Van Allen, M.D., Soo-Hong Uh, M.Sc., R. Brian Lowry, M.D., Barbara Sibbald, M.Sc., Jane A. Evans, Ph.D., Michiel C. Van den Hof, M.D., Pamela Zimmer, M.H.S.A., Marian Crowley, M.N., Bridget Fernandez, M.D., Nora S. Lee, M.Sc., et al. New England Journal of Medicine (N Engl J Med): 2007; 357:135-142. 12 Jul, 2007. Online:
https://www.nejm.org/doi/full/10.1056/NEJMoa067103