Many people want to know the difference between an epidemic and a pandemic. The basic rule is that if an outbreak is confined to one country it’s an epidemic and if it’s international it’s a pandemic. So first, a definition of “outbreak” has to be established. An outbreak occurs when the incidence of disease has increased beyond what is normal for an area.[1] The CDC has thresholds for certain illnesses, like the flu, and when the proportion of deaths in an area is greater than this threshold an outbreak is declared.[2]
Epidemiologists use a definition of “epidemic” that is broader than the general public’s definition: “An epidemic is the occurrence in a community or region of cases of an illness, specified health behavior, or other health-related events clearly in excess of normal expectancy.”[3] The terms “outbreak” and “epidemic” are considered synonymous by epidemiologists.[4] However, some dictionaries describe outbreaks as “local epidemics.”[5] This puts “outbreaks” lower on the severity scale than “epidemics.” In common usage, an epidemic does not need to be contagious,[6] but an outbreak does. Think about the difference between an “obesity epidemic” versus an “obesity outbreak.” The first is familiar while the second seems rather silly. So the difference between “outbreak” and “epidemic” is determined largely by common usage, but officially could be thought of as occurring when disease levels in an area are higher than expected.
And then there’s the difference between “epidemic” and “pandemic.” WHO’s Assistant Director-General in 2009, Dr. Fukuda, stated regarding H1N1 influenza that it’s considered a pandemic if there’s community transmission in many parts of the world.[7] This means that it’s not enough for the virus to spread to many parts of the world but that it has to also start transmitting in the local populations for it to be considered a pandemic. While Ebola has been transmitted once in both Europe and North America (in addition to, of course, Africa) that’s not enough to really be considered a pandemic.
The WHO has defined pandemic criteria for influenza: a pandemic occurs when there is community-level transmission in at least two counties in a WHO region and in a country in another WHO region.[8] If we end up with person-to-person transmission in the US or Spain, or any other country outside of WHO’s Africa region, we will have reached the pandemic stage. Hopefully with good containment measures and an understanding of how the virus is spread we will never end up at that point.
Sources below the cut.
[1] Medical News Today, What is a pandemic? What is an epidemic?; http://www.medicalnewstoday.com/articles/148945.php.
[2] Brendan Koerner, Slate, Outbreaks vs. Epidemics; http://www.slate.com/articles/news_and_politics/explainer/2003/12/outbreaks_vs_epidemics.html
[3] A Dictionary of Epidemiology, Oxford University Press, 1995, via Manfred S. Green et al, Israeli Medical Association Journal, When is an Epidemic an Epidemic?, 2002; http://www.ima.org.il/FilesUpload/IMAJ/0/55/27606.pdf.
[4] Green, Id.
[5] Id.
[6] Id.
[7] WHO, Press Conference, 2009; http://www.who.int/mediacentre/influenzaAH1N1_presstranscript_20090526.pdf
[8] WHO, Current Who phase of pandemic alert for Pandemic (H1N1) 2009; http://www.who.int/csr/disease/swineflu/phase/en/
