[Definition] What is Epiweek

What is an epidemiological week and why do we use them?

An epidemiological week, commonly referred to as an epi week or a CDC week, is simply a standardized method of counting weeks to allow for the comparison of data year after year. Many people know these as CDC weeks because CDC uses them and has published them in the past. Nevertheless, they are used throughout the world by epidemiological teams in different countries. Unfortunately, not all countries calculate epi weeks in the same manner, and this discrepancy causes some difficulty in comparing data internationally. 

The question is, how do we define epi weeks? The heart of the matter is how we define the first epi week. Is it the week in which the first of January falls? Is it the first complete week in January? To solve this, there is a definition (in the United States and many Latin American countries) as to how we establish epi week one.

The first epi week of the year ends, by definition, on the first Saturday of January, as long as it falls at least four days into the month. Each epi week begins on a Sunday and ends on a Saturday.
(http://www.paho.org/english/sha/be_v21n3-calendar.htm)
The following figure provides the epi week calendar for 2006, with epi week one starting (by coincidence) on the first of January.

So why do we bother to determine and use epi weeks? Many times, particularly for mosquito surveillance programs or epidemiological studies, daily increments are too frequent and too varied to be able to be managed and analyzed, or there are many factors that make it impossible to compare daily results. On the other hand, the monthly time interval is too great, and data interpretation is needed at more frequent intervals. Therefore we seek an intermediary period of time to analyze the data, and this brings us to the week. By defining the epi week, we are able to maintain harmony and ensure that all parties count weeks the same way, understanding that epi week n refers to the same time period.

Jorge R. Arias, Ph.D.

Disease Carrying Insects Program

Fairfax County, Virginia

(source: https://www.cmmcp.org/sites/cmmcp/files/uploads/spring_skeeter_06.pdf)