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What is an
“Epi Week?”
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.
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
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