CDC Week System
The CDC (MMWR) week system is based on the methodology used by the US CDC.
Overview
The CDC week system, also known as MMWR (Morbidity and Mortality Weekly Report) weeks, is the epidemiological week methodology used by the US Centers for Disease Control and Prevention for disease surveillance. The first day of any CDC week is Sunday. Week numbering is sequential beginning with 1 and incrementing with each week to a maximum of 52 or 53.
Geographic Usage
The CDC week system aligns with countries where the week starts on Sunday. This includes most of the Americas (United States, Canada, Brazil), and parts of Asia (India, Indonesia, Japan, Pakistan, Bangladesh), Africa (Nigeria, South Africa, Egypt), and the Middle East (Saudi Arabia).
Week 1 Definition
CDC week 1 of a year is the first week that has at least four days in the calendar year. Based on this rule:
- When January 1 occurs on Sunday–Wednesday, that week is CDC week 1
- When January 1 occurs on Thursday–Saturday, that week is the last CDC week of the previous year (week 52 or 53)
- Consequently, December 29–31 may fall within CDC week 1 of the following year
53-Week Years
Some years have 53 CDC weeks instead of 52. Examples include 2020 and 2025.
Comparison with ISO System
Key differences from the ISO week system:
- CDC weeks start on Sunday; ISO weeks start on Monday
- Week 1 definitions differ, so the same date may fall in different week numbers
- The week containing January 1 may differ between systems