MMWR Weeks

MMWR weeks have the following characteristics: - Weeks start on Sundays. - Week 1 of a year is the first week that contains at least four days in that year, in other words, the week containing the first Wednesday of the year. As a result, some years have 53 MMWR weeks. - Each influenza season starts with MMWR Week 40. The columns were calculated with the following Excel formulas (expressed in terms of the second data row where the first row is column headers and copied to all subsequent rows): - Year: =YEAR(A3+3) -- The calendar year of the Wednesday following the Sunday on which the week begins. - Week: =IF(D2=D3,C2+1,1) -- If the year is the same as the previous week, increment by 1.

If not, start over at Week 1 of the new year. - Influenza Season: =IF(C3 -- If the week is 39 or earlier, the influenza season is the previous calendar year through the current calendar year. If the week is 40 or later, the influenza season is the current calendar year through the next calendar year. Although not referred to explicitly at the time, MMWR week numbers effectively began with the first issue of the MMWR in January 1952.