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An MMWR week (Morbidity and Mortality Weekly Report week) is the standardized epidemiologic week designated by the Centers for Disease Control and Prevention (CDC) and state or local health departments to aggregate, track, and publish national notifiable disease incidence data. In epidemiological research, tracking disease transmission across strictly defined 7-day cycles removes calendar-month distortions and stabilizes comparisons across reporting jurisdictions.
Definition and Official CDC Rules
The National Notifiable Diseases Surveillance System (NNDSS) governs the business rules for assigning MMWR weeks across public health surveillance systems in the United States:
- First Day of the Week: Every MMWR week begins on Sunday at 00:00 and ends on the following Saturday at 23:59.
- Sequential Numbering: Weeks are numbered sequentially from 1 to 52 or 53 within each epidemiologic year.
- The 4-Day / First Wednesday Rule: MMWR Week 1 of any given year is defined as the first week of the calendar year that contains at least four days in that year. Equivalently, MMWR Week 1 is the week containing the first Wednesday of January.
- Year Boundary Handling: If January 1 occurs on a Sunday, Monday, Tuesday, or Wednesday, that calendar week is designated as MMWR Week 1 of the new year. If January 1 occurs on a Thursday, Friday, or Saturday, the calendar week containing January 1 is designated as the final week (Week 52 or Week 53) of the previous MMWR year.
- December Days in Week 1: Because of the 4-day rule, December 29, 30, and 31 frequently fall into MMWR Week 1 of the following calendar year.
- 52 vs 53 Weeks: While most years comprise 52 weeks, years with 53 weeks occur every 5 to 6 years to compensate for the fractional day in solar years and leap-year shifts. MMWR year 2025 contains 53 weeks, whereas MMWR year 2026 contains 52 weeks.
The Epidemiologic Date Hierarchy
In public health surveillance systems, disease events undergo multiple processing checkpoints before arriving at CDC. According to the CDC NNDSS technical standards, reporting jurisdictions assign MMWR weeks using an established hierarchy of dates:
- Date of disease onset: The date clinical symptoms first manifested in the patient.
- Date of diagnosis: The date a medical provider established the clinical or syndromic diagnosis.
- Date of laboratory result: The date diagnostic confirmatory findings (e.g., PCR or serology) were recorded.
- Date of first report to the public health system: The earliest timestamp when a local, county, district, or state health authority received notice.
- State or MMWR report date: The administrative transmission date to state surveillance databases or CDC.
Under National Electronic Disease Surveillance System (NEDSS) messaging architectures, surveillance platforms capture illness onset, diagnosis date, specimen collection date, laboratory confirmation, public health notification, and CDC transmission timestamps. Assigning the earliest robust date in the hierarchy prevents sporadic spikes in reported incidence caused by batched administrative processing.
Official CDC MMWR Week Lookup Calendar (2025–2026)
The following lookup table is copied directly from the official CDC/NNDSS MMWR Week Log (2025–2026). It documents the precise start (Sunday) and end (Saturday) dates for all weeks in 2025 and 2026:
| 2025 Week | 2025 Start Date | 2025 End Date | 2026 Week | 2026 Start Date | 2026 End Date |
|---|---|---|---|---|---|
| 1 | 12/29/2024 | 1/4/2025 | 1 | 1/4/2026 | 1/10/2026 |
| 2 | 1/5/2025 | 1/11/2025 | 2 | 1/11/2026 | 1/17/2026 |
| 3 | 1/12/2025 | 1/18/2025 | 3 | 1/18/2026 | 1/24/2026 |
| 4 | 1/19/2025 | 1/25/2025 | 4 | 1/25/2026 | 1/31/2026 |
| 5 | 1/26/2025 | 2/1/2025 | 5 | 2/1/2026 | 2/7/2026 |
| 6 | 2/2/2025 | 2/8/2025 | 6 | 2/8/2026 | 2/14/2026 |
| 7 | 2/9/2025 | 2/15/2025 | 7 | 2/15/2026 | 2/21/2026 |
| 8 | 2/16/2025 | 2/22/2025 | 8 | 2/22/2026 | 2/28/2026 |
| 9 | 2/23/2025 | 3/1/2025 | 9 | 3/1/2026 | 3/7/2026 |
| 10 | 3/2/2025 | 3/8/2025 | 10 | 3/8/2026 | 3/14/2026 |
| 11 | 3/9/2025 | 3/15/2025 | 11 | 3/15/2026 | 3/21/2026 |
| 12 | 3/16/2025 | 3/22/2025 | 12 | 3/22/2026 | 3/28/2026 |
| 13 | 3/23/2025 | 3/29/2025 | 13 | 3/29/2026 | 4/4/2026 |
| 14 | 3/30/2025 | 4/5/2025 | 14 | 4/5/2026 | 4/11/2026 |
| 15 | 4/6/2025 | 4/12/2025 | 15 | 4/12/2026 | 4/18/2026 |
| 16 | 4/13/2025 | 4/19/2025 | 16 | 4/19/2026 | 4/25/2026 |
| 17 | 4/20/2025 | 4/26/2025 | 17 | 4/26/2026 | 5/2/2026 |
| 18 | 4/27/2025 | 5/3/2025 | 18 | 5/3/2026 | 5/9/2026 |
| 19 | 5/4/2025 | 5/10/2025 | 19 | 5/10/2026 | 5/16/2026 |
| 20 | 5/11/2025 | 5/17/2025 | 20 | 5/17/2026 | 5/23/2026 |
| 21 | 5/18/2025 | 5/24/2025 | 21 | 5/24/2026 | 5/30/2026 |
| 22 | 5/25/2025 | 5/31/2025 | 22 | 5/31/2026 | 6/6/2026 |
| 23 | 6/1/2025 | 6/7/2025 | 23 | 6/7/2026 | 6/13/2026 |
| 24 | 6/8/2025 | 6/14/2025 | 24 | 6/14/2026 | 6/20/2026 |
| 25 | 6/15/2025 | 6/21/2025 | 25 | 6/21/2026 | 6/27/2026 |
| 26 | 6/22/2025 | 6/28/2025 | 26 | 6/28/2026 | 7/4/2026 |
| 27 | 6/29/2025 | 7/5/2025 | 27 | 7/5/2026 | 7/11/2026 |
| 28 | 7/6/2025 | 7/12/2025 | 28 | 7/12/2026 | 7/18/2026 |
| 29 | 7/13/2025 | 7/19/2025 | 29 | 7/19/2026 | 7/25/2026 |
| 30 | 7/20/2025 | 7/26/2025 | 30 | 7/26/2026 | 8/1/2026 |
| 31 | 7/27/2025 | 8/2/2025 | 31 | 8/2/2026 | 8/8/2026 |
| 32 | 8/3/2025 | 8/9/2025 | 32 | 8/9/2026 | 8/15/2026 |
| 33 | 8/10/2025 | 8/16/2025 | 33 | 8/16/2026 | 8/22/2026 |
| 34 | 8/17/2025 | 8/23/2025 | 34 | 8/23/2026 | 8/29/2026 |
| 35 | 8/24/2025 | 8/30/2025 | 35 | 8/30/2026 | 9/5/2026 |
| 36 | 8/31/2025 | 9/6/2025 | 36 | 9/6/2026 | 9/12/2026 |
| 37 | 9/7/2025 | 9/13/2025 | 37 | 9/13/2026 | 9/19/2026 |
| 38 | 9/14/2025 | 9/20/2025 | 38 | 9/20/2026 | 9/26/2026 |
| 39 | 9/21/2025 | 9/27/2025 | 39 | 9/27/2026 | 10/3/2026 |
| 40 | 9/28/2025 | 10/4/2025 | 40 | 10/4/2026 | 10/10/2026 |
| 41 | 10/5/2025 | 10/11/2025 | 41 | 10/11/2026 | 10/17/2026 |
| 42 | 10/12/2025 | 10/18/2025 | 42 | 10/18/2026 | 10/24/2026 |
| 43 | 10/19/2025 | 10/25/2025 | 43 | 10/25/2026 | 10/31/2026 |
| 44 | 10/26/2025 | 11/1/2025 | 44 | 11/1/2026 | 11/7/2026 |
| 45 | 11/2/2025 | 11/8/2025 | 45 | 11/8/2026 | 11/14/2026 |
| 46 | 11/9/2025 | 11/15/2025 | 46 | 11/15/2026 | 11/21/2026 |
| 47 | 11/16/2025 | 11/22/2025 | 47 | 11/22/2026 | 11/28/2026 |
| 48 | 11/23/2025 | 11/29/2025 | 48 | 11/29/2026 | 12/5/2026 |
| 49 | 11/30/2025 | 12/6/2025 | 49 | 12/6/2026 | 12/12/2026 |
| 50 | 12/7/2025 | 12/13/2025 | 50 | 12/13/2026 | 12/19/2026 |
| 51 | 12/14/2025 | 12/20/2025 | 51 | 12/20/2026 | 12/26/2026 |
| 52 | 12/21/2025 | 12/27/2025 | 52 | 12/27/2026 | 1/2/2027 |
| 53 | 12/28/2025 | 1/3/2026 | — | — | — |
Which MMWR Week Ends in 2026?
A frequent query in surveillance operations is determining which MMWR weeks end within the 2026 calendar year:
- MMWR Year 2025, Week 53: Runs from Sunday, December 28, 2025, and ends on Saturday, January 3, 2026. This is the first MMWR week that concludes in calendar year 2026.
- MMWR Year 2026, Weeks 1 through 51: Week 1 begins on Sunday, January 4, 2026 (ending Saturday, January 10, 2026). Weeks 1 through 51 conclude consecutively on Saturdays throughout 2026, with Week 51 ending on Saturday, December 26, 2026.
- MMWR Year 2026, Week 52: Begins on Sunday, December 27, 2026, and ends on Saturday, January 2, 2027. Therefore, MMWR 2026 Week 52 does not end in 2026; it concludes in calendar year 2027.
Mathematical Formula and Calculation Rules
To calculate the MMWR week and year algorithmically from any given date D:
- Find the Sunday preceding or equal to D:
Sunday(D) = D - DayOfWeek(D), where Sunday = 0, Monday = 1, …, Saturday = 6. - Identify the Wednesday of that week:
Wednesday(D) = Sunday(D) + 3 days. - The MMWR year Y assigned to the date is the calendar year of that Wednesday:
Y = Year(Wednesday(D)). - Determine the first Wednesday of year Y, denoted W1(Y).
- Compute the MMWR week number:
MMWR_Week = 1 + floor((Wednesday(D) - W1(Y)) / 7 days).
Worked Example 1: Date January 1, 2026
Evaluating calendar date 2026-01-01:
- Day of week: January 1, 2026 falls on a Thursday.
- Preceding Sunday:
2025-12-28. - Associated Wednesday:
2025-12-31. - MMWR Year assignment: Because the Wednesday belongs to 2025, the assigned MMWR Year is 2025.
- First Wednesday of 2025: January 1, 2025 (
2025-01-01). - Days elapsed between Wednesdays:
2025-12-31 - 2025-01-01 = 364 days. - Week calculation:
1 + (364 / 7) = 1 + 52 = 53. - Result: January 1, 2026 belongs to MMWR Year 2025, Week 53.
Worked Example 2: Date January 6, 2026
Evaluating calendar date 2026-01-06:
- Day of week: January 6, 2026 falls on a Tuesday.
- Preceding Sunday:
2026-01-04. - Associated Wednesday:
2026-01-07. - MMWR Year assignment: The Wednesday belongs to 2026, so the MMWR Year is 2026.
- First Wednesday of 2026: January 7, 2026 (
2026-01-07). - Days elapsed between Wednesdays:
2026-01-07 - 2026-01-07 = 0 days. - Week calculation:
1 + (0 / 7) = 1. - Result: January 6, 2026 belongs to MMWR Year 2026, Week 1.
Surveillance Code Implementations (Python, R, Excel)
Data engineering and surveillance analytics pipelines in biostatistics and epidemiology automate MMWR week calculations across data intake feeds.
Python Implementation
import datetime
def get_mmwr_week(dt: datetime.date):
# Sunday is index 0
day_of_week = (dt.weekday() + 1) % 7
sun = dt - datetime.timedelta(days=day_of_week)
wed = sun + datetime.timedelta(days=3)
mmwr_year = wed.year
# First Wednesday of mmwr_year
jan1 = datetime.date(mmwr_year, 1, 1)
jan1_dow = (jan1.weekday() + 1) % 7
first_sun = jan1 - datetime.timedelta(days=jan1_dow)
first_wed = first_sun + datetime.timedelta(days=3)
if first_wed.year != mmwr_year:
first_wed += datetime.timedelta(days=7)
mmwr_week = 1 + ((wed - first_wed).days // 7)
return mmwr_year, mmwr_week
# Example
year, week = get_mmwr_week(datetime.date(2026, 1, 6))
print(f"MMWR Year: {year}, Week: {week}") # Returns 2026, Week 1
R Implementation
# Using the lubridate package
library(lubridate)
sample_date <- as.Date("2026-01-06")
epi_week_num <- epiweek(sample_date)
epi_year_val <- epiyear(sample_date)
cat(sprintf("MMWR Year: %d, Week: %dn", epi_year_val, epi_week_num))
Excel Formula Implementation
In Microsoft Excel, when date cell A2 contains the observation date, calculating the epidemiologic week uses the midpoint Wednesday logic. The Wednesday date formula is: =A2-MOD(A2-1,7)+3. Comparing this midpoint to the first Wednesday of that calendar year yields the exact MMWR week without requiring third-party macros.
Role in Public Health Practice and Research Administration
Public health reporting jurisdictions transmit aggregated case files through NNDSS feeds, which feed national surveillance platforms including CDC WONDER and the weekly MMWR summary publications. While distinct from broader public health administration frameworks, consistent week numbering guarantees that seasonal pathogen curves—such as respiratory syncytial virus, seasonal influenza (conventionally tracked from Week 40 to Week 39), and vector-borne diseases—remain directly comparable across calendar decades. Institutional studies linking pathogen surveillance datasets to observational cohorts should formalize week assignment methods within their research data management plans.
Frequently Asked Questions
Which MMWR week ends in 2026?
MMWR Year 2025 Week 53 is the first MMWR week ending in 2026 (concluding on Saturday, January 3, 2026). MMWR Year 2026 Weeks 1 through 51 end consecutively on Saturdays through December 26, 2026. MMWR Year 2026 Week 52 concludes in 2027 (on Saturday, January 2, 2027).
How does an MMWR week differ from an ISO 8601 week?
An MMWR week begins on Sunday and ends on Saturday, with Week 1 requiring at least 4 days in the new calendar year (the week containing the first Wednesday). An ISO 8601 week begins on Monday and ends on Sunday, with Week 1 defined as the week containing the first Thursday of the year.
Why do some MMWR years have 53 weeks?
A standard 52-week calendar contains 364 days. Because solar years contain 365 or 366 days, approximately 1.25 extra days accumulate annually. To keep epidemiologic weeks synchronized with the solar calendar, the CDC inserts an extra 53rd week approximately every 5 to 6 years (e.g., 2020 and 2025).
Which date field should researchers use to assign MMWR week?
Following NNDSS guidelines, researchers and state health agencies prioritize Date of Illness Onset when available. If onset is unrecorded, the established hierarchy proceeds to Date of Diagnosis, Date of Laboratory Result, and Date of First Report to the Public Health System.
Sources
- Centers for Disease Control and Prevention (CDC), National Notifiable Diseases Surveillance System (NNDSS). “MMWR Weeks Overview: Business Rules and Date Hierarchy.” Published by CDC NNDSS. URL: https://ndc.services.cdc.gov/wp-content/uploads/MMWR_week_overview.pdf (Read: 2026-11-07).
- Centers for Disease Control and Prevention (CDC), National Notifiable Diseases Surveillance System (NNDSS). “MMWR Week Log (2025-2026).” Published by CDC NNDSS. URL: https://ndc.services.cdc.gov/wp-content/uploads/MMWR-Week-Log-2025-2026.pdf (Read: 2026-11-07).
- Centers for Disease Control and Prevention (CDC), National Notifiable Diseases Surveillance System (NNDSS). “Surveillance Resources.” URL: https://www.cdc.gov/nndss/ (Read: 2026-11-07).
