COVID-19 Data and Definitions

Data & Definitions

COVID-19 Data & Definitions

Information about COVID-19 including daily data updates can be overwhelming and confusing. MCW and industry experts break down the lingo to help you understand the meaning behind the numbers and technical terms.

Quick Glance Stats

Updated 10/20/20


new daily positive cases in wisconsin


positive cases as % of total cases for past 7 days - wisconsin


new daily positive cases in milwaukee


positive cases as % of total cases for past 7 days - milwaukee

COVID-19 Data Definitions

Burden (Case Rate): Total number of cases per 100,000 Wisconsin residents in the last two weeks. Low = case rate is less than or equal to 10; Moderate = case rate is greater than 10, but less than or equal to 50; Moderately High = case rate is greater than 50, but less than or equal to 100; High = case rate is greater than 100.

Case Fatality Rate: Proportion of deaths from a disease compared to the total number of people diagnosed with the disease.

Deaths: Deaths per day, caused by COVID-19 Deaths per week, caused by COVID-19.

Hospitalizations/ICU use: Number of COVID-19 patients who have been hospitalized or put in the ICU.

Infection Fatality Rate: Proportion of deaths from a disease compared to all infected individuals. Differs from Case Fatality rate in that it tries to account for undiagnosed and asymptomatic cases.

Mortality Rate: Deaths caused by a disease in a population over a period of time. Mortality rates are hard to predict, and may be better for looking at the past.

Non-surge capacity of hospital & ICU beds & ventilators: Hospital and equipment capacity for normal, day-to-day hospital needs.

Positivity Rate: Percentage of people who test positive for the virus out of the total number of people who have been tested.

R0 or Reproductive Number: The expected number of cases directly caused by one case. Or, how many people are infected by one sick person.

Testing Capacity: The number of tests able to be performed per day in Wisconsin. Limited by availability of supplies (reagents and swabs) and number of tests that can be analyzed in a certain period.

Trajectory (Percent Case Change): Percent change from previous week to current week. Includes a “p-test”, which indicates probability. The smaller the number, the more probable it is that the trajectory is shrinking. Shrinking = percent change in cases is less than or equal to -10%, and the p-value is less than 0.025; Growing = percent change in cases is greater than or equal to 10%, and p-value is less than 0.025; No Significant Change = any percentages or p-values besides those described by shrinking and growing.

Learn more about COVID-19 indicators (PDF)

Common COVID-19 Terms

Cluster: A group of COVID-19 cases identified at a specific location and time suspected to be greater than expected. Not all clusters are considered outbreaks. Clusters should be quickly and thoroughly investigated to prevent an outbreak or to implement health and safety measures to prevent further spread.

Contact: An individual who comes within 6 feet of an infected person for at least 15 minutes starting from 2 days before the start of COVID-19 symptoms. An infected person can spread COVID-19 starting from 48 hours (or 2 days) before they exhibit any symptoms or test positive.

Contact Tracing: An identification and notification process used by state and local health departments to slow or stop the spread of COVID-19 by letting people know they may have been exposed to someone with the virus. Through contact tracing, individuals are advised to self-monitor for symptoms, receive help in getting tested, be told to self-isolate or self-quarantine.

Herd Immunity: When a large percentage of the population becomes immune to a disease through vaccination and/ or recovery from infection. If enough people are immune, it limits spread and protects the entire community. The ability to achieve herd immunity for COVID-19 is unclear because it is unknown if infection results in immunity. Experts estimate that at least 70% of the population (more than 200 million people) would have to recover from, and be immune to, COVID-19 to achieve herd immunity. This could increase the risk of death, long-term health problems and healthcare system overload.

Isolation vs. Quarantine: Isolation separates those with COVID-19 from people who do not have the virus. Quarantine separates and confines those potentially exposed to prevent spread and see if they test positive or become sick. Both isolation and quarantine may be voluntary or required by federal, state, or local public health officials.

Outbreak: More cases of a disease at a specific time or place than expected. If the disease is new, rare, or has serious public health implications, an outbreak may be declared with as few as one or two cases.

Learn more about common COVID-19 terms (PDF)


Incidence and Growth Rate

These data show incidence of COVID-19 cases and testing over time. The maps show incidence in the Milwaukee region and the graph shows incidence in Wisconsin. 

Incidence of Cases

This map shows the total number of cases in Milwaukee County, and where the people who were diagnosed live. Many of the “hotspots” are places like prisons and nursing homes, where people are close together, and infection is spread easily.

COVID-19 Incidence of Cases Cumulative

Data Sources: Wisconsin Electronic Disease Surveillance Systems (WEDSS) (incidence data); 2018 American Community Survey (population data); City of Milwaukee Map Milwaukee Portal (neighborhood boundaries); Census Bureau TIGER/Line Shapefiles (census place boundaries)

Incidence of Testing

This map shows where tests are being conducted in Milwaukee County. More tests are being conducted at high-risk locations, like prisons and nursing homes, which can be seen when comparing the two maps.

COVID-19 Incidence of Testing Cumulative