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 10/20/21


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: Two main measures that may vary from one data source to another. Person-based measure: calculated as percentage of people who test positive divided by total number of people tested (used by the Medical College of Wisconsin due to data availability). Test-based measure: calculated as percentage of tests completed that are confirmed positive.

Reproductive Number (R0): Number of people infected by one sick person. Number changes as behavior changes or immunity develops. If R0 is higher than 1, the number of cases increases exponentially. If R0 is lower than 1, the disease will eventually stop spreading.

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 how to navigate the numbers (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)