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How do we count our COVID-19 cases?

Anytime we see an outbreak of any disease, public health identifies the people who have the disease and monitors the number of cases. This work helps guide how we prevent and control the disease and tells us when the outbreak is over. But diseases can affect every person uniquely. We find this especially true with COVID-19.

So, what do we do? We create case definitions based on a combination of clinical symptoms, laboratory testing, and epidemiologic criteria to classify diseases uniformly. Public health officials develop case definitions and consistently use them for all reportable infectious diseases, such as measles and foodborne illnesses. COVID-19 is the same.

Case definitions help us better track community disease. While they are essential to inform public health interventions to manage outbreaks, they don’t change individual patient diagnosis or care.

COVID-19 has three standard case definitions: suspected, probable, and confirmed. People who are confirmed or probable cases get the same public health guidance and recommendations. We ask them to self-isolate to prevent disease spread.

In Pierce County, we only report confirmed positive cases of COVID-19. To date, we have a total of 11,641 confirmed cases. We track probable and suspected cases, but we do not report them publicly. This may change in the coming weeks as we expect Department of Health to change how it reports probable cases.

As of Nov. 10, we have 1,210 people considered probable cases in Pierce County who reported symptoms and have a link to a confirmed case but no confirmatory test. We also have 128 probable cases who have a positive antigen test and no PCR test.

Learn more about the three cases definitions:

A confirmed COVID-19 case is someone who has a positive PCR (polymerase chain reaction) test. This test specifically detects genetic material from SARS-CoV-2, the virus that causes COVID-19. A positive test means the person is infected with SARS-CoV-2 and is contagious to others.

A probable case is the most complicated classification. It could mean one of two things: 1) The patient likely had a COVID-19 infection. Or 2) We believe the patient has COVID-19 but a PCR test has not confirmed it. A probable COVID-19 case must meet one of the following criteria:

  • Have symptoms of COVID-19 and an epidemiologic link to a positive laboratory case with no confirmed test result. An epidemiologic link means the person was in close contact with a confirmed case either by living in the same household or being within 6 feet for 15 minutes or longer over 24 hours.
  • Have a positive COVID-19 antigen test, which also detects the presence of the virus. These tests identify protein fragments found on or within the virus and are quicker than PCR tests but may not detect active infection. FDA approved antigen tests for Emergency Use several weeks after PCR tests were readily available.  Positive results from antigen tests are highly accurate, but negative results can be false and do not rule out an active infection with SARS-CoV-2.
  • Have COVID-19 listed as the cause of death on the death certificate but never had a confirmatory test.

A suspected case is someone who tests positive for antibodies to COVID-19 using a serology test and has no previous history of being a confirmed or probable case. The presence of antibodies in the blood occurs when the body responds to a specific infection, like COVID-19. Serology tests detect the body’s immune response to the SARS-CoV-2 virus’ infection rather than seeing the virus itself in the body. Antibodies can be present after a person’s immune system already cleared the virus. The presence of antibodies does not tell you if you have an active infection or a past infection.

Working from what’s called an open data set, every day we receive data from sources across the country:

  • Laboratories.
  • Hospitals.
  • Other local health jurisdictions.
  • State Department of Health.
  • Federal Centers for Disease Control and Prevention.

We must protect patient privacy while providing the most complete, most accurate data possible to you, the public. We will continue to re-examine how we present data to you as we move through the pandemic.

We will continue to work hard to be your reliable local source for public health information and data. During COVID-19 and beyond, we want to help you make choices to protect yourselves and your families.

To learn more about our data reporting, see past Your Reliable Source blogs, including “We’re your reliable local source for COVID-19 data.”

To learn tips on how to gather safely this holiday season see our Safe Gatherings webpage.

A chart showing the COVID-19 cases by day since February in Pierce County. The number are rising recently.