Make sure everyone counts: Change the way we define COVID-19

This is what the public health officials need to do to provide more order in this catastrophe and to more accurately reflect the size of the pandemic.

Rather than only counting “confirmed” cases as we do in the U.S., we should adopt the COVID-19 case definitions recommended by WHO and the European CDC.  In those settings there are three tiers or types of cases depending on the degree of certainty.

Suspect case

A. A patient with acute respiratory illness (fever and at least one sign/symptom of respiratory disease, e.g., cough, shortness of breath), AND a history of travel to or residence in a location reporting community transmission of COVID-19 disease during the 14 days prior to symptom onset;


B. A patient with any acute respiratory illness AND having been in contact with a confirmed or probable COVID-19 case (see definition of contact) in the last 14 days prior to symptom onset;


C. A patient with severe acute respiratory illness (fever and at least one sign/symptom of respiratory disease, e.g., cough, shortness of breath; AND requiring hospitalization) AND in the absence of an alternative diagnosis that fully explains the clinical presentation.

Probable case

A. A suspect case for whom testing for the COVID-19 virus is inconclusive.1


B. A suspect case for whom testing could not be performed for any reason.

Confirmed case (this is the only category that we have now in the US)

A person with laboratory confirmation of COVID-19 infection, irrespective of clinical signs and symptoms per WHO technical laboratory guidance.

What does this approach offer us?

At this point with such limited testing available in the U.S., we are missing many cases. This means that our public health tools—including quarantine and isolation—are being used in limited circumstances and are confusing. 

Every question I got today—from community members and health workers—represented people trying to navigate the instructions for self-isolation and quarantine in the midst of this chaos. It does not have to be chaotic (or not this chaotic). Having more categories in the COVID-19 case definition, based on the degree of certainty, is how we epidemiologists usually do it.  That is how we did it with Ebola and cholera.  I don’t know why we are not using the usual approach in the U.S., but it is ill-considered.

Today, a person who tested negative last week returned to a local health care facility and was hospitalized with severe symptoms and a positive test. The nurses involved wondered why the early symptoms that the patient exhibited last week could not have been used to have the person start quarantine rather than have them moving about in the community and potentially exposing others.

At the very least we should combine the suspect and probable case definitions into one category and include everyone in it who meets one of those WHO definitions.  In addition, we cannot ignore clinical signs and symptoms and health worker judgment. Chinese doctors and public health officials complained mightily about the limited case definition they were forced to use. If you review the Chinese data, you will see that the day the number of cases spiked was when doctors revolted and treated COVID-19 clinically compatible cases in spite of negative test results or lack of available testing. They began to use their experience with cases and chest CT scans to advise their treatment and provide care.

This approach would not cause problems with analysis because suspect and probable cases would be counted separately—stratified—from the “confirmed” cases that have a positive laboratory test.  It would be better if we could follow persons as they changed from a suspect to a confirmed case definition and then re-categorize their status. At this time, when they are “suspect” cases and simply ignored and not counted.

Moreover, current limitations of much of the SARS-CoV-2 testing —relatively poor validity—are important. The RT-PCR-based testing has high specificity but low sensitivity.  Thus, it can miss a lot of cases.  In fact, based on consistent data in China and Italy, it is estimated that at least 30% of COVID-19 patients may have a negative test.  The reasons for this are not certain, but based on other diseases we can speculate.  Timing matters. For example, a person needs to have the viral infection long enough to produce enough virion-babies in the throat and nose to make a positive test. A test done too early may produce a false negative result.  Similarly, a test done late in the illness, as the patient is recovering, may also be too late to produce a positive result.

Specimen collection quality is also an issue. The RT-PCR test is performed using a swab specimen from the nose (best) or throat, and the swab must hit the right spot to provide an adequate sample. So, variation in specimen collection quality can be important.

Finally, characteristics inherent to the test may affect its sensitivity (the proportion of people with the infection that it actually picks up). Hopefully with time and further study, we will learn more about testing and be able to address these limitations.

As more testing becomes available, we need to be clear about which people are likely to have a false negative result because this information is needed to guide decisions about who can return to work and who should be quarantined or isolated.

At this point with so much community transmission and relatively poor testing performance, we cannot rely on it as the sole source of information. 

What are the costs of not doing this?

In the U.S. the instructions given to persons without a confirmatory laboratory test are not well enough defined.   Community members need to know when their contact with a COVID-19 case—suspect, probable or confirmed—or their symptoms warrant self-quarantine and when they can be released from it. 

Ultimately, perhaps not much longer from now, we will have more tools to use in our decision making.  When we have more testing available, we can more easily decide which patients we think should be tested; however, we cannot use the test results as the only factor in deciding whether patients should be in quarantine. In addition, when a test for antibodies to SARS-CoV-2 is available, suspect and probable cases can be given clear information about whether they are now immune and what precautions, if any, they might need as they go about their lives.  This information will continue to be important over the years to come. We want an army of known immune people to help us move through this crisis.

Finally, the psychology is important here, and it is dispiriting to those in NYC, Seattle, and soon, many other cities and states, to not be counted or validated.  By counting only persons with positive tests as cases, we are participating in a kind of “abandonment” of individuals who could not get tested or who might have had a false negative test. They are also building our experience and our herd immunity, and they deserve to be recognized and counted. 

This is not that hard, people. Whoever makes these decisions— please, just do it.

Published by sharon Mcdonnell

Medical Epidemiologist and Community Memeber

2 thoughts on “Make sure everyone counts: Change the way we define COVID-19

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: