Are cases important?

News reports in the U.S. of coronavirus test positivity do not necessarily interpret the metrics consistent with the public health objectives for collection of these data. Other metrics easily reviewed on data reporting websites can better serve the typical news viewer.

Why do we track a disease or, in public health terms, conduct surveillance? How do we define a case of a disease, and to what ends?

Surveillance can have many objectives, and the objectives can differ by disease. For example, the WHO objectives for influenza surveillance include these.

  • Determine when and where influenza activity is occurring, and who is affected.
  • Detect changes in the antigenic and genetic characteristics and antiviral sensitivity of influenza viruses.

These objectives can be used in decision making, such as to inform policy for prevention and treatment and to inform selection of virus strain for vaccine production. The objectives for a given surveillance system drive the definition of a disease case. The WHO case definitions for Influenza-Like Illness (ILI) and Severe Acute Respiratory Infection (SARI) are not necessarily intended to capture all cases, but rather seek to describe trends over time. Their intention is not for diagnosis and treatment in a clinical setting. Thus, the case definitions for Influenza-Like Illness and Severe Acute Respiratory Infection are based on symptoms of disease, like fever and cough, etc.

Notably, the surveillance case definition of COVID-19 adopted by the National Notifiable Diseases Surveillance System (NNDSS) of the CDC differs from the WHO surveillance case definition of influenza. The CDC adopted the position of the Council of State and Territorial Epidemiologists on April 5, 2020. The surveillance objectives include these.

  • Measure the potential burden of illness.
  • Detect community transmission.

The position of CSTE states that individuals who test positive while presenting no symptoms of disease need to be counted as cases. This differs from a case definition like the WHO’s because the objectives are different. The WHO surveillance objectives do not include measurement of the burden of disease.

What does this mean for the typical news viewer in the U.S.?

The first consequential event occurred on April 5, 2020. On that date, the CDC adopted the position of the CSTE, accepting a COVID-19 case definition that includes asymptomatic individuals. Thus, confirmed cases exhibited a jump in frequency due to a change to a more inclusive definition of a case.

There is another consequence of the change in definition that affects how the statistics of cases can be interpreted. The CSTE position of measuring the potential burden of illness appears to have driven the inclusion of asymptomic individuals in its case definition, perhaps over the question of consequences yet to be identified from asymptomatic infection. Counting the asymptomatic also appears to satisfy the objective of measuring community transmission. However, there is arguably some conflict between these objectives.

The sub-population of infected individuals who do not exhibit symptoms of COVID-19 have no observed burden of disease. The objective of detection of community transmission appears meaningful, but inclusion of the asymptomatic in case counts inflates the measurement of disease burden.

Still, it could be said that we do not know yet the long-term consequences of asymptomatic infection. This could be the point of the language in the position of the CSTE about measuring the potential burden of illness. The word “potential” introduces a challenge for intepretation that persists since it refers to an unknown. The most transparent reporting would quantify that uncertainty. It would be even more helpful to reduce that uncertainty with serological surveys of coronavirus antibody prevalence.

Given the multiple objectives in COVID-19 surveillance and the challenges of interpreting the COVID-19 case definition, the typical news viewer is better served by seeking reports of other measurements, like hospitalizations, deaths, ICU capacity, etc. These are better and more meaningful measures of disease burden than cases. The events of hospitalization and death are grave. It is unnecessary to inflate the gravity of those events with case counts that include asymptomatic individuals who appear not to suffer a disease burden. Focusing more incisively on the burdens of disease we can observe yields a clearer indicator of the status of the health of our communities.

Suggested sites for reviewing the data of hospitalizations and deaths:

  • Excess Deaths Associated with COVID-19. I find excess deaths the most meaningful gauge for the level of normalcy. When deaths return to a level that is no more than expected before coronavirus, the burden on the population has returned to normal. This chart plots expected deaths based on prior years and overlays it with experience. It skirts the whole question of COVID-19 case definition by presenting data for all-cause mortality. Vital statistics registries provide a very accurate picture of the burden of mortality, and this chart can be viewed by region.
  • Coronavirus in the U.S.: Latest Map and Case Count. New York Times. Links to each state with tabular summaries and plots of deaths.
  • COVID-19: Data. NYC Health. Reports hospitalizations and deaths.
  • Key Metrics by State. The COVID Tracking Project. Easier to navigate to individual states than the New York Times charts, but one advantage of the Times presentation is the time series are vertically aligned.