Five Things to Remember When Interpreting Epidemiologic Data
1. Data changes over time.
VDH collects data on COVID-19 from a number of different sources. Laboratory, morbidity and case reports, vital statistics (i.e., death certificates), medical and vaccination records, and information collected during case investigations (i.e., patient interviews) are many of the data sources used. Sometimes these sources contain different information. For example, we may get a lab report that does not include the patient’s address. To count this case, we use the address of the patient’s doctor, but later when the local health department performs a COVID-19 case investigation, we may learn of the patient’s home address and update their COVID-19 case record. As a result, the case may be moved to a different county based on the change in address. VDH performs ongoing and comprehensive quality assurance on COVID-19 cases, hospitalizations, and deaths. Every time VDH reports data, we are reporting the most up-to-date information we have at that time. All data are preliminary and subject to change. More information can be found in the Quality Assurance Steps for COVID-19 Data.
2. Data we share underrepresents the true number of COVID-19 cases in Virginia.
The number of COVID-19 cases reported by VDH is likely less than the true burden of COVID-19 in Virginia. Not everyone with COVID-19 develops illness; as a result they may not seek medical treatment and then are not tested. For persons who do get sick, not everyone has access to or will seek medical treatment. Additionally, not everyone who is sick will seek testing to confirm their illness is caused by COVID-19. VDH may never know about these cases.
3. Cases are classified using a standardized case definition.
Public health uses standardized case definitions to determine who is a case and who is not for epidemiological surveillance. Having a case definition makes it possible to compare data trends over time, across counties, and between Virginia and other states. A case definition is different from a diagnosis and is used for a different purpose. A diagnosis is helpful for treatment and medical billing while a case definition is used for public health surveillance to monitor the burden of disease. For COVID-19, Virginia uses the national CDC COVID-19 case definition for counting cases for surveillance.
Most often, a case is only counted once, even if a person has multiple positive tests. However, there are instances where a person may be counted as more than one COVID-19 case. This may happen when a person tests positive again after 90 days from the first test, or when a person tests positive for COVID-19 with a different SARS-CoV-2 variant, which could indicate potential re-infection. On September 1, 2021, VDH adopted the Centers for Disease Control and Prevention (CDC) COVID-19 2021 Case Definition. This definition provides criteria for counting a new case of COVID-19 in the same person separately from a case that was already counted in Virginia. Re-infection of COVID-19 is still being analyzed in Virginia.
4. Our surveillance data are intended to answer questions about the epidemiology of COVID-19.
There are different data sources used to summarize the COVID-19 pandemic, and the numbers may be different for things that sound the same. For example, the Virginia Hospital and Healthcare Association has a dashboard with the number of hospitalizations for people who have tested positive for COVID-19 or who have test results pending. These data help measure the current burden on the healthcare system and prepare hospitals for a possible surge in new cases. These data do not have the same kind of case definition that epidemiologic case data do because they are not intended for the same purpose. VDH reports hospitalizations using COVID-19 case investigations to estimate severity of disease. The hospitalization of a case is captured at the time the local health department performs the case investigation. This underrepresents the number of hospitalizations in Virginia because a person may become hospitalized after the case investigation. It is important that VDH uses the same case definition to report all COVID-19 cases, including those hospitalized.
5. There are limitations to the data we share.
Public health epidemiologists work hard to make sure data are presented as accurately and timely as possible, but there are limitations to any data source. We have presented some of the surveillance challenges above, but there are many other limitations to our data. VDH has experts in infectious disease epidemiology, community health, data visualization, and public communication working to make the data we share as accurate, useful, and easy to understand as possible.
*Originally posted May 1, 2020