Five Things to Remember When Interpreting Epidemiologic Data
Data will change some over time.
VDH gets data on COVID-19 from a number of different sources. Laboratory results, morbidity reports, death certificates, medical records, and patient interviews are a few of the ways we collect data. Sometimes these different sources will disagree on something. For example, we may get a positive lab result that doesn’t have the patient’s address. To count this case, we use the address of the doctor who ordered the lab test. During the course of the interview, we may find out that the case-patient sought care from their doctor in one county, but actually lives in a different county. In another example, we may receive a report of a case- patient who has all of the symptoms of COVID-19 and meets the criteria for a ‘Probable’ case. If later laboratory testing comes back negative, then we won’t count that person as a case anymore. Every time that we report data, we are reporting the most up-to-date information we have, even if it’s different from what we reported before.
The data we share is an underrepresentation of COVID-19 in Virginia.
We know that the number of cases we have on record is an underrepresentation of the true burden for several reasons. Some underrepresentation is because testing for SARS-CoV-2 might not be available for the infected person.. Another factor is that not everyone will need to see a doctor for COVID-19. The World Health Organization (WHO) published a very detailed report about the outbreak of COVID-19 in China and found that 80% of cases were mild or moderate. Since then, there have been studies that have identified infections in people who never develop symptoms. If someone gets infected and recovers on their own, then public health may never find out about the case.
This data is based on a case definition.
Public health uses standardized case definitions to count cases. These case definitions make it easier to compare data over time, across states, or even between different counties. 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. For COVID-19, Virginia uses the CDC COVID-19 confirmed and probable case definitions. These definitions suggests that we report two case statuses:
- Confirmed cases – Confirmed cases include anyone who tests positive for SARS-CoV-2 RNA in a clinical or autopsy specimen using a molecular amplification test.
- Probable cases – There are a few ways to identify a probable case. In Virginia, anyone who is positive using an approved antigen test or anyone who displays a specific set of symptoms and has an epidemiologic linkage (contact with another confirmed or probable case or part of a risk cohort), or anyone whose death certificate mentions COVID-19 or SARS-CoV-2 without a positive lab result counts as a probable case.
Our data are intended to answer questions about the epidemiology of COVID-19.
There are a few different sources that provide data on the COVID-19 pandemic, and the numbers may be different for things that sound the same. That’s because these different sources have different purposes. The Virginia Hospital and Healthcare Association (VHHA), for example, has a great dashboard that includes the number of hospitalizations for people who have tested positive for COVID-19 or who have tests pending. These data are intended to help measure the current burden on the healthcare system and to help hospitals prepare for a surge. These data do not have the same kind of rigorous case definition that epidemiologic case data do because they are not intended for the same purpose. For healthcare system preparation, an overestimation is better than an underestimation. VDH reports hospitalization data among identified cases so that we can measure the relative severity of the disease. For our purposes, it’s important that the same case definition be applied to the numerator (the number of cases that result in hospitalization) and the denominator (the total number of cases).
There are limitations to the data we share.
Public health epidemiologists work hard to make sure we can present the best data possible, but there are limitations to any data source. We’ve presented some of the issues above, but there are many other complexities that we work with on a daily basis. 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.