About the Data
Conceptualized by: International Vaccine Access Center, Johns Hopkins Bloomberg School of Public Health (www.jhsph.edu/ivac/): Melissa Higdon, Maria Deloria Knoll, Maria Garcia Quesada, Julia Bennett
This page was last updated on Thursday, August 6, 2020 at 03:00 AM EDT.
These charts lay out the key metrics for understanding the reach and severity of COVID-19 in a given area: number of new daily cases, tests per 100,000 people (testing rate), and percentage of tests that are positive (positivity rate).
As testing capacity increases, considering confirmed new cases, testing rates, and percent positivity together gives us a fuller picture of COVID-19 in a particular state or region. Under these conditions and stable testing practices, trends in daily cases can be cautiously interpreted as trends in transmission of the virus. Leaders can then make informed decisions about lifting social distancing and other transmission control measures.
Positivity rates can tell us whether a state’s testing capacity is sufficient. Ideally, a state should be meeting or exceeding the recommended positivity rate, which the WHO has set at 5%. A positivity rate over 5% indicates a state may only be testing the sickest patients who seek out medical care, and are not casting a wide enough net to identify milder cases and track outbreaks.
Percent positivity can also help us determine if an increase in cases is simply the result of expanded testing or if it signals increased transmission of the virus. If we see the percentage of positive tests begin to rise, it indicates insufficient testing to find infections that may be occurring. Not finding these infections may mean that the virus is transmitting without intervention, which can lead to future case growth.
If a rise in cases is the result of increased testing, the percent positive line could look flat or like it is decreasing over the time period when cases increased.
If a rise in cases is the result of increased transmission, the line could appear to be increasing over that same time period.