Updated September 14
Note: Sacramento County halted updates to the Laboratory Testing data section of their COVID-19 dashboard on September 1, 2020 because the state of California’s tiered system for the risk level of counties uses different parameters to determine test positivity rate, as well as the number of new positive cases per 100,000 residents. The county is currently evaluating their methodology for updating this dashboard, but may resume updates in the future, according to a county spokesperson. This story has been updated to reflect the change.
As the COVID-19 pandemic in California and across the U.S. continues to rapidly evolve, data around the transmission of the virus are a key piece in understanding the spread.
But with the intermittent availability of coronavirus testing and recent technical problems with California's disease reporting system, it can be hard to understand what these data do (and don’t) tell us.
Understanding how public health data are collected and how epidemiologists work with that information can help shed light on virus transmission. Here are answers to some questions CapRadio’s journalists and audience have had about how Sacramento County collects and displays its COVID-19 data, and what it means.
Where can I get the latest data on COVID-19 in Sacramento County?
Sacramento County Public Health provides a data dashboard that contains regularly updated information on the number of cases, deaths and hospitalizations in the county.
When the county updates its dashboard each day, it doesn’t simply add new cases and deaths to the data that already exists on the dashboard. Every day, Sacramento County Public Health totally overwrites the dashboard with the latest, most up-to-date version of the county’s COVID-19 data. This can lead to changes in numbers from one day to the next as more up-to-date information becomes available, including the numbers of cases in certain places or demographic groups going down from one day to the next.
What data points should people look like to best understand the extent of COVID-19’s spread?
To get the most complete picture of COVID-19’s spread in the county, it’s best to look at a few metrics together to get a more nuanced understanding, according to Sacramento County Public Health. Each individual metric is a piece of the bigger picture.
The major indicators are the total number of reported cases, as well as the total number of deaths due to COVID-19. These numbers, like a lot of data related to the virus, are entirely reliant on people being tested and those test results making it to the county.
Another thing to watch is the number of people hospitalized or in the ICU currently. This, combined with the number of people who have died, gives an understanding of the number of severe cases of the disease, according to Sacramento County.
The count of new cases by episode date is also a good metric to monitor. The episode date is the closest date officials can get to when a person got sick. For some cases, this may be the diagnosis date or the date symptoms showed up. But for others, it may just be the date the case was reported or the date that their test sample was taken.
How can I visualize the spread of the virus, and the state of transmission now versus in past weeks?
Sacramento County provides something called an epidemic curve under the heading “Cases by Episode Date” on the county data dashboard. It shows the progress of the pandemic through the number of new cases by the episode date, which is health officials' best estimate of when a person got sick. That could be the date symptoms began, the date a person was diagnosed, the date a person was tested or the date the case was reported to the county, depending on the situation.
Ideally, these cases would be sorted by the date that a person was diagnosed with an illness. But in this case, the county often does not have a diagnosis date or symptom onset date for people, so officials try to get as close as possible to the date a person was infected.
If a person gets tested more than once, how do they show up in the data?
Even if a person tests positive multiple times, their case will only be counted once in the county’s total number of cases.
However, every test that a person takes is counted as part of the county’s test positivity rate, even if the tests are taken by the same person.
How should I interpret the county’s map of cases by ZIP codes?
Sacramento County’s map of COVID-19 cases by ZIP code shows the number of residents of each ZIP code in the county who have tested positive for COVID-19.
While it can be helpful and interesting to know how many people in a specific city or zip code can be helpful, Sacramento County Public Health said it’s important to remember that COVID-19 is everywhere at this point. One zip code having fewer cases than another doesn’t necessarily mean less risk of transmission there.
Also, because the county overwrites its dashboard each day with new data, these updates can sometimes lead to the cumulative number of confirmed cases in a ZIP code to go down from one day to the next. These data are dynamic, as sometimes county public health officials get more accurate information and end up correcting the data.
Why does this data change so much and come with so many caveats?
Real-time epidemiological data are often incomplete, according to the county. When more information becomes available about cases and test results, data from previous days may change as the county updates it with the latest information.
Typically, epidemiologists have a holding time before they release disease data, so that what is released is the most complete version, according to the county. In a situation like the COVID-19 pandemic where there’s a need and demand for real-time information, the data are more dynamic.
How reliable are day-to-day increases in the number of total cases and deaths in showing the recent spread of the virus?
While these increases do accurately tell us how many new cases and deaths were reported in a day, they’re not necessarily an accurate show of how many recent cases and deaths there have been.
Data on total cases and deaths are dependent on laboratories and providers reporting these instances and those reports making it to the county. That means day-to-day increases may not always show the actual number of cases or deaths that occured on a given day.
An example of this is the recent technical issues with CalREDIE, the state’s electronic disease reporting system. The problems caused a backlog of 295,000 test results, which led to undercounts of cases and then large numbers of newly reported cases as counties processed backlogged results.
When does the county inform people of possible exposure to COVID-19?
The county notifies people of possible exposures if it has reason to believe a person has had direct contact with someone who has tested positive. Direct contact means they have been within six feet of them for fifteen or more minutes. But people who hadn’t been in direct contact with the person would not necessarily be notified. People would not be notified by officials if someone in their neighborhood who they hadn’t had direct contact with tested positive, for example.
But this contact tracing and notification relies on people being willing to provide information on people they’ve had contact with outside their household when they test positive, which the county said more and more people have been reluctant to do.
How can we distinguish a surge in testing availability from a surge in cases?
When CapRadio first spoke with Sacramento County, the county suggested that residents look at the number of new cases, the number of tests that have been done recently and the county’s test positivity rate in combination to provide this context around what’s driving a surge in numbers.
For example, if there were a large number of new cases reported recently, it would be a good idea to examine the county’s laboratory testing data along with that increase. If there were also more tests performed than usual and the test positivity rate had remained somewhat the same, that increase in cases may be tied to an increase in testing.
However, the county halted updating their Laboratory Testing data dashboard on September 1 because the state of California’s tiered system for the risk level of counties uses different parameters to determine the test positivity rate, as well as the number of new positive cases per 100,000 residents. Sacramento County Public Health says it is evaluating the methodology it uses for its dashboard, and may restart these updates later on.
Because of this, as of September 14, the most recent test positivity number available for Sacramento County was for September 1. The county’s Laboratory Testing dashboard was updated on September 1, and the state’s data was updated on August 31.
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