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Tracking COVID-19 in California Counties

by Monice Wong | April 24, 2020

In mid-March when COVID-19 was beginning to spread at increasing rates across the United States, a classmate and I recorded and mapped positive cases in California counties from March 12 to March 19. We were shocked by the exponential increase in positive cases across the state over that week. Realizing the importance of tracking the novel coronavirus’s spread in California as well as the lack of organized, accessible county data, I decided to take the project further on my own. Every day since March 14th, I have tracked the number of COVID-19 cases in each of California’s 58 counties and compiled the data into an easy-to-use website. The project includes bar charts with the daily confirmed case counts for each county over time as well as maps with the case and mortality rates in each county. The data on daily case numbers is sourced from reports by county health departments, news outlets, and organizations like Johns Hopkins University. The simple graphs allow for easy tracking of the virus’s spread across all California counties. They also indicate whether the “curves” of the numbers of confirmed cases over time for each county are flattening following California’s shelter-in-place measures, which were implemented on March 19th. It is important to note however that there are other factors that may influence the shapes of the curves, such as when COVID-19 tests are returned and reported. The curves for the majority of counties are gradually flattening, reflecting a statement made by California Governor Gavin Newsom in a press conference on March 16th that “[Californians] have successfully bent and arguably flattened the curve in the state of California.” However, the curves for a number of counties, such as Los Angeles, continue to climb steadily. This is confirmation that we must continue social distancing measures in order to see the daily numbers of active cases decrease across California counties.

The confirmed number of cases in each county is a cumulative number including active cases, recovered cases, and fatal cases. Therefore the number of active cases is lower than the reported cumulative total. However, we must also remember that the actual number of total cases is likely higher than the reported number, as a result of testing inaccuracies, undertesting, and lags in test results. Tests for COVID-19 still have relatively high degrees of inaccuracy. Furthermore due to a shortage of tests, only some of the population is eligible to receive tests, such as hospitalized patients with symptoms or individuals with chronic medical conditions. People who have the virus but have mild to no symptoms may not get tested. Test results take days to weeks to be returned. Comparison between counties is also difficult because there are a number of factors determining the extent of virus spread within various counties, such as population density, the strength of the local healthcare system, and the extent to which local residents are following shelter-in-place measures.

There is still so much that we do not know about COVID-19, and while this pandemic is severe and frightening, it is also an opportunity for scientific research and discovery. I began this project with the goal of utilizing the public health knowledge and skills I’ve gained at UC Berkeley School of Public Health to do something impactful during this pandemic. I will continue tracking COVID-19 in each California county daily, and I hope that my work will be a useful resource for the public and researchers across California and beyond.

Monice Wong
Public Health Undergraduate

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