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Visualized: COVID-19 cases in the Bay Area

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(Updated April 7 6:35 p.m. PT)

As cases of COVID-19 continue to develop in the Bay Area and at Stanford, The Daily is tracking the number of cases, including their reported causes and outcomes. All Santa Clara County data comes from daily reports by Santa Clara County Public Health and is available in full on the Stanford Open Data Portal. Other counties’ data are collected from their respective public health sites and are also available on the Stanford Open Data Portal. Check our live blog for updates on the coronavirus outbreak and Stanford’s response. For each of visualizations below, feel free to use the arrows in the top right corner to navigate to different views of the depicted data.

Bay Area COVID-19 Cases

Out of the Bay Area counties, Santa Clara County has consistently had the most cases, currently at 1,285 confirmed cases. Napa County has the fewest, at 23 confirmed cases. When adjusting case counts to each county’s population, San Mateo County has the largest case density, with about 76.5 confirmed cases per 100 thousand residents.

Santa Clara County COVID-19 Cases

Outcomes & Causes

In Santa Clara County, the number of hospitalized or isolated individuals makes up a vast majority of outcomes of COVID-19 cases. The recovery count is low since the disease takes approximately two weeks to run its course in mild cases (six weeks for severe cases), meaning we will not see a steady increase of recovered individuals a decent amount of time after cases began growing in earnest around March 3. Both due to latency and COVID-19’s mortality rate ranging from  1% to 3.4%, deaths also won’t begin growing until later, but not nearly as much as the number of recovered individuals.

Before April, the number of currently hospitalized individuals had been steadily growing alongside non-hospitalized cases of COVID-19 in Santa Clara County. When April recently began, however, currently hospitalized cases started to stagnate at around 280 hospitalizations. Fortunately, this does not seem to be a factor of Santa Clara County hospital beds being at full capacity, although ICU beds are seeing significantly increased occupancy due to COVID-19.

The cause of infection in Santa Clara County seems to have begun with a relatively tiny spike in international travel, where other case causes were due to close contact with known cases. Afterwards, international travel became a minor cause of contracting COVID-19. Presumed community transmission and close contact with known cases quickly overtook as the leading and fastest growing causes. Starting on March 25, SCC Public Health stopped tracking case causes, citing the recent influx of newly confirmed cases as being “likely associated with community transmission.” Santa Clara County Public Health told The Daily on March 29 that it plans “to release additional elements of [its] data dashboard in the coming days.” Until then, we have frozen our reported causes visualizations.

To mitigate these causes, most Bay Area counties, including Santa Clara, announced “shelter in place” orders to begin on March 17. The state of California quickly followed two days later. With several other states issuing equivalent statewide directives, one in four Americans is now under “shelter in place” or “stay at home” orders.

Age Breakdown

On March 25, SCC Public Health began reporting the age breakdown of confirmed cases. To give context, the age distribution of Santa Clara County is provided, where the difference in age distributions represents increased or decreased risk of having a confirmed case of COVID-19. As expected, adults over 50 are more at risk, and younger people (under 30) are less at risk. However, even adults from ages 31-50 seem to be at a similar increased risk, matching reports that a majority of confirmed COVID-19 cases in California are under 50.

Reports suggest that younger people are more likely to experience milder symptoms when contracting COVID-19, sometimes even experiencing no symptoms at all. Therefore, as a note of caution when interpreting these data, this distribution could possibly suffer from more of a bias due to limited testing than the other data depicted in this article. With limited testing capabilities, those with milder symptoms will likely be less represented in this age breakdown, meaning that the reduced risk of younger people may be overstated in SCC Public Health’s data. In South Korea, where testing capabilities are vastly increased, about 27% of confirmed cases were in their 20s as of April 6, compared to only 12.1% of confirmed cases in Santa Clara County.

Daily Cases & Hospitalizations

Based on current data, we see that the average daily growth rate of confirmed COVID-19 cases in Santa Clara County is about 15% since March 3. Assuming exponential growth, confirmed cases will double roughly every 5 days at this rate. For the total count of COVID-19 cases in Santa Clara County to be growing exponentially, the percent increase of daily cases should be relatively stable (and nonzero) and the daily number of COVID-19 cases should grow exponentially. For it to be growing linearly, the percent increase should be diminishing to zero and the daily number of cases should remain somewhat constant.

Given the rapidly changing nature of testing in the Bay Area and across the United States, the confirmed case counts and their growth might not be fully representative of how actual cases, including those that are unconfirmed, are developing. Confirmed cases can grow not only because of the spread of COVID-19, but also “as testing capacity increases” according to Santa Clara County Public Health. One way to analyze how actual cases are growing is to analyze the growth of hospitalizations; one can assume that a certain, relatively constant, percentage of people who contract COVID-19 develop cases severe enough to require hospitalization. When analyzing the hospitalization growth rate, we see a lower average daily percent increase of about 13% since March 3 in Santa Clara County. As COVID-19 continues to increase stress on hospital capacities across the Bay Area, hospitalizations may incrementally become less representative of actual case growth in the Bay Area.

In the data we have thus far, we see that the daily percent increase of confirmed cases has been recently fluctuating around 10%, but it’s difficult to tell if daily cases are growing exponentially or linearly and if confirmed case growth owes more to the spread of COVID-19 or the expansion of testing. In order to determine this, we unfortunately must wait to see at which rate cases are reported in the coming days as testing expands.

Testing

Santa Clara County Public Health posted testing data on April 2. After accounting for 208 pending results and 449 undated tests, a total of about 11,607 individuals were tested, 10,175 of which came out negative. Assuming that COVID-19 spreads at an exponential rate, tests must match that rate in order to properly document the number of cases in a given area. Looking at the daily increases of tests, early to mid-March showed signs of a quickly expanding testing infrastructure, increasing around 50% every day from March 6 to March 19. Testing increase rates then began steadily decreasing, with daily testing capping out at around 650 in late March. Since March 27, the daily number of tests has actually been decreasing.

With a ceiling on how many tests can be administered in a given day, a strained testing infrastructure exacerbates how much of an underestimate reported cases in Santa Clara County are. Another sign of a strained testing infrastructure is a high positive cases to total tests ratio. With a high amount of positive cases per test, there is a higher likelihood that tests are being used for individuals with more obvious and progressed symptoms and leaving out those with mild or no symptoms. While such a strategy is an efficient use of a low amount of tests, it is also imperative to test, isolate and report asymptomatic individuals with COVID-19 since such individuals can “expose a far greater portion of the population to [COVID-19] than would otherwise occur” according to a March 16 study in Science by Li et al. According to SCC Public Health, their reported number of cases “represent only a small portion of the total number of likely cases in the county.”

As more cases develop and more data is collected, The Daily will continue adding analysis to this post tracking the local spread of COVID-19. In other areas affected by COVID-19, the number of cases has grown at an exponential, not linear, rate. Precautionary steps such as staying home, washing your hands and practicing social distancing can reduce the overall growth rate and “flatten the curve.”

A previous version of this article mistakenly attributed the 43 cases from March 9 to March 8. Cases from March 14 were also collected too early in the day, meaning 79 total cases were reported instead of 91. This led to a misrepresentation of how cases grew the following day. The Daily regrets these errors.

Contact Dylan Grosz at dgrosz ‘at’ stanford.edu.

Dylan is a senior majoring in Symbolic Systems-AI and minoring in Economics. He very much enjoys playing guitar, listening to music, and reading FiveThirtyEight. As a Senior Data Team Writer for The Stanford Daily, Dylan hopes to offer his data-driven approach to journalism as a vessel for others to navigate the vast, stormy seas of society. He will also usually do so in an overly dramatic metaphor.