Finding Hidden COVID-19 in Urgent Care: Santa Clara County, March 2020

On February 27, 2020, Santa Clara County, California, found its first case of COVID-19 with no known source — no travel and no contact with a confirmed case — suggesting the virus was already spreading in the community. At the time, testing was reserved for people with lower respiratory illness plus travel or known contact, or for hospitalized patients with no other diagnosis. So the county health department, the California Department of Public Health and CDC set up sentinel surveillance to see how far it had spread.

How it worked

From March 5 to 14, 2020, four urgent care centers — chosen for their spread across the county, their mix of adult and child patients, and their capacity to collect and ship specimens — took part.

1. County residents with respiratory symptoms (fever, cough, shortness of breath), no recent international travel to an affected area and no known contact with a COVID-19 case were swabbed for influenza, as part of routine care, and told their sample might also be tested for SARS-CoV-2.
2. Because testing capacity was limited, the first 5–10 flu-negative specimens each day went to the county public health laboratory for SARS-CoV-2 testing by CDC's RT-PCR assay.

What they found

If no one had both flu and COVID-19, and if the untested flu-negative patients were like those tested, then about 8% (19 of 226) of people with respiratory symptoms at these centers had COVID-19 — similar to the 5% found among patients with mild flu-like illness at a Los Angeles medical center around the same time.

People who tested positive were told, isolated, and their contacts traced.

What the county did

The results, with a growing number of cases of unknown source, confirmed community transmission and prompted action:

The county also issued guidance for people at high risk, long-term care facilities and hospitals.

Why it matters

Knowing whether a virus is spreading in the community shapes testing criteria, quarantine advice, investigations and mitigation — and mitigation works best when it starts early. Health departments can use sentinel surveillance to judge community spread and decide the scale, timing, length and focus of their measures. Combined with hospital and mortality surveillance and antibody surveys, sentinel sites in clinics and emergency departments give a robust picture of an epidemic.

Limits

1. SARS-CoV-2 testing used a convenience sample of flu-negative specimens.
2. The numbers are small and come from four sites, so they may not represent the whole community.

Sources

Based on Zwald ML, Lin W, Sondermeyer Cooksey GL, et al., "Rapid Sentinel Surveillance for COVID-19 — Santa Clara County, California, March 2020," MMWR Morbidity and Mortality Weekly Report volume 69, number 14, Centers for Disease Control and Prevention, first posted as an Early Release on April 3, 2020; a work of the United States government in the public domain.