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Volume 26, Number 10—October 2020

High Proportion of Asymptomatic SARS-CoV-2 Infections in 9 Long-Term Care Facilities, Pasadena, California, USA, April 2020

Matt FeasterComments to Author  and Ying-Ying Goh
Author affiliations: City of Pasadena Public Health Department, Pasadena, CA, USA

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Our analysis of coronavirus disease prevalence in 9 long-term care facilities demonstrated a high proportion (40.7%) of asymptomatic infections among residents and staff members. Infection control measures in congregate settings should include mass testing–based strategies in concert with symptom screening for greater effectiveness in preventing the spread of severe acute respiratory syndrome coronavirus 2.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel human coronavirus that causes coronavirus disease (COVID-19). The disease was detected in the United States on January 20, 2020, and had caused >1.7 million cases and >100,000 deaths as of June 1, 2020 (1,2). As the pandemic continues, data consistently show that older adults, particularly those with >1 underlying medical conditions, experience higher hospitalization rates and increased vulnerability to in-hospital death (3,4).

Long-term care facilities (LTCFs) in the United States, including skilled nursing facilities (SNFs) and assisted living facilities (ALFs), are populated by older adults and adults needing residential care for underlying medical conditions who are at increased risk of more severe COVID-19–associated illness (4,5). ALF residents generally require a limited amount of care, such as help getting dressed or assistance with medications, whereas SNF residents have acute or chronic health conditions, or both, that require 24-hour onsite medical care and often rehabilitative care and therapy.

The city of Pasadena, California, USA, is an independent local public health jurisdiction that has a disproportionately high representation of older adults compared with other southern California local public health jurisdictions. More than 12.0% of Pasadena’s population is >70 years of age, compared with 9.1% in Los Angeles County (6), and Pasadena has >1,253 licensed SNF beds, which is 2.4 times the rate (per 100,000 residents) of SNF beds as in the Los Angeles County public health jurisdiction (7). The Pasadena Public Health Department (PPHD) recognized that this large population of medically fragile adults was at high risk for illness and death from COVID-19 as it spread through California, especially after early reports of nursing facility outbreaks in late February (8).

Extensive COVID-19–specific outreach and education efforts with skilled nursing facilities by PPHD staff began in late January 2020. In the second week of March, the PPHD received a report of laboratory-confirmed COVID-19 in a Pasadena resident (not facility-associated); a report of COVID-19 in a LTCF employee was received on March 31. By mid-April, the PPHD had opened investigations for facilities with >1 confirmed COVID-19 case in 14 of 15 SNFs in the city jurisdiction and 3 of 26 ALFs. By the end of April, 19 facilities in Pasadena had completed mandatory facilitywide screenings for SARS-CoV-2 to aid in the investigation and control of COVID-19 transmission.

The Study

Facilitywide testing of staff and residents was completed in all facilities by the end of April, with thousands of test results available by early May. This analysis was restricted to facilities with >3 linked cases. Facilities excluded from the analysis had singular cases, non–epidemiology-linked cases, or no reported COVID-19 cases at the time of the initial mass testing. Of the 19 facilities, 9 (8 SNFs and 1 ALF) had evidence of sustained transmission by investigation within the facility and were included in this analysis. Residents were included if they were listed on the facility’s census sheet on the day the investigation was opened. All types of staff, both clinical and nonclinical, were required to participate.

A case-patient was defined as a person with a nasopharyngeal swab specimen that tested positive for SARS-CoV-2 by real-time reverse transcription PCR (rRT-PCR) (9) at a commercial laboratory or the Los Angeles County Public Health Laboratory (Downey, CA, USA). Laboratory results were combined with case investigation data collected by PPHD public health nurses. Symptom data were extracted from case reports compiled during the case investigation (10), patient medical records (hospital and physician notes), and facility clinical staff assessments and records for residents. Residents and staff were classified as symptomatic if they had had >1 new or worsened signs or symptoms of COVID-19 in the 14 days before nasal swab specimen collection. Persons with subjective fever or temperature >100.0°F (37.8°C), muscle aches, cough, shortness of breath, fatigue, headache, new loss of taste or smell, sore throat, runny nose, nausea or vomiting, diarrhea, low oxygen saturation, or clinical oxygen requirement (as determined by the patient’s physician) were classified as symptomatic (11).

A total of 1,093 persons (608 residents and 485 staff members) were eligible for rRT-PCR testing for SARS-CoV-2 based on facilitywide testing strategies at the 9 LTCF sites (Table 1). Test results for 85.9% (938/1,093) of the staff and residents were obtained by PPHD, specifically 95.7% (582/608) of residents and 73.6% (356/485) of staff. The overall population (residents and staff) prevalence of SARS-CoV-2 among these 9 facilities was 67.3% (631/938). The overall prevalence of asymptomatic infection among those who tested positive was 40.7% (257/631). The prevalence of SARS-CoV-2 infection among staff involved with direct patient care, such as certified nursing assistants (CNAs), licensed vocational nurses (LVNs), registered nurses (RNs), and other caregivers (68.5%, 150/219) was higher than among those not providing direct patient care, such as activity, dietary, and maintenance workers (48.1%, 25/52). A larger percentage of female staff (62.5%) than male staff (46.5%) functioned in clinical roles. The prevalence of SARS-CoV-2 infection among all residents was 70.1% (408/582); among female residents, the prevalence was 71.4% (237/332), and among male residents, it was 68.4% (171/250). Female residents had a higher rate of asymptomatic infection (51.0%, 121/237) than male residents (47.4%, 81/171).

Varying levels of SARS-CoV-2 prevalence were identified across facilities. The lowest levels were among residents and staff in facility E (30.6% of residents [11/36], 20.0% of staff [7/35]), the highest among residents in facilities A (89.5%, 77/86), B (88.5%, 46/52), and C (87.8%, 65/74) and among staff in facilities D (26/26), F (25/25), and G (16/16) (Table 2). The prevalence of asymptomatic infection among staff members ranged from 17.4% (facility B, 4/23) to 30.6% (facility H, 11/36) (Table 2). The prevalence of asymptomatic infection among residents ranged from 19.0% (facility F, 8/42) to 85.7% (facility A, 66/77) (Table 2).


The ability of SARS-CoV-2 to spread rapidly among residents and staff in congregate settings poses a major infection control challenge. Our findings demonstrate a high proportion of asymptomatic infection, even within moderately affected facilities, and support the use of mass testing-based strategies in concert with symptom screening. Data from the facilitywide screenings indicate that the rate of asymptomatic infection among staff, on average, was 1 in 4, and among residents was 1 in 2.

Early in the COVID-19 pandemic, the supply of both nasopharyngeal swabs and test kits for SARS-CoV-2 rRT-PCR testing in the United States was extremely limited and made available only for symptomatic persons meeting certain criteria determined by the Centers for Disease Control and Prevention (CDC) (12). Diagnostic testing remained limited for many weeks, and LTCFs relied on symptom screening to exclude potentially infectious staff from work. On March 30, CDC published a change for the COVID-19 period of exposure risk from onset of symptoms to 48 hours before symptom onset (13). This change meant that symptom screening alone could be insufficient in protecting LTCF residents from contracting COVID-19 from asymptomatic, but infectious, staff, and studies have suggested a role for asymptomatic transmission in COVID-19 outbreaks (14).

Our findings demonstrate a high prevalence of both symptomatic and asymptomatic COVID-19 infection among residents and staff in 9 LTCFs. Because the potential for asymptomatic transmission of SARS-CoV-2 is concerning, for greater effectiveness, infection control efforts in LTCFs should include both mass testing–based strategies and symptom screening.

Dr. Feaster is the lead epidemiologist for the City of Pasadena Public Health Department, Pasadena, California, USA, and is responsible for communicable disease investigation and chronic disease health assessment for the city of Pasadena. Dr. Goh is the health officer and director for the City of Pasadena Public Health Department.



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Cite This Article

DOI: 10.3201/eid2610.202694

Original Publication Date: July 02, 2020

Table of Contents – Volume 26, Number 10—October 2020

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Matt Feaster, Pasadena Public Health Department, 1845 N Fair Oaks Ave, Pasadena, CA 91103, USA

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Page created: June 30, 2020
Page updated: September 17, 2020
Page reviewed: September 17, 2020
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