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Low yield of severe acute respiratory coronavirus virus 2 (SARS-CoV-2) asymptomatic routine screen testing, despite high community incidence

Published online by Cambridge University Press:  24 June 2021

Dylan C. Kain
Affiliation:
Department of Medicine, University of Toronto, Toronto, Canada
Nathan M. Stall
Affiliation:
Department of Medicine, University of Toronto, Toronto, Canada Division of General Internal Medicine and Geriatrics, Sinai Health, Toronto, Canada
Allison J. McGeer
Affiliation:
Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Canada
Gerald A. Evans
Affiliation:
Department of Medicine, Queens University, Kingston, Canada
Vanessa G. Allen
Affiliation:
Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Canada Public Health Ontario, Toronto, Canada
Jennie Johnstone*
Affiliation:
Department of Laboratory Medicine and Pathobiology, University of Toronto, Toronto, Canada Department of Infection Prevention and Control, Sinai Health, Toronto, Canada
*Corresponding
Author for correspondence: Dr Jennie Johnstone, E-mail: jennie.johnstone@sinaihealth.ca
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Abstract

Type
Letter to the Editor
Copyright
© The Author(s), 2021. Published by Cambridge University Press on behalf of The Society for Healthcare Epidemiology of America

To the Editor—Serial asymptomatic screen testing of staff with reverse-transcriptase polymerase chain reaction (RT-PCR) has been used in some jurisdictions as one strategy to help prevent severe acute respiratory coronavirus virus 2 (SARS-CoV-2) outbreaks in high-risk settings. 1 Results from other serially screened populations during periods of low community SARS-CoV-2 incidence demonstrated very low percent positivity (<0.5%) Reference Snoeck, Vaillant and Abdelrahman2,Reference Olalla, Corrrea and Martin-Escalante3 ; the yield in settings with high SARS-CoV-2 community incidence is less well understood. We investigated the percent positivity of routine asymptomatic screen testing in nursing-home staff during periods of both low and high SARS-CoV-2 incidence.

Methods

Beginning July 1, 2020, all nursing-home staff in Ontario, Canada, were required to undergo serial asymptomatic screen testing for SARS-CoV-2 by nasopharyngeal RT-PCR twice per month, 1 which then increased to once per week on November 22, 2020, in regions with weekly SARS-CoV-2 incidence of >25 per 100,000 population. Reference Steele4 Staff with a history of coronavirus disease 2019 (COVID-19) were exempt from testing. Data were collected centrally and included reporting from all 623 nursing homes in the province. Staff tested while symptomatic and homes experiencing COVID-19 outbreaks were excluded from the analyses. The study period was June 28, 2020, until March 13, 2021. Analysis and graphing were conducted using Microsoft Excel (Microsoft, Redmond, WA). The study was approved by the University of Toronto Research Ethics Board.

Results

In total, 705,370 SARS-CoV-2 screen tests were collected during the study period; of these, 1,147 were positive (positivity rate, 0.16%). During July and August 2020, when the weekly SARS-CoV-2 incidence in Ontario was <10 cases per 100,000 individuals, the positivity rate in asymptomatic nursing-home staff was <0.1%. During December 2020–January 2021, when weekly SARS-CoV-2 incidence in Ontario was >100 cases per 100,000, the asymptomatic screening positivity rates in nursing-home staff peaked at 0.36%.

Discussion

Our study confirms previous work showing that, during times of low SARS-CoV-2 incidence, the yield of routine asymptomatic screen testing is very low. We further show that in times of high SARS-CoV-2 incidence, positivity rose only modestly to a peak of 0.36% in contrast to the rise in percent positivity observed in symptomatic individuals. This finding contrasts an earlier study showing 13.7% positivity in systematically screened pregnant women in New York in late March and early April of 2020. Reference Sutton, Fuchs, D’Alton and Goffman5 This finding may be explained by the fact that, at the time, access to SARS-CoV-2 testing for the general public was limited. Also, without reporting on previous COVID-19–compatible illness associated with this testing, positive cases may have in fact had recovered illness with prolonged viral RNA persistence. Thus, staff in the study underwent serial symptomatic screening and screen testing and also had access to symptomatic testing. Although SARS-CoV-2 vaccination of nursing-home staff began December 14, 2020, the roll out was slow, with only 55% of staff receiving their first dose by February 23, 2021. Vaccination would not have impacted the results at peak in December 2020. Reference Brown, Stall and Vanniyasingam6

Our findings demonstrate that outside an outbreak or high-risk contact setting, the pretest probability of routine asymptomatic SARS-CoV-2 screen testing remains low, even in settings with high SARS-CoV-2 incidence. When pretest probability is low, the rate of false positives increases, further increasing the opportunity costs of such testing strategies (eg, unnecessary isolation of residents). Although routine screen testing has theoretical benefits, the low yield seen in our study, even during high SARS-CoV-2 incidence, means that in sectors with limited resources, these costs must be weighed against the limited benefit of such testing. The yield in those fully vaccinated against SARS-CoV-2 will be even further reduced, suggesting that any benefit from asymptomatic screen testing in fully vaccinated individuals unlikely apart from high-risk exposures and outbreaks.

Fig. 1. Ontario nursing-home staff screen testing SARS-CoV-2 percent positivity compared to provincial percent positivity, from June 28, 2020, to March 13, 2021. Line graph showing percent SARS-CoV-2 test positivity for routine asymptomatic nursing-home staff screening compared to provincial test positivity between June 28, 2020, and March 13, 2021. During the low SARS-CoV-2 incidence summer months of 2020, test positivity rates were consistently <0.1%. There was a small increase in SARS-CoV-2 test positivity during the second wave (September 1, 2020–March 14, 2021) peaking at 0.36% in December 2020.

Acknowledgments

Financial support

No financial support was provided relevant to this article.

Conflicts of interest

All authors report no conflicts of interest relevant to this article.

References

COVID-19 testing for long-term care home staff. Government of Ontario website. https://www.health.gov.on.ca/en/pro/programs/ltc/memo_20200716.aspx Published July 16, 2020. Accessed March 15, 2021.Google Scholar
Snoeck, CJ, Vaillant, M, Abdelrahman, T, et al. Prevalence of SARS-CoV-2 infection in the Luxembourgish population: the CON-VINCE study. medRxiv 2020. doi: 10.1101/2020.05.11.20092916.CrossRefGoogle Scholar
Olalla, J, Corrrea, AM, Martin-Escalante, MC, et al. Search for asymptomatic carriers of SARS-CoV-2 in healthcare workers during the pandemic: a Spanish experience. QJM 2020;113:794798.CrossRefGoogle Scholar
Steele, R. Updated long-term care sector surveillance testing strategy. Ontario Ministry of Long-Term Care website. https://ohwestcovid19.ca/wp-content/uploads/2020/11/20201124-DM-Memo-Updated-Long-Term-Care-Sector-Surveillance-Testing-Strategy-Nov-22-2020.pdf. Published online November 22, 2020. Accessed March 15, 2021.Google Scholar
Sutton, D, Fuchs, K, D’Alton, M and Goffman, D. Universal screening for SARS-CoV-2 in women admitted for delivery. N Engl J Med 2020;382:21632164.CrossRefGoogle ScholarPubMed
Brown, K, Stall, NM, Vanniyasingam, T, et al. Early impact of Ontario’s COVID-19 vaccine rollout on long-term care home residents and health care workers. Ontario Science Table 2021. doi: 10.47326/ocsat.2021.02.13.1.0.CrossRefGoogle Scholar
Figure 0

Fig. 1. Ontario nursing-home staff screen testing SARS-CoV-2 percent positivity compared to provincial percent positivity, from June 28, 2020, to March 13, 2021. Line graph showing percent SARS-CoV-2 test positivity for routine asymptomatic nursing-home staff screening compared to provincial test positivity between June 28, 2020, and March 13, 2021. During the low SARS-CoV-2 incidence summer months of 2020, test positivity rates were consistently <0.1%. There was a small increase in SARS-CoV-2 test positivity during the second wave (September 1, 2020–March 14, 2021) peaking at 0.36% in December 2020.

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