Serial Testing of Asymptomatic Health Care Workers

Last Updated: March 11, 2021

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This Briefing Note was completed by the Research, Analysis, and Evaluation Branch (Ministry of Health)
based on information provided by members of the COVID-19 Evidence Synthesis Network. Please refer to the Methods section for further information.


This note summarizes available evidence and recommendations on the serial testing of
asymptomatic health care workers (HCWs) in health care and congregate settings with direct patient contact before or after COVID-19 vaccination.

*The full version of the Briefing Note including the Appendix can be accessed in the PDF file at the top of the page*

Key Findings

Testing asymptomatic HCWs in congregate care

Vaccine effectiveness:

Transmission upon vaccination:

Recommendations for post-vaccination testing:

Analysis for Ontario

Overall, additional data is needed to inform an approach to serial testing of symptomatic and asymptomatic HCWs following COVID-19 vaccination.

Supporting Evidence

This section below summarizes findings from research and grey literature associated with testing symptomatic and asymptomatic individuals who work in health care settings and have direct patient contact (e.g., physicians, nurses, personal support workers, orderlies, etc.) following SARS-CoV-2 vaccination. For ease, these workers will be referred to as health care workers (HCWs) throughout this Evidence Synthesis Briefing Note. Details of the specific professions involved in each study are presented where available.

Scientific Evidence

Vaccine Effectiveness

  • Decreased viral load:
    • Israel: Emerging research (2021) on the Pfizer-BioNTech COVID-19 vaccine, BNT162b2, reported that vaccination reduces viral load by 1.6x to 20x in older patients (60 years and older) who are positive for SARS-CoV-2, potentially affecting viral shedding and contagiousness; additional research suggests viral load is reduced four-fold for SARS-CoV-2 infections occurring 12-28 days after a single dose of vaccine.
  • Infection prevention:
    • England: Emerging research (2021) suggested that the BNT162b2 mRNA vaccine effectively prevented both symptomatic and asymptomatic infection among HCWs and administrative staff working in hospitals; among antibody negative HCWs, vaccine effectiveness was 72% 21 days after the first dose, and 86% seven days after two doses.
    • Israel: After a single dose of the BNT162B2 vaccine, HCWs working at the Sheba Medical Centre had substantial early reductions in SARS-CoV-2 infection and symptomatic COVID-19 rates; of the 170 SARS-CoV-2 infections, 89 (52%) were unvaccinated, 78 (46%) tested positive after the first dose, and three (2%) tested positive after the second dose.
    • Israel: Among members of a state-mandated health provider, the first dose of the BNT162b2 vaccine is associated with an approximately 51% reduction in the incidence of PCR-confirmed SARS-CoV-2 infections at 13 to 24 days after immunization; similar levels of effectiveness were found across age groups, gender, sub-populations, and patients with various comorbidities.
  • Vaccine uptake:
    • England: A recent preprint study (2021) on the effectiveness of BNT162b2 mRNA vaccine against infection suggested that significantly lower coverage among HCWs in publicly funded hospitals was associated with prior COVID-19 infection, female gender, aged under 35 years, being from minority ethnic groups (especially Black), being employed as a porter/security guard, or midwife, and living in more deprived neighbourhoods.
  • Transmission upon vaccination:
    • Three top global evidence synthesis teams (i.e., COVID-NMA, the Copenhagen Trial Unit (CTU), and McMaster Health Forum) are currently developing living evidence syntheses that address questions about COVID-19 vaccine safety and effectiveness. They are committed to reporting the data if/when they become available.
  • Serial testing of HCWs:
    • No identified academic or grey literature addresses recommendations on serial testing of asymptomatic HCWs following COVID-19 vaccination.

International Scan

Impact of Post-Vaccination on HCWs

  • The US Centers for Disease Control and Prevention (CDC) provides recommendations, including viral testing, to minimize the impact of post-vaccination systemic signs and symptoms on health care staffing. These include:
    • Developing a strategy to provide timely assessment of HCWs with systemic signs and symptoms post-vaccination, including providing or identifying options for SARS-CoV-2 viral testing.
  • Suggested approaches to evaluating and managing new-onset systemic post-vaccination signs and symptoms in HCWs are provided in Table 2 in the Appendix.

Routine Aysmptomatic Screening of HCWs in Congregate Care Settings

  • Reports from five US jurisdictions (Washington State; Los Angeles, CA; Minnesota; Massachusetts; Detroit, MI) describe testing asymptomatic HCWs in long-term care facilities, in the early phase of the COVID-19 pandemic (March to June 2020).
    • US reports suggest that RT-PCR testing leading to rapid isolation of both symptomatic and asymptomatic staff and residents in long-term care facilities led to the prevention of COVID-19 outbreaks (Los Angeles, Minnesota).

Recommendations for Routine COVID-19 Testing

  • In August 2020, the US Centers for Medicare and Medicaid Services required nursing homes to test staff weekly in states that had a 5% or greater COVID-19 testing positivity rate to prevent outbreaks.
  • An article on repeat testing in skilled nursing facilities in Detroit, MI, suggests that with the increased availability of SARS-CoV-2 testing, repeated point prevalence surveys and enhanced and expanded infection prevention and control (IPAC) support should be standard tools for interrupting and preventing COVID-19 outbreaks in skilled nursing facilities (SNFs).

Canadian Scan

  • The BC Centre for Disease Control (BC CDC) suggests that HCWs, including those working in assisted living facilities, who experience symptoms other than local injection site reactions are advised to refer to the BC COVID-19 Self-Assessment Tool. This will inform HCWs if they must get tested. If HCWs require a COVID-19 test, the BC CDC advises the worker is not to return to work until they receive a negative test result.


The COVID-19 Evidence Synthesis Network is comprised of groups specializing in evidence synthesis and knowledge translation. The group has committed to provide their expertise to provide high-quality, relevant, and timely synthesized research evidence about COVID-19 to inform decision makers as the pandemic continues. The following member of the Network provided an evidence synthesis product that was used to develop this Evidence Synthesis Briefing Note:

  • COVID-End in Canada; and
  • Ontario Health.

This is version 3.0 of the Evidence Synthesis Briefing Note; the previous version was completed in early February, 2021.