Asymptomatic COVID-19 Testing and Vaccination: Always Learning

Reviewed by Clare Marlin, MS, BSN, CIC, Shirley Ryan AbilityLab and Erica Stohs, MD, MPH, University of Nebraska Medical Center 

A new systematic review reports that an estimated one-third of COVID-19 infections are asymptomatic while two studies examining routinely screened healthcare workers demonstrate vaccination reduced both asymptomatic and symptomatic COVID-19 infections.

Throughout the COVID-19 pandemic, we have come to learn some individuals with a SARS-CoV-2 infection remain asymptomatic for the duration of their illness while still being able to spread the infection to others. Using extensive databases, the authors of a systematic review in the Annals of Internal Medicine reviewed 61 eligible studies, which met their inclusion criteria, of mass screening for SARS-CoV-2 infection to determine at least one-third of such infections are asymptomatic. The data reviewed by the authors included results from either polymerase chain reaction (PCR) testing of nasopharyngeal swabs to detect infection or antibody testing to detect current, or prior, infection with the authors acknowledging antibody testing is an evolving field of testing. The authors call for additional longitudinal studies using PCR testing that samples individuals who represent national populations, in addition to the use of an infection containment strategy such as frequent, in-home testing in order that asymptomatic and pre-symptomatic individuals can isolate. The prevention of the spread of the SARS-CoV-2 infection by asymptomatic individuals through surveillance testing, along with vaccination, are two critical actions which can be taken to curb the COVID-19 pandemic.

Demonstrating the value of surveillance testing for SARS-CoV-2 infection, data from routinely screened healthcare workers (HCWs) suggests vaccination reduced both asymptomatic and symptomatic COVID-19 illness. Two studies in the Journal of the American Medical Association this month examined incidence rates of COVID-19 in routinely screened HCWs who were vaccinated vs. those who not vaccinated. The first, by Angel et al., compared 5,953 (89%) vaccinated HCWs to 757 (11%) unvaccinated HCWs. Symptomatic COVID-19 developed in 8 vaccinated and 38 unvaccinated HCWs (IR: 4.7 vs. 159/100,000 person days) with a statistically significant adjusted IRR 0.03. Asymptomatic COVID-19 occurred in 19 vaccinated and 17 unvaccinated HCWs (IR: 11.3 vs 67/100,000 person days) with a statistically significant adjusted IRR 0.14. The authors also compared outcomes based on whether the HCWs were fully vaccinated and the timing post-vaccination.

The second study, by Tang et al., also examined routine screening of asymptomatic HCWs and targeted testing of symptomatic HCWs. The authors compared 5,217 vaccinated vs. 2,776 unvaccinated HCWs. Symptomatic COVID-19 developed in 22 vaccinated vs. 106 unvaccinated HCWs, again with a statistically significant adjusted IRR 0.16. Asymptomatic COVID-19 developed in 29 vaccinated vs. 79 unvaccinated HCWs (adjusted IRR 0.28, statistically significant).

The incidence rate ratios in both of these large samples of HCWs provide compelling evidence for preventing asymptomatic, and symptomatic, COVID-19 with vaccination. However, what is either unknown or incomparable are infection prevention and control practices at these facilities and their surveillance strategies. Asymptomatic COVID-19, which can be identified with testing, remains an important source of transmission but the emerging data on the impact of vaccination to reduce asymptomatic infection is encouraging.


  1. Oran DP and EJ Topol. The proportion of SARS-CoV-2 infections that are asymptomatic, a systematic review. Annals of Internal Medicine 2021;174(5):655-63.
  2. Angel Y et al. Association between vaccination with BNT162b2 and incidence of symptomatic and asymptomatic SARS-CoV-2 infections among healthcare workers. JAMA 2021;Epub May 6, 2021.
  3. Tang et al. Asymptomatic and symptomatic SARS-CoV-2 infections after BNT162b2 vaccination in a routinely screened workforce. JAMA 2021;Epub May 6, 2021.
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