Molecular Testing Plays Large Role in Preventing COVID-19 Transmission

A mathematical modelling study conducted in the United Kingdom analyzed the reduction in the effective reproduction number achieved by polymerase chain reaction (PCR) testing, isolation of symptomatic individuals, screening of high-risk groups, and contract tracing.

Since the beginning of the pandemic, the main priority was to test hospitalized patients with suspected COVID-19 to ensure proper infection control and treatment. As testing becomes more common, the role of wider testing should be expanded to prevent transmission. Within the hospital setting, health care workers can contribute to the nosocomial spread of SARS-CoV-2. Requiring regular screening for high-risk groups, such as health care workers, could identify individuals who are asymptomatic or have a mild COVID-19 infection and reduce transmission if these individuals self-isolate.

The mathematical model was developed to evaluate the impact of self-isolation and quarantine of contacts of laboratory-confirmed cases. The model assumed lower infectiousness in persons who were asymptomatic. Based on reports by the Foundation for Innovative New Diagnostics, the specificity of the PCR testing was assumed to be 100%.

If individuals with COVID-19 symptoms self-isolated, and no further transmission occurred, there would be a reduction in the effective reproduction number of 47%. Utilization of PCR testing would decrease the number of symptomatic individuals self-isolating. A negative test could release any persons who were quarantined due to exposure to household members with COVID-19. However, this would also reduce the effectiveness of self-isolation as some persons would receive false negative tests.

The introduction of regular PCR testing to high-risk groups like health care workers could reduce transmission. For regular testing to be effective, the testing depends on frequency, timeliness, and sensitivity. The model demonstrated that a weekly screening of health care workers with a 24-hour delay from testing to self-isolation reduces the effective reproduction number by 23%, in addition to any reduction already achieved by self-isolation following symptoms. If testing were done at the end of each shift and results were available prior to the start of the next shift, the effectiveness of the PCR test would increase to 32%. If testing sensitivity were to decrease, so too would the effectiveness of the weekly screening. A 15% reduction in test sensitivity decreases the effectiveness of a weekly screening from 23% to 19%.

Contract tracing efficacy is highly dependent on the number of symptomatic individuals who are tested, the success in tracing contacts, and the timeliness of test results and tracing contacts. The model assumed a contract tracing coverage of 80% and a period of 24 hours from sample collection to quarantine of contacts. This results in a reduction of secondary infections by 26%. Combining the impact of quarantining both symptomatic individuals and the contacts, the model demonstrated a 61% reduction in the effective reproduction number.

The researchers suggested that contract tracing should only be conducted based on symptoms and should not wait for a positive test result. This would avoid false negatives and would decrease the time from exposure to self-isolation. If the time from exposure to a symptomatic individual to quarantine was 12 hours, the reduction in transmission would increase from 26% to 31%.

However, the researchers suggest that in times of high disease incidence, contacts should be tested and only those who are positive should be put into self-isolation. If the time to testing is increased, the probability of a false negative decreases. If contract tracing remained at 80% and there were a 48-hour delay to contract tracing, the deduction in transmission is 10%. However, if coverage decreases to 50%, the reduction in transmission is 4%.

PCR testing could additionally be used to determine the immune status to COVID-19. Individuals who possess detectable antibodies could return to work and could be exempt from further self-isolation following exposure. This implication of an “immunity passport” presents several challenges. If a test has poor sensitivity, individuals who truly do have immunity may not show a presence of antibodies on the test results. Additionally, it is currently unknown how long antibodies to COVID-19 protect an individual.

Overall, PCR plays an important role not only in monitoring the COVID-19 pandemic but also in identifying the extent of transmission.

—Audrey Amos

Reference:
Grassly NC, Pon-Salort M, Parker EP, et al. Comparison of molecular testing strategies for COVID-19 control: a mathematical modelling study. The Lancet Infectious Diseases. 20(12); 1381-1389. PrePrint August 18, 2020. doi:10.1016/S1473-3099(20)30630-7