The rapid spread of the Omicron variant has served as a reminder that vaccination alone may not prevent new waves of COVID-19 infection. Nonpharmaceutical interventions like isolation are still playing a key role in controlling the epidemic, but this can force people to isolate for unnecessarily lengthy periods, placing a burden on both patients and society.

Now, a new study has demonstrated that isolation times can be reduced through the use of high-sensitivity antigen tests. Undertaken by a multinational team led by researchers at the Tokyo Foundation for Policy Research, the study simulated different isolation guidelines for COVID-19 patients.

It found that utilizing viral tests, particularly rapid antigen tests, to determine when to end isolation could shorten the isolation period while controlling the risk of releasing patients who are still infectious.

Currently, most countries prescribe a fixed isolation period for those testing positive. The CDC has recently changed its isolation guidelines, shortening the period from 10 to 5 days after a positive result. This update has been criticized by some experts, who note that patients could still be infectious after 5 days of isolation.

The updated CDC guidelines, moreover, do not include viral tests: “We opted not to have the rapid test for isolation,” CDC Director Rochelle Walensky said, “because we actually don’t know how our rapid tests perform and how well they predict whether you’re transmissible during the end of disease.”

Differences in Virus Persistence

The study’s author Keisuke Ejima, a senior fellow at the Tokyo Foundation for Policy Research and an assistant research scientist at Indiana University Bloomington’s School of Public Health, explains that isolation time could be reduced when high-sensitive antigen tests are used in determining end of isolation.

“Given that virus persistence differs from one patient to the next, a fixed isolation period may be too long for some while being too short for others,” Ejima said. “We found that using rapid antigen tests to ascertain loss of infectiousness is a better determinant of when to end isolation. This will help reduce the isolation period for those who are noninfectious while avoiding prematurely releasing infectious patients.”

False negative rates for antigen tests are not negligible, however, partially because they are less sensitive compared with PCR tests. Ending isolation, therefore, would require multiple, consecutive negative tests. How many negative results are needed for tests conducted at what intervals, though, requires further study.

Ejima and his colleagues assessed the risk and burden of different isolation guidelines using rapid antigen tests in a simulator mimicking the dynamics of a SARS-CoV-2 viral load. They found that when the detection limit of antigen tests is higher than infectiousness threshold values, additional consecutive negative results are needed to determine loss of infectiousness.

Additionally, to control the risk of releasing infectious individuals, rapid antigen tests need to be designed to have lower detection limits than infectiousness threshold values. The length of isolation would increase when the detection limit is higher than the threshold values.

“Antigen tests tell us if a viral load is above or below the detection limit,” Ejima points out. “If the detection limit is close or below the infectiousness threshold values, antigen tests could become reliable tools in ascertaining loss of infectiousness, enabling the optimization of the isolation period for each patient.”

This would benefit not just the patients themselves but society as well. “These findings have direct implications for our economic and educational activities,” Ejima added. “Should new variants emerge henceforth, additional research would be needed to ascertain the effectiveness of vaccines, treatments, and other factors influencing COVID’s viral dynamics.”

“Designing Isolation Guidelines for COVID-19 Patients Utilizing Rapid Antigen Tests: A Simulation Study Using Viral Dynamics Models,” medRxiv
https://www.medrxiv.org/content/10.1101/2022.01.24.22269769v1

Profile of Senior Fellow Keisuke Ejima
https://www.tkfd.or.jp/en/experts/detail.php?id=681