epidemiology-public-health-implementation, science-brief
| December 9, 2021
Rapid Antigen Tests for Voluntary Screen Testing
Individuals infected with the provincially and globally dominant SARS-CoV-2 Delta variant demonstrate viral loads that peak sooner after exposure compared to those infected with previous strains of SARS-CoV-2. High SARS-CoV-2 viral loads are likely present before symptoms.
These characteristics make the Delta variant challenging to control; however, they may also improve the performance and expand the practical utility of rapid antigen tests.
In this Science Brief, we determined whether cases with high viral loads/low cycle thresholds are likely to be infectious, and if so, whether rapid antigen tests can reliably detect those cases.
Voluntary screen testing entails regular voluntary testing of asymptomatic individuals to find cases in moderate-risk settings such as schools and workplaces. We found that rapid antigen tests can be a useful tool to reduce transmission in schools when used for voluntary screen testing to identify infectious cases in Public Health Units or neighborhoods using the following thresholds:
Once a Public Health Unit or neighborhood approaches 50 new COVID-19 cases per million per day (corresponding to 35 cases per 100,000 per week) and is in sustained exponential growth, we advise weekly voluntary screen testing of unvaccinated and incompletely vaccinated individuals in elementary schools. Public health units in this situation may also decide to deploy rapid antigen testing in other settings, such as workplaces and congregate settings.
If a Public Health Unit’s or neighborhood’s new daily case rate approaches 250 COVID-19 cases per million per day (corresponding to 175 cases per 100,000 per week), weekly testing is likely not frequent enough to reduce spread effectively. In that case, we advise testing unvaccinated and incompletely vaccinated individuals 2 to 3 times per week. Public health units in this situation may also consider voluntary screen testing of fully vaccinated individuals at the same frequency.
Rapid antigen tests may also present a valuable alternative to individual isolation after exposure in schools. Implementing voluntary “test to stay” protocols, where exposed students remain in school as long as daily tests are negative for SARS-CoV-2, could help prevent the harms of isolation without increasing transmission.
Individuals with positive rapid antigen tests should immediately isolate and undergo confirmatory Polymerase Chain Reaction (PCR) testing. Evaluation of the performance of rapid antigen tests for the diagnosis of the Omicron variant is urgently needed.
