GUEST POST BY DR. JONATHAN DAVIDS
Conflicting advice in media stories may have led some to wonder whether or not rapid antigen tests are effective at detecting the Omicron COVID-19 variant.
The bottom line is that rapid antigen tests are an effective tool in detecting the latest variant when used correctly. Before Omicron, rapid antigen tests were most accurate two days before symptoms started and up to three days after symptoms develop. What has changed with Omicron is that the highest viral load of an individual with the virus corresponds to when symptoms develop rather than during an asymptomatic phase.
With Omicron, rapid antigen tests are most accurate when people are exhibiting symptoms as this is when the viral load is the highest. So the best time to use a rapid antigen test is two to three days after a known exposure or every two to three days for ongoing monitoring.
Rapid antigen tests continue to maintain a high rate of accuracy (90%¹) when used correctly and are an effective monitoring tool when used on an ongoing basis.
For ancestral strains of COVID like Alpha and Delta, rapid antigen tests picked up the virus in the asymptomatic phase, as early as three days after exposure. We knew that with the ancestral strains, people started shedding the virus two days before symptoms developed (the pre-symptomatic period). Since most people developed symptoms on day five after exposure, they started shedding virus as early as day three of exposure.
With Omicron, however, the incubation period is shortened to two-to-three days; individuals exposed to Omicron start shedding virus on day two or three after exposure. They also show symptoms at the same time. The rapid antigen tests still pick up disease approximately three days after exposure but, with Omicron, this corresponds to when symptoms develop rather than an asymptomatic phase like Alpha and Delta.
So, to summarize, the best time to use a rapid antigen test hasn’t changed – it is still two to three days after a known exposure. If we don’t know when exposure has happened, the ongoing testing strategy for surveillance screening (for workers in an employer-sponsored program, students in school, or individuals and families) remains testing every two to three days.
Ultimately, rapid antigen tests remain useful in confirming when symptoms are Omicron versus other non-COVID respiratory infections.
Dr. Jonathan W. Davids is an occupational medicine physician and Corporate Medical Director at COVIDdetect’s parent company, DriverCheck. He is involved in conducting COVID-19 testing and has overseen more than 600,000 COVID-19 lab-based, rapid on-site PCR, and rapid antigen tests for employers across Alberta, British Columbia and Ontario including testing done for migrant farmworkers, truckers and the general public on behalf of Ontario Health.
¹The sensitivity (ability to correctly identify a positive case) of the Rapid Response™ is 90.2%*.
Test kit manufacturer specifications:
Quantity: 5 tests/box
Result Time: 15 minutes
* When performed on a symptomatic individual by a trained individual. Performance has not been established for serial asymptomatic testing.