Drugstore rapid COVID-19 tests, less sensitive than gold-standard PCR tests, might yield a negative result in 15 minutes while failing to detect virus particles but those same particles might pose no risk of transmission in the very short term, according to a report posted on Thursday on medRxiv ahead of peer review.

The researchers first performed these antigen tests on swab samples from 181 individuals with PCR-confirmed SARS-CoV-2 infections. Next, they took the virus from the swabs and tried to grow it in test tubes. When viral loads were below the antigen tests' level of detection, the virus particles were often incapable of growing.

People with low viral loads and negative antigen tests may become infectious "a day or two or three days later," said Dr. James Kirby of Beth Israel Deaconess Medical Center in Boston. "Therefore, to be most effective, antigen tests should be used immediately before an event or contact with those at greater risk from infection." The swabs must be collected for testing carefully, following the instructions provided with the testing kits, he added. "In other words, you want a really good sampling of the inside of your nose."