A: For most people, the risk of spreading to COVID to others is likely minimal after Day 10 – but not so low that you can throw caution to the wind.
While rapid antigen tests can be useful as proxies for contagiousness, their value is unclear after Day 10. Infectious disease experts generally support exiting isolation after Day 10, even when antigen positive – if you’re feeling well. People who are immunocompromised or have moderate to severe disease are often contagious for more than 10 days. Masking after day 10 can help protect vulnerable people from exposure in indoor settings and in close proximity. As always, be sure to follow local guidelines and consult your health provider with questions.
🕐Note: Day 0 of your COVID infection is the first day of symptoms, or your first positive test, whichever comes first.
Unfortunately, we can’t tell you your risk of spreading COVID, because the ideal study has not been done – one that follows antigen positive people after Day 10 and tracks onward transmission.
The limited evidence we do have is reassuring for those who can’t shake that vexing test line. Several studies of the Omicron variant suggest that the overall transmission risk is very low at this late stage of infection. These studies inform isolation policies and help explain why even healthcare workers must often return to work after 10 days (or less) regardless of antigen test results.
➡️ A Spanish study reported that only 2% (8/356) of secondary Omicron cases were due to transmission after the 5th day of symptomatic disease. Antigen testing was not done.
➡️ A Japanese study of the Omicron variant could not grow any viable virus in the lab from anyone after Day 9 post diagnosis or symptoms. Antigen testing was not done.
➡️ A US study reported that roughly 1 in 4 symptomatic Omicron cases had viable virus in lab culture experiments at Day 8, dropping to 1 in 8 by Day 10. This study also compared rapid antigen tests and lab-based “culture” tests for viable virus. They found that after Day 5, a negative antigen test was a clear green light – it strongly predicted the absence of viable virus, but a positive antigen test was murky – it did not strongly predict the presence of viable virus (in lab tests).
At the same time, most experts won’t go so far as to say that there is no risk of transmission after 10 days. Why? Rapid antigen tests only light up when the swab has loads of fresh viral protein. It seems unwise to dismiss this fact until more definitive studies are done.
This quote from infectious disease expert Dr. Emily Landon captures the tricky situation:
👉”If you’re past day 10, you feel better and you’re not immunocompromised, and the rapid test line isn’t very dark or it’s taking longer to turn positive each day, you’re probably safe to be out in the world.”
❗”But if you’re going to be around vulnerable people, … then even a very faint positive line should make you think hard about being in contact with these folks”
🤓 Nerd Note: Neither rapid antigen tests nor lab “viral culture” experiments directly measure transmission risk. They are both imperfect proxies, but they are the best we have.
👉According to Dr. Amy Barcak, lead author of a recent study on Omicron viral shedding: “Although we all consider a positive culture the best proxy for contagiousness, the precise relationship between positive culture and the capacity to transmit infection hasn’t been defined. It’s possible that cultures may be able to detect virus at levels that are too low to actually be transmitted to other people, or conversely that our assays aren’t sensitive enough to detect low levels of virus that are still capable of infecting other people. But to date they are the best indicator we have.”
🤗Whether you’re hunkered down in your bedroom, or feeling uncomfortable returning to work, we see you. Many of the Nerdy Girls have been there, too. We are sending well wishes your way and hope this helps you navigate this challenging time.
Stay safe and stay sane!
Those Nerdy Girls
Thanks to Nerdy Girl Chana at Fueled by Science for this post. She tested positive with Omicron in January and knows how long 10 days can be!
NOTE: Edited on 5/5/22 to clarify the definition of Day 0.
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Amy Barczak, MD. Ragon Institute of MGH, MIT and Harvard
Personal email communication