Antigen tests are less sensitive than PCR. They are less likely to detect the low levels of antigen present in a recently cleared infection. Therefore positive rapid antigen test = ongoing viral replication.
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So while a positive antigen test can’t tell you if you are making enough virus to actually transmit to someone else, it can tell you if you are making detectable levels of N protein, which probably means you have a productive, actively replicating viral infection.
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And if someone has ongoing viral replication, doesn’t it make sense to extend isolation until they do not? Shouldn’t we assume that means a realistic transmission risk even if we can’t quantify it?
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Apparently not for the CDC, who instead looks at data that is quantified, which suggests transmission risk is highest a few days before and a few days after symptom onset. All that data is from other variants, not from omicron.
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And that matters a lot, because omicron is different from the previous circulating variants. It spreads much faster, including among vaccinated people, and also has unique characteristics in the lab in terms of growth and fusion.
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And this especially applies to making recommendations based on symptoms. Omicron is generally causing less severe disease in vaccinated people (as expected—vaccines work even when they aren’t sterilizing), but they clearly can still transmit.
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Omicron also may have different tissue tropism (infecting cells in a different part of the respiratory tract), which can also lead to different symptoms and different levels of viral shedding in different parts of the respiratory tract. Confounding variables, all.
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So we also don’t have “predictive capability” for how contagiousness is linked to symptoms for omicron. I’d argue that a rapid test provides better guidance for where an individual person looking to leave isolation is in the moment.
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While we’re still learning a lot about omicron, I feel comfortable saying it’s sufficiently different both in terms of its epidemiology & virology that we should be very cautious making assumptions about transmission based on other variants, especially for informing policy.
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So let’s review:
1. Rapid antigen tests indicate actively replicating virus
2. We can’t assume that the kinetics of infection/transmissibility are the same for omicron as they are for the original recipe SARS-CoV-2 or alpha through delta.
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IMO this is not strong or sufficient justification for excluding a rapid antigen testing component from reduced isolation times. Keeping essential services running is, of course, essential. But if we can do that in a way informed by evidence in real-time, shouldn’t we?
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Guten Tag, you can read it here: I respect Dr. Fauci enormously, particularly for his candor. In the… threadreaderapp.com/thread/1476219 Enjoy :) 🤖
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Hi! please find the unroll here: I respect Dr. Fauci enormously, particularly for his candor. In the… threadreaderapp.com/thread/1476219 Talk to you soon. 🤖
It's simple. In the USA, tests are too expensive and are in short supply.
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