If you get a positive antigen rapid test, it’s best to assume that you are contagious.
A positive PCR test doesn’t tell you whether you’re contagious, because PCR can be positive for weeks after the infectious period.
Neither PCR nor rapid tests directly measure contagiousness, but they both shed light on what’s going on inside you. The key is to understand what they can, and cannot, tell you.
RAPID TESTS
🧪 Rapid tests detect specific proteins inside the SARS-CoV-2 virus, and only light up when there is a ton of it on your swab, to the tune of millions of copies. Thus, a positive rapid test means that your swab is teeming with virus, and you are likely contagious.
⚠️ A negative rapid test is trickier to interpret. In the early days of an Omicron infection, you can be contagious yet test negative, because the virus hasn’t taken off *yet* in your nose. In the later days of infection, a change from positive to negative sends a strong message that you’ve cleared the virus.
⏰ How long will I be positive? With rapid antigen tests, people only test positive during the infectious period of 1-2 weeks. A recent study of NBA players tested daily found high viral levels (likely contagious) for an average of 10 days with Omicron, and 11 days for the Delta variant, with significant person-to-person variation.
PCR
🧪 PCR can detect whispers of residual virus, long after the contagious period. Think of PCR as a sleuth checking out the scene of the crime to see if there is any lingering evidence of a villain. Thus, a positive PCR test doesn’t tell you whether you’re teeming with virus, or cleaning up the last little remnants. Your contagiousness depends on where you are in this journey.
⚠️ A negative PCR test means that your swab didn’t have even a small amount of virus. Just like a negative rapid test, this is not necessarily a green light, especially in the first few days of an infection, when the virus may not *yet* have taken off in your nose. False negatives PCR tests can also happen due to issues with sampling, processing, or analysis.
⏰ How long will I be positive? With PCR, you can expect to test positive for weeks, or even months (rarely), well after the infectious period.
🤓 Nerd Notes: PCR is super sensitive because it involves a signal “amplification” step (while rapid antigen tests do not). Viral RNA is first converted to DNA, then the DNA is copied many times in a process called polymerase chain reaction (PCR).
In reality, COVID PCR tests don’t just give “yes” or “no” answers. They give a value, called a Ct or cycle threshold value, that indicates many amplification cycles it took to get a strong signal. A PCR test is reported as positive when the Ct value is below a certain cutoff (e.g, Ct<40). In general, samples with more viral RNA have lower Ct values and are more likely to be contagious, though this link is fuzzy. PCR Ct values are used for research, and are not provided to patients, because this metric is imperfect and has not been approved by regulatory bodies as a diagnostic tool.
UNCERTAINTIES
There are many unanswered questions about testing and contagiousness. For example, it’s unclear whether a Day 0 positive rapid test is equally contagious to a positive Day 10 rapid test. Studies suggest that most transmission happens in the first five days, but we don’t know how much of this is due to viral load or other factors that could influence contagiousness, like the immune response. We also don’t know how vaccination impacts the course of infection for Omicron. Stay tuned, as we are learning more every day!
FROM SCIENCE TO POLICY
🇺🇸 The CDC’s current policy – 5 days of isolation, then five days of masking (if resolving symptoms), does not mean that we are not contagious after five days. It suggests that the CDC thought the benefits of a longer period were outweighed by the societal costs. Notably, the US CDC recently added an optional rapid test after “Day 5”. If positive, another five days of self-isolation are advised, because a positive rapid test means you are likely contagious.
🌎 Isolation policies vary around the globe. In England, you must self-isolate for 5 days, then provide two negative rapid tests a day apart (after day 5). In this Nerdy Girl’s hometown of Vancouver, Canada, unvaccinated people are required to self-isolate for 10 days after testing positive.
As our policy Nerdy Girl Lindsey recently discussed, health policies involve difficult tradeoffs, and often require working with incomplete, evolving, information. The UK’s policies, for example, were shaped by modeling studies that estimated that roughly 3 in 10 people released from quarantine after Day 5 would be contagious if no testing was used.
THE BOTTOM LINE
If you test positive, it’s safest to assume that you are contagious for ten days. The best way to prevent further spread is to self-isolate for a full 10 days or until you test negative with a rapid test. If this is not feasible, take steps to protect others, including strict masking and limited time in confined public spaces.
⚡US residents: Don’t forget to order your free rapid tests from the US government at covidtests.gov!
DP Links:
UPDATE on detecting Omicron with Rapid Antigen Tests
Is the new CDC isolation policy good or bad?
Welp, I have COVID. When can I stop worrying about giving it to other people?
References:
COVID PCR Ct Explainer (Public Health Canada)
Viral dynamics and duration of PCR positivity of the SARS-CoV-2 Omicron variant
COVID-19 self-isolation changes: scientific summary (UK HSA)
CDC isolation and quarantine guidelines
UK Guidance for households with possible or confirmed coronavirus (COVID-19) infection
