Thread by @jeffgilchrist

COVID-19: New 5 day isolation rule & real-world rapid test results

Both the CDC in the USA ( cdc.gov/media/releases… ) and Ontario in Canada have introduced new 5 day self-isolation rules if you test positive for COVID-19 ( news.ontario.ca/en/backgrounde… ). 🧵1/

They both claim the the change in guidelines is based on science but I have yet to see anything released that actually shows the science they used for their decision making. 2/

For background, you may want to read this thread about when it is appropriate to use PCR, rapid tests, and antibody tests with their strengths and weaknesses (

https://twitter.com/jeffgilchrist/status/1476886860723101698

). 3/

Before Omicron, evidence from 113 studies in 17 countries found the mean period of infectiousness and risk of transmission could be somewhere between 2-3 days before and 8 days after symptom onset, well past the 5 days in the new guidance ( thelancet.com/journals/lance… ). 4/

For some real-world data, Dr. Erin Bromage oversees testing of thousands of people each week in workplace surveillance programs (

https://twitter.com/ErinBromage/status/1475598716681793541

). Some people are tested weekly and some daily with millions of tests over the past 18 months. 5/

They have been using PCR tests since the beginning but over the last 9 months have been supplementing with rapid tests. As we saw above the ability of PCR tests to detect very low levels of virus meant they saw people test positive for months after recovery. 6/

They can now use RATs as a good balance to help determine when someone has a very low likelihood to transmit the virus to others. 7/

He provides a recent example of someone who was asymptomatic. The rapid test started showing they were positive 3 days after exposure to the virus when viral loads became high enough to be detected. 8/

As you can see from how dark the test line is, the person still had a large viral load in their nose 8 days after testing positive. Since this is a rapid test, it only shows positive when someone has a viral load high enough for the person likely to be infectious. 9/

Looking at 59 people in their surveillance program with rapid tests, the fastest someone cleared the virus was 6 days and the longest was 8.5 days. 10/

Dr. Bromage is baffled by the new CDC guidelines that allow people to stop isolating 5 days after testing positive for COVID-19 unless you have a fever since everyone in his surveillance program still has high viral loads consistent with being infectious at 5 days. 11/

Dr. Jennifer Glass who had 3 doses of Pfizer (3rd received 2.5 months ago) and still had a breakthrough infection documented her rapid test results (

https://twitter.com/methanoJen/status/1476276591219064836

). 12/

She believes she was exposed to COVID-19 at a conference in New Orleans and tested negative 3 days before her symptoms started when she traveled home. She had a faint positive line the first day she experienced a slight headache, stuffy nose, and muscle aches. 13/

An hour and a half later she retested and had a much stronger positive line. Six hours later she had an even stronger positive result with the rapid test. The following day she tested positive from a PCR test, and continued to test positive on a rapid test for 10 days. 14/

She no longer had symptoms on day 5 so by the new CDC guidelines could have stopped isolating, but continued to have viral loads high enough on day 10 for a rapid test to turn positive which means she is likely still infectious. 15/

On Day 11 she finally tested negative on a rapid test (

https://twitter.com/methanoJen/status/1476564875639934977

). While testing positive on a rapid test is not a guarantee that a person is infectious, there is a much higher probability that the person is currently infectious than with a PCR test. 16/

There are some studies looking at the sensitivity of rapid tests with Omicron and the best place to swab now:

https://twitter.com/jeffgilchrist/status/1477282003733078016

17/