Thread by @ClaireBerlinski

Good morning. I have questions about antigen-and PCR tests, probability, and common sense. Studies and hard numbers would help me a lot, if you have them; anecdotes, not so much.

Consider 4 parties: A, B, C, D. All vaxxed and boosted. A has had Covid before, confirmed by PCR.

The PCR-confirmed case was at the outset of the pandemic & probably original or alpha strain. The 2nd, last fall, was confirmed by the "I recognize these symptoms" test. A *thinks* he had it.

A, B, and D were sharing an apartment.
3 days ago, A woke up with a sore throat.

He thought, "How could it be Covid? I'm vaxxed, boosted; and I've had it twice."
And lo, two antigen tests were negative.
But 1 day later the third test--PCR--was positive.

Q.1) How often does the PCR return a false positive?

Q.2) Is it possible the PCR test was picking up a viral fragment from a past Covid infection?

Q.3) What are the odds of two antigen tests serially coming up with a false negative? If the rate of a false negative in test 1 is, e.g, "0.2",

would I be right to think the sample space for the first test is

TTTTT TTTFF and the second test, likewise
TTTTT TTTFF
so the odds are 0.1? Roughly the odds of a false positive on a PCR test?

(Remember, he had a high enough viral load to be symptomatic.)

Q.3 What are the current, most robust figures we have for "false negative rate" on antigen tests and "false positive rate" on PCR?

Is it more likely, given these numbers and chronology, that A had two false negatives on on the antigen tests or a false positive on the PCR?

Q.4 If A is right that he had Covid (a 2nd time) last fall, is it possible the PCR test picked up viral fragments from that infection, as opposed to a current, transmissible infection?
Q.5 If it is possible, is it likely?

Q.5. Until A developed Omicron-like symptoms, A, B, C, and
D were in the same apartment. When A developed symptoms, he isolated himself in a hotel. He took C with him.

B developed a fever, fatigue, nausea 48 hours after A.
B is in a number of high-risk categories.
B took an antigen test: negative. Then two PCR tests, 24 hours apart: both negative.
B says he's already feeling better.

C and D are completely asymptomatic. C is a child. C tested negative on a PCR test, 3 days ago. D also tested negative on a PCR test 3 days ago, negative on an antigen test yesterday, and negative on an antigen test today.

Q6: Assume: A, B, C, and D are rarely in the same country at once, love each other very much, and want very much to spend time together.
Since A became symptomatic, he's been isolated.
C has been staying with D.

Assume: 1. A, B, C, and D want to maximize the time they spend together and their enjoyment of this time.
2. They want to minimize the risk of inadvertently kill each other.
3. They'd like to stop haemorrhaging money on an isolation hotel. So:

How many more days should they isolate and test before they can prudently and responsibly do the following:
a. Go out for a walk together, masked.
b. Go out to lunch, outdoors, unmasked while eating.
c. Go out to lunch indoors, unmasked while eating.
d. Sleep in B's apartment?

All serious thoughts welcome. @zeynep @PeterHotez @DrEricDing @EricTopol @MinkaOceane_Dr.

(0.2, sorry)

@ASkarimbas, perhaps you have a good sense of this ... and one more question: It seems Paxlovid works best when given as early as possible. But it also seems that false negatives are not unusual in the early stage of infection. How do you reconcile this when it's your patient?