Negative RATs and infectiousness

The Ministry of Health's recommendation that one can end isolation despite providing a positive Rapid Antigen Test flummoxed me.

I had understood that, while a positive nasopharyngeal PCR test could be expected after infectiousness had passed, because of dead viral fragments' RNA still being picked up, that wasn't true for a RAT. RATs trigger on antigens, and if they're still around at levels high enough to generate a positive test, you're probably still infectious.

But what do I know? I haven't taken biology since 1993. I had a minor refresher when the kids were into Cells At Work, but that's it. Well, that and paying minor amounts of attention to all of this over the past couple of years.

So I asked Anne Wyllie at Yale. She helped us out last year when we were, again, arguing with the Ministry of Health about appropriate testing protocols.

She knows this stuff. She got an award at Yale for it last week. Yale outranks any of the universities New Zealand has to offer. 

This is what she said about it, with minor edits from me to add the article details that go with the links:

Below and attached, some evidence showing that if you're still positive by antigen test, you're most likely still contagious. There are likely still more out there.

You'll see a couple are more about Ct values from PCR - it is generally accepted that on a broad range of assays a Ct value of 30 or below also means you're likely still infectious, hence why I've included them as they show dynamics at the back end of infection and that this can last 10-12 days.

• Ke et al, 2022. Daily longitudinal sampling of SARS-CoV-2 infection reveals substantial heterogeneity in infectiousness. Nature Microbiology 7: 642-52.
• Chu et al. 2022. Comparison of Home Antigen Testing with RT-PCR and Viral Culture During the Course of SARS-CoV-2 Infection. JAMA Intern Med 29 April.
• Sikka, Wyllie et al. 2022. "COVID Testing in the Workplace: Return to Work Testing in an Occupational Cohort." medRxiv preprint. 10 February.
• Hay et al. 2022. Viral Dynamics and the Duration of PCR Positivity of the SARS-CoV-2 Omicron variant. Harvard pre-print.

Radio New Zealand had an interview with the Prime Minister this morning. She's preparing for a trip to the United States. She tested positive on 14 May, 9 days ago. She's back at work today. Discussing a potential meeting with the President, she said:

Ardern: It's a little more complex than that. It's all around Covid clearance and so on. The rules are generally built around picking up infections rather than dealing with existing ones.

Dann: Do you just need to have a test clearance or something like that?

Ardern: That's part of it but not the only part of it.

I would hope that the President would at least be requiring a couple of negative tests, a day apart, before meeting someone who is less than two weeks in from a Covid infection. 

Dann could have asked whether she actually has had a negative test yet, or whether she's just going by the inadequate MoH guidelines that say to ignore a positive RAT. 

The US requires a negative test to travel, or documentation of having recovered from COVID-19 in the past 90 days including a letter from a licensed healthcare provider or public health official stating that the passenger has been cleared for travel.

That latter bit is fun because the PM is the boss of the boss of all the public health officials. If Ashley Bloomfield says she's cleared for travel, based on his Ministry's advice to ignore positive tests, should the US government trust that?