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The Ministry of Health has established a panel of experts to find the best way to treat cases where people give what is called a weak positive result when tested for Covid-19.
One of those cases was that of the former Stuff reporter Tom Kitchin, now working for Radio New Zealand.
Kitchin first became ill with the virus in March during the first outbreak. In May, he was acquitted.
The former Christchurch reporter said Morning report He had recently had a sore throat, was feeling unwell and tired, symptoms similar to those he experienced in March, so he got tested for the virus and got a weak positive result.
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“The DHB called me and I was in complete shock, I just couldn’t believe it,” he said on RNZ.
He has been in isolation and his close contacts were asked to isolate themselves and get tested. All of those contacts have yielded negative evidence, he said. Morning report.
ALDEN WILLIAMS / THINGS
Former Stuff reporter Tom Kitchin was diagnosed with Covid-19 in March, was approved in May, and recently tested a weak positive.
Health officials believed he was showing remnants of the ancient virus he had in March. The same day that you came back with a weak positive test, it was cleaned again and that test came back negative.
On Tuesday, Chief Health Officer Dr. Ashley Bloomfield listed four types of situations in which tests often produced such weak positives, suggesting an old infection.
In those cases, the results had what Bloomfield described as a high cycle threshold or Ct value.
The Ct value is related to the polymerase chain reaction (PCR) process widely used to analyze Covid-19.
According to the Center for Evidence-Based Medicine at the University of Oxford, PCR works by amplifying genetic material exponentially, doubling the number of virus molecules over and over again.
Stacy Squires / Stuff
Chief Health Officer Dr. Ashley Bloomfield said more tests for Covid-19 were underway “and it appears we are seeing only a few more” false positives.
The Ct value is the number of amplification cycles necessary for the genetic material of the virus to be detectable.
Speaking on Radio New Zealand on Wednesday, Bloomfield said that early in an infection the Ct value was lower, below 25.
In other words, it would take 25 or fewer cycles of amplification for virus genetic material to be detected in those samples.
Around 30 “you start to wonder,” continued Bloomfield. “If you’re over 35, then you’re pretty sure it’s an old infection.”
Bloomfield previously said that international research had shown that people with historical infections, weak positive test results and a high Ct value were not infectious.
RNZ
The Health Ministry says that tests have revealed historical cases of Covid-19 among some people in this country. It says all the cases have involved a “weak positive” test result, suggesting an old infection in people who are no longer infectious and pose no risk to others.
At RNZ, he said a precautionary approach was being taken in the case of weak positives for now.
“We treat them as if they were a positive infectious case initially, until we retest.”
The main evidence that those weak positive cases were not infectious was a very good study conducted in South Korea, where researchers followed several hundred people who returned weak positive results, he said.
Contact tracing did not find anyone else infected by one of those cases.
“The scientific consensus is that these are viral fragments of the previous infection that seem to be detectable especially when someone contracts another respiratory infection and perhaps because there is inflammation in the throat and the cells are perhaps breaking down a bit,” Bloomfield said.
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“That then releases these virus fragments that are detected, but it is also not uncommon to have a weak positive and then a negative test.”
Canterbury Health Laboratories clinical director of microbiology, Dr. Joshua Freeman, said genetic material can be detected long after infection with many respiratory viruses.
The question was whether the people involved were contagious. “That’s something we’re still learning about and we have to exercise some judgment on these questions,” Freeman said.
The Ct value was just one of many things that were taken into account when trying to determine if someone could be contagious. Some other factors included a person’s exposure history and whether they had antibodies to the virus.
The PCR process was incredibly sensitive in detecting the virus, Freeman said.
With the weak positives, it was thought that people likely had bits of dead virus on the membranes at the back of their throats that persisted long after the people stopped being infectious.
Some of those membranes can be collected when a swab is taken from the back of someone’s nasal passage.
Researchers were still learning about how long people were infectious, but the pieces of the puzzle were slowly coming together, and more was known now than was known three months ago, Freeman said.
There can be a variety of reasons for having a high level of confidence that someone who tested weak positive was not infectious, although the reasons could vary from case to case.
There was always the possibility that a weak positive could be infectious, which is why additional tests and investigations were carried out.
“It could be someone very early in the infection, it could be on the rise. Repeating the tests should solve that, ”he said.
Whether someone who returned a weak positive should isolate himself was a judgment based on several factors, including the implications of doing it wrong.
“I don’t think there is a simple answer to that question. You always need to take a precautionary approach and that’s what I think the (health) ministry has been doing, appropriately, ”Freeman said.
Weak positives were becoming a problem now because some people without symptoms, who may have had an infection before, were being tested.
“Sometimes if you keep testing people, sooner or later you could give a positive result.
“It’s a bit like dipping the net in a pond full of goldfish,” he said. “If you submerge your net enough, eventually you’re going to get a goldfish.”
In his briefing on Tuesday, Bloomfield said more tests were being done “and it looks like we’re seeing just a few more” false positives.
“There is no doubt that during that March / April period when we entered alert level 4, there was another infection in the community that was not identified and reported at the time, but because we broke the chain of transmission, it did not continue. spread.”
