Why clusters’ missing link may never be found
A potential "missing link" between the Brisbane Youth Detention Centre coronavirus cluster and last month's Logan outbreak may always remain a mystery after genomic sequencing proved inconclusive.
Queensland Chief Health Officer Jeannette Young said the testing had found just one nucleotide's difference between the genetic code of the virus sequenced in the first case identified out of the detention centre and one of two young Logan women who had the virus.
It's the scientific equivalent of finding a single-letter difference between otherwise identical books - they virtually match, but not quite.
Both clusters have been linked to the B.1.1.25 lineage of SARS-CoV-2, the virus that causes COVID-19, with just one molecule difference in genetic sequencing between the two outbreaks.
"The problem is, there's lots of other samples out there that are exactly the same and we know it's the most frequent lineage that's circulating in NSW and Victoria," Dr Young said.
"We can't say definitively that those two cases are linked but we can't say they're not."
Using apples as analogy, in science terms, it's like having two similar looking apples, knowing that they are a particular variety and originate from the same orchard but not knowing whether they are from the same tree.
Although further genetic sequencing is unlikely to uncover a connection between the two clusters, Dr Young said Queensland Health's experienced disease detectives, or contact tracers, continued to work the phones, trying to find a possible link between the outbreaks.
If the "missing link" exists - implying that someone infected as part of the Logan cluster, but never identified, has gone on to trigger the detention centre outbreak - the concern would be that they may still be spreading the disease.
But it's also possible other chains of transmission fizzled out through people staying home, social distancing or wearing a mask, preventing them from infecting others.
The Logan cluster was sparked when the two women tested positive last month after returning from Melbourne, but allegedly failed to quarantine.
Dr Young said the most infectious time for the virus was in the three days before the first symptom developed.
"It peaks around half a day or so before the first symptom," she said.
"Then it steadily comes down.
"This is why quarantine is absolutely rigid and essential, because if you don't quarantine people then you're always going to have people out in the community in those three days before they have their first symptom and have no idea that they've got anything."
University of Queensland virologist Ian Mackay said the ongoing hunt for the so-called missing link connecting the Logan and detention centre clusters, if one existed, would "likely need some luck" to be successful.
"The odds of success seem quite long," he said.
"Tracers would have to find those infected people who were missed because people weren't tested or weren't shedding virus at the time of being tested."
Associate Professor Mackay said public health experts did not need to find the link in order to contain a cluster.
Instead, he said they needed to make sure there was no ongoing community transmission of the virus and that came down to people getting tested.
Originally published as Why clusters' missing link may never be found