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“If we had a person who developed COVID-19 who hadn’t been overseas, we would ask if they had had contact with family or a friend who had travelled and we could do serology testing on that person and find that they were positive, so that’s very reassuring.”

Dr Scalley said serology testing helped piece together the chains of transmission for a cluster within an institution in Perth.

“We found two unknown source cases and the way they were spaced in time meant they couldn’t have given it to each other, so there had to have been a case that was missed in the middle,” he said.

“Through that investigation, we found a couple of missed cases that had no symptoms at the time.”

Dr Scalley said most people who tested positive were asymptomatic or could only recall having mild symptoms.

In WA, of the 658 cases recorded, 15 remain locally acquired from an unknown source, with contact tracers continuing to try and find the missing link.

The cases developed symptoms between March 6 and April 3.

WA faced its harshest social gathering restrictions between March 31 and April 27, which helped extinguish community transmission of the virus in the state.

Another technology helping WA scientists piece together the COVID-19 transmission jigsaw puzzle is genome sequencing, which can determine the specific strain of virus a person has contracted to determine where it originated.

“At times when the sources were coming from overseas cruise ship infections, you could see it was coming from those areas and other times you could see it coming from other areas,” Dr Scalley said.

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“We haven’t done a whole lot of genome sequencing on all our samples, we tend to save it for when there is a benefit such as if we have an unknown source and a possible link between two cases.”

Genome sequencing has been used successfully in Victoria to determine the seeding event for 90 per cent of its second wave as being a family in hotel quarantine whose infections spread to security guards.

The method has also been used by New Zealand authorities in an attempt to determine how coronavirus re-entered the country after 100 days of no transmission.

WA has not recorded a case of community transmission since April 12, with cases since that time limited to hotel quarantine, the Al Kuwait livestock ship, and one man who entered WA from Victoria and tested positive in self-isolation.

Around 3000 COVID-19 standard swab tests are being carried out in WA each day, which includes mandatory testing in hotel quarantine and for truck drivers, hospital tests, and walk-ins to COVID clinics.

Heather McNeill is a senior journalist at WAtoday.

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