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The Ministry of Health has revealed it is investigating after a person who has returned from the United States recorded a "weak positive" test for Covid-19 in Wellington.
The Ministry also revealed there are four new cases of Covid-19 in managed isolation.
It said it had been investigating the weak positive result - which was not included in today's reporting numbers because the Ministry believed it was a historical infection - overnight.
The individual who returned the weak positive was a recent returnee from the US and completed their full 14 days in a managed isolation facility (MIQ) in September, the Ministry said.
They were asymptomatic during their time in MIQ and returned two negative tests during this time. They were released from MIQ on September 21.
After becoming unwell this week, they sought medical care. They were subsequently tested for Covid-19, which has returned a weak positive result, indicating an old infection.
"We have undertaken further testing today to fully confirm this is a historic infection."
A test conducted today returned a negative result. This follow-up testing result strongly suggested it was a historical infection.
"We are awaiting further results from other tests including serology.
"We are taking precautionary measures as we always do in these instances and the individual has been transferred to an MIQ facility in Wellington."
The Ministry said as it had seen with other cases, individuals could return weak positive tests if they were infected earlier in the year.
"This case follows a typical pattern of what we have seen in other historical cases."
These cases could emerge after the person had a respiratory illness that was not Covid-19, such as a cold or influenza.
Residual remnants of the virus could be picked up through swabbing, with inflammation often bringing forward virus particles that were not previously picked up.
The first imported case arrived from the Philippines via Hong Kong on October 3 and was taken to a managed isolation facility in Auckland.
They tested positive as part of routine day 12 testing and have been transferred to the Auckland quarantine facility.
The second imported case arrived from Peru via Mexico and the USA on October 10 and was taken to a managed isolation facility in Christchurch.
They tested positive as part of routine day 3 testing.
The third and fourth imported cases both arrived from Hong Kong on October 11 and were taken to a managed isolation facility in Auckland. They tested positive as part of routine day 3 testing and have been transferred to the Auckland quarantine facility.
The total number of active cases was 46 – all imported cases. There were no active community cases of Covid-19.
Further details about the suspected historical case under investigation are expected to be released later today.
Yesterday laboratories processed 5078 tests, bringing the total number of tests completed to date to 1,021,931.