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A possible new Omicron case, detected in the community despite five negative tests in managed isolation, is raising questions about what we know about the incubation period of the Covid-19 virus.
The case was detected in Palmerston North on Thursday, after an MIQ stay in Christchurch and five negative tests before being released on January 16.
The person became symptomatic on Wednesday, three days after leaving MIQ, and returned a positive test that evening.
WARWICK SMITH/Stuff
A possible new Omicron case has been detected in Palmerston North, but its origins are unexplained.
Urgent genome sequencing is under way, and the Ministry of Health is treating it as a suspected Omicron case. Investigations are ongoing to determine the source of infection, including possible in-facility transmission.
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University of Canterbury associate professor of epidemiology and health science Dr Arindam Basu said there were a couple of other possibilities.
The Omicron variant could have an incubation period anywhere between 5-15 days. It was possible this person was infected before their stay in MIQ, and the virus lay dormant in their body for almost two weeks before rearing its head.
ROBERT KITCHIN/STUFF
ACT leader David Seymour has come out criticising the government over a lack of an Omicron plan.
Travellers to New Zealand, even vaccinated, must return a negative PCR or RT-PCR test within 48 hours before departure.
The MIQ system was based on the average incubation period. It would be “impossible” to cater for the “extreme” 15-day incubation period in every case.
Second, it had been noted that 10 per cent of people diagnosed with Covid-19 became reinfected after further exposure. It was possible the person did not have the virus when tested, but had been previously infected, and when further exposed, either in MIQ or the community, the illness was reignited.
Professor Kurt Krause, an infectious diseases physician at the University of Otago, said the incubation for all viruses, including coronaviruses, was variable. While the average time was five days for Covid-19, it was fair to say Omicron and Delta would be one day faster on average.
However, there was a “long tail”, with some people returning positive results on day 10, 11, or 12, he said, which could explain the case. One naval vessel in the United States ended up discovering an Omicron case about 20 days into the journey, with nobody getting on or off the boat in that time, and nobody showing symptoms when the ship set sail, he said. “Strange things happen in medicine.”
It is possible the case was picked up in MIQ, experts say.
Krause wondered whether the virus was picked up in MIQ. Genome sequencing would reveal this, he said. If this was the case, health staff would need to determine whether the cases had contact with one another. A border worker could have passed on the variant, he said.
The positive case also could have contracted Omicron within the community. While this was not impossible, it was unlikely, Krause said.
Krause said a late presentation of symptoms of Omicron, and the case possibly being an outlier in this regard, was not strong enough grounds to extend MIQ stays for all returnees.
If the person was tested with a rapid antigen test, the result may also have not shown up, as these tests often did not work with Omicron in the first two days of infection, he said.
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