Managed isolation and quarantine hotels may become "swamped" with Omicron cases unless the number of arrivals is cut, a public health professor is warning. 

The government has announced three measures intended to fend off the more transmissible variant for "as long as possible". It has pushed back the first phase of the border reopening, tightened pre-departure tests, and extended time in managed isolation. 

So far, managed isolation is working as intended. Twenty-two cases of Omicron have been identified in MIQ facilities. One has recovered and been released. None have been identified in the community. 

But Otago University professor of public health Michael Baker said one other measure should be considered: limiting or even halting arrivals from certain countries with high rates of Omicron. 

"We should look at the [positive case] numbers coming in every day... [if] we're getting potentially 10 or 20 travellers a day coming from some countries or regions, that would just swamp our MIQ system, so we've got to do something to turn that tap down." 

There are 46 people with Covid-19 in MIQ at the moment. Twenty-five of them have been reported in the last four days alone, and 11 of those have come from the UK. No other country of origin has contributed more than two cases in that time period. 

Dr Matthew Hobbs, of GeoHealth Laboratory and the University of Canterbury, said it would be "sensible" for the government to consider limiting arrivals. 

"It's something I think the government can look at as an extra layer of protection over the next couple of months, to get us through that summer period, so ... allowing for boosters and paediatric vaccines we're in the best position possible." 

Baker said any decisions about which countries could be limited should be based purely on case numbers. 

"I think we need to look at having a threshold for the number of cases we can tolerate, and actually taking action if we're approaching that. It does mean using the principles we've already established - that it is reasonable to limit travel from the highest risk countries." 

That principle saw a complete travel ban on people coming from India for over two weeks in April this year, as the Delta wave took off there. 

At the time the rolling seven-day average of cases arriving into MIQ was five a day. 

In the last seven days there's been an average of 4.7 cases reported in MIQ a day, but things are now different: in April there was no Covid in the community, and the government was firmly after elimination. 

The other measures - lengthening MIQ stays, shortening pre-departure test times, and pushing back Australian travel - have been welcomed by Dr Lesley Gray of Otago University, a senior lecturer in primary health care. 

She said the move to 10 days in managed isolation, rather than seven and a further three at home, reduces the risk of a case leaking into the community. 

"I think seven days was just a little bit cutting it fine. There's every possibility that someone could have tested negative [on leaving MIQ] very early on in an infection, especially if they did pick it up in transit somewhere." 

There's also doubt cast on the initial assumption than Omicron is less severe than Delta, Dr Matthew Hobbs said. 

"There was a study that came out of Imperial College London, and actually that showed that there was no clear evidence that Omicron has a lower severity than Delta. So it's really too early to say one way or the other, but I think we should move away from this perspective that it's milder than Delta." 

The Ministry of Health's Chief Science Advisor, Dr Ian Town, said the country needs to keep Omicron at bay as long as possible, but notes the Delta outbreak started with just one case from MIQ. 

- by RNZ