Amid worries over international arrivals spreading Covid-19 into the community, a group of scientists who have been modelling the threat say the risk is low, provided the travellers are quarantined for two weeks – especially the first eight days. Science reporter Jamie Morton posed these questions to Te Punaha Matatini investigators, Professor Michael Plank and Dr Alex James of the University of Canterbury, Professor Shaun Hendy and Nic Steyn at University of Auckland, and Dr Rachelle Binny and Dr Audrey Lustig at Manaaki Whenua-Landcare Research.
Epidemiologist Professor Michael Baker has suggested that, before the Government moved to test all arrivals this month, there likely would have been some positive cases of Covid-19 who slipped by because they weren't symptomatic, or simply weren't tested. With what we know about the general prevalence of Covid-19 among international travellers, would you agree with this?
It's likely some cases were missed but it's very unlikely any of them were still infectious after completing their 14 days quarantine.
The people who were released from quarantine early for compassionate reasons could have posed a greater risk, but if they never developed symptoms the risk of infecting others would still be very low.
It's also possible an infectious arrival could have passed the virus on to someone else during quarantine, and for that person to still be infectious after they were released.
The Ministry of Health has reported that 2,159 people left managed isolation between June 9 and 16 without being tested. But Director-General of Health Dr Ashley Bloomfield said these people would have posed "a very, very low risk" to the community because they'd completed the 14 days of isolation. Is this also a fair comment? And can you explain how two weeks is enough to break the chain of transmission?
Provided they'd completed their 14 days, Dr Bloomfield is right these people would have posed a very low risk. The reason is that people are at their most infectious between two and eight days after getting infected. More than 14 days after getting infected, there is a very low risk of passing the virus on to others.
There have been suggestions of testing people on arrival, along with on days three and 12. Do you see any merit in this? Or is there a risk that testing at the border itself could actually heighten the risk, by creating another point of potential transmission?
The biggest drawback to testing on arrival is that people could be incubating the virus but not yet be infectious enough to show up in a test.
That's why testing two or three days after arrival is a good idea. Of course we could do both, but this would place an increased demand on the labs doing the test and it's not clear how much extra benefit it would provide.
Since it takes time to do the tests and process the results, it makes more sense to focus on day-three and day-12 testing.
With the rising numbers of arrivals coming into New Zealand – as a result of more airways opening up – along with current resurgent rates in the pandemic, how might this change our risk profile?
It's possible that we'll see an increase in the number of new cases of the coming weeks and months as international travel resumes and the pandemic continues to grow around the world.
So far we have seen about four cases per 1000 international arrivals. But as long as our current border restrictions are adhered to, these should still pose a low risk to the community.
Professor Baker has also suggested it would be worthwhile to formally classify active cases into those either detected and contained at the border or detected within the community. He'd further like to see a publicly available, up-to-date and searchable database of the world regions from which all arrivals were coming from. Do you see merit in these suggestions - and would having geographic data help us to better quantify risk?
It would be useful to know how many cases, if any, were infected during quarantine.
If there are a lot this would suggest we need to reduce the amount of mixing within the quarantine facilities.
Knowing where people are travelling from is one of the most important factors in quantifying the risk because some countries have much higher prevalence of Covid-19 than others.
Another question about data: ministry officials have been struggling to track down how many arrivals haven't been tested, because this requires matching people's National Health Index (NHI) number with data from the border facilities which didn't previously record NHI numbers. Was this surprising to you? Does it show that the Government needs to urgently improve its data collection and collation?
Not many people would know their NHI [number] off the top of their heads, particularly if you have been overseas for a long time, so it is not surprising that this has to be tracked down.
That being said, the Government certainly needs to get better at collecting and sharing data amongst agencies, but it also has to protect people's privacy.
Do you feel the move to also test flight crew will make a difference?
This is definitely important because flight crew are exempt from quarantine so it makes sense to test them to reduce the risk they pose to the community.
Assuming that we rigorously test arrivals twice – and ensure they're contained within managed isolation or quarantine – is there any chance of the virus spreading into the community? Are there any further border measures which could be put in place?
The combination of 14 days isolation with day-three and day-12 testing is a strong system for keeping our border secure.
Our modelling shows that the other important thing is to minimise mixing and mingling within the quarantine facilities themselves to prevent new infections from occurring just before people are released.
If we follow these rules we will reduce the risk to a very low level. But it's impossible to eliminate the risk altogether.
Especially when you multiply it up over the long period of time we're likely to need to keep border measures in place, it's likely we will see a case breach the border sooner or later.
While we don't currently have any community transmission within New Zealand, are there measures we should be improving in the meantime, such as further beefing up our contact tracing and testing capacity?
We definitely need to make sure we are ready for a community case if and when we get one.
This means keeping our guard up and not forgetting the lessons we learned at the start of the epidemic: That is: practice good hand hygiene and social distancing; trace where you've been and who you've seen; and if you feel unwell, get tested and stay home.