How to strengthen the post-isolation Covid rules

Over the weekend, the Ministry of Health reported a case of Covid-19 in Auckland that is not related to the current Auckland cluster. Before we start to panic, here’s how I think the case happened and how we can strengthen our current border controls.

The new Covid-19 case is someone who returned to New Zealand from India on August 27. They spent their two weeks in managed isolation in Christchurch and returned two negative tests before flying home to Auckland on September 11. Five days later, on September 16, they developed symptoms and got tested again. That test was positive. Two of their household contacts have also tested positive. They have all been moved to the Auckland quarantine facility and their close contacts are isolating.

So how could someone who has been through managed isolation and returned two negative tests then come down with Covid-19? Just to be clear, I don’t think their day 12 test result was wrong. As I’ve explained before, the PCR test we use is a critical tool that looks for the presence of the virus. We know that virus levels start to build in the few days before people develop symptoms. If our case didn’t develop symptoms till five days after they left managed isolation, then a test two days before that wouldn’t have been positive. So, what else could it be?

This is where the genome sequencing gives us our first clue. The virus the person is infected with matches that of other people with Covid-19 who arrived on the same flight from India. That means they could have been infected before boarding their flight or while on the flight. Another possibility is that they could have become infected while in managed isolation if they were in the same facility as the people with the matching virus. I imagine officials will now be checking CCTV footage to see if there was any contact before the people who tested positive were moved into quarantine.

I think the more likely scenario is that this case is our first proper example of someone incubating the virus for far longer than most people. Fourteen days is used as the ‘gold standard’ for managed isolation and quarantine because the majority of people incubate the virus for less than this time. Most people develop symptoms within 10 days. But we do know there are outliers. The longest I could find reported was 24 days. Several recent studies have suggested that about 1 in 100 people infected will develop symptoms after 14 days:

• Incubation period of COVID-19: a rapid systematic review and meta-analysis of observational research
• The Incubation Period of Coronavirus Disease 2019 (COVID-19) From Publicly Reported Confirmed Cases: Estimation and Application 
• Estimation of incubation period distribution of COVID-19 using disease onset forward time: A novel cross-sectional and forward follow-up study 

This case came to light because the person involved did the right thing and got tested as soon as they developed symptoms. They didn’t assume their symptoms weren’t Covid-19 just because they had already returned two negative tests. As I’ve said before, I think we should bring in further testing of people after they leave managed isolation. One test could be sometime between day 17 and 21 of arriving in New Zealand, and then a final one around day 28. I also think we should bring in saliva testing rather than swab testing as it looks like it could be both more practical and more accurate.

But what could we do apart from more testing? Just as I don’t think it’s feasible to keep people in managed isolation for more than 14 days, I can’t see people being forced to self-isolate at home for another two weeks after they have left. But they could act as though they were at a higher alert level than the rest of us to minimise the chances of any transmission happening if it turns out they were infectious. Acting as if we were at alert level 3 would be good, but I’d settle for a 2.5. That would mean working from home if they could, wearing a mask when outside of their home, and not attending any events or gatherings, or if they must, limiting themselves to those with less than 10 people.

While our managed isolation and quarantine procedures are currently following global guidelines, to follow an elimination strategy means being better than the gold standard. I have every confidence we can do it.