On the one hand, the ramifications of the Sydney Covid-19 case who visited Wellington should be easy to handle. On the other, the Delta variant could throw health officials for a loop, Marc Daalder writes.
ANALYSIS: New Zealand has dealt with Covid-19 outbreaks before and always succeeded in eliminating the virus. But the potential of an outbreak in Wellington after a person with the virus visited the city from Sydney over the weekend differs in a handful of key ways from previous incidents.
To start with, there isn’t strictly an outbreak in Wellington, yet. The escalation of the region to Level 2 was more precautionary than previous alert level changes because the case that has prompted alarm is back in Sydney. It’s entirely possible – given Covid-19’s uneven transmission dynamics – that there are no active community cases in Wellington or anywhere else in the country.
But that technicality aside, two more critical differences are worth highlighting as officials grapple with how to deal with the phantom menace. These are that we know exactly how long the threat has been here for, which improves our chances of ring-fencing any transmission, and that it involves the far more transmissible Delta variant of the coronavirus, which increases the risk of the virus escaping our containment measures.
The greatest difficulty with calibrating the right response to previous outbreaks has been uncertainty. Uncertainty around the number of cases out there and how long the virus has been in the community.
This is the reasoning behind the “go hard and go early” response, which has seen Auckland lock down over just a handful of cases in the past. The risk is that any individual case is just the tip of the iceberg and that, by the time our testing catches up and reveals the true extent of the outbreak, it will be too late for conventional measures to put the virus back in its box.
While we still don’t know whether there are any undetected Covid-19 cases in Wellington or elsewhere in New Zealand, we at least have the benefit of knowing when the virus was introduced to the community: just after midnight on Saturday. That gives it precious little time to have jumped from person to person. At worst, we might be two links deep in the chain of transmission – but we’re certainly not in the scenario we saw in August, where people had been symptomatic with the virus nearly two weeks before it was first detected.
The longer the virus is undetected in the community, the better its chances of spreading. But now that we know it’s here, we can apply tried-and-true methods to break any existing chains of transmission, identify cases and their close contacts and isolate everyone involved. That includes the alert level escalation, as well as greater hand-sanitising, mask-wearing and QR code-scanning.
But will those familiar methods hold up against what is in many ways a novel threat? The man who travelled to Wellington is likely to have been infected with the Delta variant of the coronavirus, which was first detected in India. This variant has been found to not only be more transmissible than the original virus, but even more transmissible than other variants that emerged around the end of 2020.
The Alpha variant, also known as B.1.1.7, which was first detected in the United Kingdom, was the variant involved in the Valentine’s Day cluster. It is thought to be around 50 percent more transmissible than the original virus. In epidemiological terms, that added transmissibility could push a Level 3 lockdown to its limits.
I wrote about this in February, when we had the first of two lockdowns in Auckland. The key number here is called the reproduction number – how many new cases any one case will create, on average. If that number is above one, then an outbreak will grow. If, through public health measures or vaccination, it is pushed below one, then an outbreak will wane and eventually vanish.
In August, the Level 3 measures pushed the effective reproduction number of the virus down to 0.7. Every 10 cases of Covid-19 led to seven new infections and the outbreak was ultimately eliminated. The greater infectiousness of the Alpha variant would increase that number by half, leaving us hovering right around the key threshold of one. If we’re dealing with just a handful of cases, then we can likely ring-fence and eliminate the virus. If the outbreak is more widespread – if, say, it had grown to 25 cases prior to detection – then Level 3 might not have been enough.
The Delta variant only makes the maths worse. Add another 40 to 80 percent transmissibility on top of the Alpha variant and you get an effective reproduction number of 1.5. Again, if the outbreak was limited in scope (or if there were no cases at all, as remains a possibility), then contact tracing and case isolation is likely to be enough.
But if one of the bars or restaurants visited by the Sydney case over the weekend ends up being a super-spreading event and if enough of the people infected have then gone on to infect housemates and co-workers and waitstaff at other venues… it’s easy to see the situation spiralling out of control into a scenario that even Level 3 can’t contain.
As it stands, the odds are probably in our favour. One of the things that makes the coronavirus so tricky is the fact that some people don’t spread the virus at all and others are responsible for the majority of the transmission.
If we get lucky and this person spread the virus to few or no people – due either to those odd transmission dynamics or the fact they had at least received one shot of the AstraZeneca vaccine – then containment should be pretty straightforward for contact tracers. Even if a few more people have been infected, public health officials could still ring-fence the cluster if secondary transmission has been limited by how quickly we caught on to the outbreak.
But on the off chance the outbreak does wrest its way out of health officials’ control, then we need to be prepared to think about responding to a more widespread cluster differently from how we would have responded to the same event with the original virus this time last year.
