There’s Covid-19 in Ōtautahi Christchurch, but the city has avoided a lockdown (for now at least). Keith Lynch explains why.

On August 17, New Zealand entered a snap Level 4 lockdown after an Auckland man tested positive for the Delta variant of Covid-19.

More than 70 days later, Auckland is still locked down. Cases have emerged in Wellington, the Waikato, and Northland. At the weekend there was a case in Blenheim and now cases have popped up in Christchurch.

So far, more than 2800 people have caught Covid in the August 2021 outbreak. More than 230 of those have been in hospital.

Christchurch people are, of course, right to be concerned. The city has not seen a case of Covid since November last year.

While anxiety is understandable, it’s worth keeping in mind that Aotearoa’s second city is facing up to the Delta variant in a relatively advantageous position, which may partly explain why there was no change in alert levels on Thursday.

Yes, of course this decision will also have a political element. The Government was very much committed to elimination when Delta hit Auckland – that lockdown was inevitable. Those days have passed.

But there’s more to it than that.

At the 1pm press conference on Thursday, Covid-19 Response Minister Chris Hipkins’ explanation was simple: the two Christchurch cases, although unvaccinated themselves, live in a highly vaccinated suburb and had no major exposure events.

To be clear, it may well be there are more cases and alert levels still change in the coming days but let’s look at why there was no sudden shift to alert level 3 or 4.

The (relatively) good news

In mid-August about 40 per cent of Aucklanders had their first dose of the vaccine. Only 24 per cent of the city had their second dose.

Today, in Canterbury close to 90 per cent of the eligible population have had their first dose; 68 per cent are fully vaccinated. The elimination strategy certainly bought the South Island valuable time.

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The two Christchurch cases live in the suburb of Bishopdale, which has pretty much the same vaccination rates as Canterbury as a whole. It goes without saying that it’s much better to face up to the virus at these levels. But why is that?

The R number, which you’ll also hear called the effective reproduction number, or Reff, tells us how many people on average one person will pass the virus on to. If the Reff is above 1 the outbreak is growing, if it’s below 1 it’s dropping off.

Delta’s R number is about 6, meaning one person will, on average, pass the virus onto six others. Professor Michael Plank says the vaccination rates in Auckland in August would have led to a reduction of the R number of about 15 per cent (pushing it down to and Reff of 5 or so).

The vaccination rates in Christchurch at present would reduce the R number by about 55-60 per cent, he says.

About 200 to 300 people were already infected before we found Delta in Auckland in August, Plank says.

“I don’t think we’ll see those types of numbers in Christchurch but nevertheless it still could have spread to potentially dozens of people.” Of course, it only takes one colleague or friend of the duo to pick it up and pass it further.

I also spoke to Australian epidemiologist Tony Blakely, who outlined a scenario showing how the vaccine can dampen down spread: the 68 per cent of the eligible population that are fully vaccinated have 80 per cent protection against infection and say the remaining 21 per cent who’ve had the first dose are 40 per cent protected against infection.

Blakely says an eligible person is about 63 per cent less likely to get infected. Allowing for kids, who are currently ineligible for vaccination, then this is about a 50 per cent reduction in chance of getting infected across all individuals.

“It halves exponential growth, making it so much easier for contact tracers. And it may even be enough to drive Reff below 1 when combined with level 2 restrictions – on average in Christchurch.”

Remember, this is just an illustration of how the vaccine works. There are no guarantees with this virus, which we’ll get back to.

Also, it’s worth keeping in mind that all of the above doesn’t take into account the fact that the vaccines hugely reduce the risk of severe illness and death. Throughout highly vaccinated countries, we’ve seen a clear decoupling of infections and deaths. For example, of all those hospitalised in the August outbreak only three were fully vaccinated (as of October 27).

The fact that Christchurch was, and is, in level 2 is also important, which is something Prime Minister Jacinda Ardern mentioned on Thursday. For example, 386 people connected to the Assemblies of God (AOG) Church of Samoa contracted the virus in a super-spreader event in mid-August. Under level 2 rules, only 100 people can gather in churches.

We’ve previously reported on the difference ongoing level 2 restrictions make using two examples.

• New Zealand is at level 1 and Delta leaks in. In this case the R number is 6, as there is nothing dampening spread. That means the first case infects six people. Those six people infect six each. And those 36 each infect another six. So, after three generations of spread, there are 259 cases of Covid-19.
• New Zealand is at level 2 and Delta leaks in. In this hypothetical, Plank suggested the R number should be 4. There are no mass gatherings, there is social distancing. This doesn’t stop Delta. But it does slow it down. The index case infects four people. Those four people infect another four. Those 16 infect another four. That’s 85 cases.

In the two scenarios above, the vaccine’s effectiveness at pushing the R number down is not accounted for. Again, just examples to show you the difference a level change can make.

And then, of course, there’s the bad news

Yes, Canterbury is in a relatively good spot, but it’s very important to note that the vaccination coverage is not uniform across everyone.

This means there is a chance of the virus leaking into unvaccinated communities and spreading widely from there. You can see the divergence in rates across Canterbury and Christchurch in the graphic below.

For example, in Aranui, the lowest vaccinated suburb, just 68.7 per cent of residents have received their first dose. Just 52.9 per cent of Māori living in Aranui have received their first dose. This is concerning.

And it means, as Blakely explains, if the virus found its way into marginalised parts of Christchurch with lower vaccination rates, the R number could easily still push up above 1 and the virus could take off. We’ve seen this play out in Auckland.

Christchurch is certainly not out of the woods yet.