Why the Covid Christmas curve may have been accidentally flattened

As case numbers subside, Keith Lynch explains the key dynamics behind the latest Covid wave and why it didn’t reach the heights of the first two.

Five days before Christmas media reports on Ministry of Health modelling suggested “a likely peak” of about 1100 people with Covid in hospital on any given day.

This particular scenario also suggested a peak of 13,000 reported Covid cases daily by the end of December.

One headline read that “the number of people in hospital with Covid-19 could hit a record high’’.

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Strangely these numbers were reported days from Christmas when cases and hospitalisations were already beginning to subside.

On December 22, for instance, the 14-day case average stood at 4634. The previous week this number was 6516.

By now, cases have fallen even further. It looks like December 12 was the high point, with 8400 daily cases recorded.

It now appears safe to say that Covid did not hit a record high in this current wave. In fact, reported case numbers (and hospitalisations with Covid) peaked at significantly lower highs than the first two Omicron surges.

Of course, as Stuff has explained, the daily reported case numbers are not an accurate representation of actual infections.

We have previously reported on the “case ascertainment rate” – simply put, the proportion of infections that researchers think are actually being reported.

That dropped as low as 33% in early November, meaning the daily reported cases only represented about a third of all infections.

It shot up to 52% by December 7. If it held steady to the December 12 high of 8400, that would signal about 17,000 infections.

This is still likely substantially fewer infections than were seen during the March wave, even though people are probably now less likely to report. While it’s probably closer to the winter wave, it’s unlikely infection totals reached similar highs.

“You would have to think that the case ascertainment rate had fallen very far to say infections during the Christmas wave were near what they were in earlier waves. And nothing really supports this – hospitalisations, etc, all show much lower peaks,” University of Auckland computational biologist and Covid-19 modeller David Welch wrote to me via email.

(A high ascertainment rate would signal reported cases more closely match the true number of infections. A very low rate would suggest there are a lot more infections out there than cases.)

Why were people predicting ‘a likely peak of about 1100 people in hospital with Covid’?

First up, it’s important to understand that modelling Covid-19, particularly now, is really challenging.

Modelling requires assumptions. Assuming, difficult at the best of times, is more challenging than ever now given the mishmash of variants and population immunity.

We don’t even know how many New Zealanders have had Covid. We can take a good guess, but that’s it.

As Stuff reported in November: “The modelling (for this wave) – done about a month ago – is not a prediction, and has a high level of uncertainty, given the unpredictable impact of new variants, population immunity and antiviral medicine use, which would further reduce hospitalisation and death.”

A recent comment from modeller Dion O’Neale to RNZ illustrates this point. When asked about the ministry’s modelling of 1100 people in hospital he noted, “admission forecasts released by the ministry were based on a ‘pretty high estimate’ of daily infections, and don’t account for the effect of antivirals”.

I spoke to O’Neale last week who explained that the modelling mentioned above contained a plausible range of a peak ranging from 2000 to 20,000 cases. (You can see it below.)

“The 13,000 scenario gave good fit to past data, but there was nothing to suggest it would be a good fit for this wave,” he said.

The bottom line is: modelling is hard, with inherent limitations. It allows officials to plan for potential scenarios, but should not be interpreted as what’s definitely going to happen.

It’s also worth looking out for, as another expert told me, words like “could reach” and “potential for” in media reporting on modelling. These words do a lot of heavy lifting, allowing for dramatic numbers that make great headlines.

Why has the wave peaked then?

Covid waves do not peak until a certain proportion of the susceptible population becomes immune to the virus, meaning it has nowhere to go (for a while at least).

Right now, we’re accumulating that immunity the hard way – via infections.

The summer holidays may well have acted as a circuit-breaker of sorts, essentially reducing the number of susceptible people by, say, getting people out of schools and offices, meaning they have fewer interactions – and fewer opportunities – to be infected.

While social interactions may be more memorable during the Christmas period, you’re probably really seeing fewer people, O’Neale says.

This may explain the drop-off in this Covid wave.

But at the same time, people will go back to their typical lives at some point, meaning we may well have a long-tail when they, once again, become – you guessed it – susceptible.

It may also be that people decided to wear masks more often, or do less socialising.

People not bothering to test over Christmas would have almost certainly contributed to a drop-off in reported case numbers.

It’s probably not that simple, though. There are other factors likely at play.

In November, O’Neale suggested more people were infected in September and October than initially expected.

This, he said at the time, could mean the wave was “lower and slower”, as the New Zealand population essentially acquired more immunity before the wave hit, meaning there were fewer susceptibles.

Yes, of course, people can be reinfected with Covid as immunity wanes. (In this wave, it’s thought about 50% of current infections are actually reinfections.)

But it’s also estimated that about 80% of people in New Zealand have probably had Covid by now.

As my colleague Kate Newton has reported, that still means those who haven’t had the virus, who only make up about 20% of the population, are more likely to be infected (all things being equal).

“While reinfections do happen, previous infection gives some degree of immunity to current variants, which means at any time only a portion of the population is susceptible,” is how Welch has explained it.

The Ministry of Health agreed, pointing me to this study before Christmas, which noted: “Individuals with hybrid immunity had the highest magnitude and durability of protection against all outcomes, reinforcing the global imperative for vaccination.”

This is important as the vast majority of New Zealanders are vaccinated.

It appears that the constant accumulation of new temporary immunity via infection after Covid restrictions were lifted may have also essentially flattened this Christmas Covid surge.

Obviously this is not ideal and not necessarily what the Government had in mind when they dropped all Covid restrictions in September. (This immunity will, of course, wane in time.)

But if widespread Covid infections at a population level are inevitable – which is what the New Zealand Government (and the whole world including China) has pretty much accepted – it is arguably better that all that illness doesn’t hit at the same time.

What may have happened is an accidental “flattening of the Christmas Covid curve”.

Yes, there were still a lot of Covid cases, but they were spread out over a longer period of time.

What next?

It’s hard to be sure.

The vast majority of the country has now had Covid and been vaccinated, which should mitigate the severity of further waves somewhat.

In a statement before Christmas, the Ministry of Health told me “second infections may be less severe due to immunity acquired from previous infections”.

(Stuff wrote about reinfection severity here. You can listen below).

The government will also be keeping an eye on XBB.1.5, a new subvariant that has emerged in the US and is present here. (Viruses are going to virus.) XBB.1.5 may well be the most transmissible version of Omicron yet. But it does not appear to be a game-changer, such as when Omicron replaced Delta in 2021.

Welch’s view is that, given we already have a subvariant here with similar immune escape (CH.1.1), its impact may be lessened.

“It appears its main advantage over previous variants is greater transmissibility – that is, it has the immune escape of others in the variant soup but has a mutation that helps it binds better to the ACE2 receptor (a protein on the surface of many cells),” he said this week.

“Definitely one to watch and likely to increase here, but still think it unlikely will lead to another wave here.”

In its release on XBB.1.5 on Monday, the Ministry of Health also made the following point: “Relative to many other parts of the world, New Zealand currently has a high level of immunity based on high vaccine uptake, combined with a recent wave of infections (so-called ‘hybrid immunity’).”

We’ll see what happens in the coming months.

It also appears the Government may roll out another bivalent vaccine programme to coincide with winter. Medsafe has granted provisional approval of these Pfizer Omicron BA.1, and BA.4/5 booster vaccines.

The rationale may well be it’s best to use scant health worker resources to maximise immunity going into the months when respiratory diseases are most prevalent.

The only certainty is, Covid is not going away. It will continue to evolve. There will be more variants and more waves this year.