Covid-19: Is our long-postponed infection survey still worth doing?

Britain has been doing them every week since May 2020.

Australia has done three this year, revealing at least two out of three Aussies have now had Covid-19.

New Zealand's Health Ministry has been promising one since March. In July, it was weeks away. Now, it's been put off again, until next year.

Covid infection and seroprevalence surveys bridge the ever-widening canyon between reported case data and reality. They can tell you how many infections you're missing, and in what age groups and communities. And they can scour your immune history to find the reminder notes left behind by symptom-free infections you never knew you had.

So why do we still have neither? And are they still worth doing?

Camp hurry up

Covid-19 Modelling Aotearoa programme co-lead Dion O'Neale said it was appalling we didn't yet have a survey measuring actual infection rates in the community.

While the Health Ministry has lumped together infection and seroprevalence surveys, they answer two different questions.

An infection survey tests a random sample of people to find out how many are infected at that time. That's a window on the effectiveness of your disease tracking, as it shows how many cases are either going undetected, or unreported.

That matters for accurate modelling, but also for understanding how severe a new variant is. If you have 50 reported cases and 50 people in hospital, you need to know whether the 50 reported cases are really a tiny fraction of total infections, or if the virus strain is incredibly dangerous.

That's more important now we're relying on self-reported RAT test results. While we were aggressively chasing every infection to keep the virus out of the community, reported case numbers were pretty accurate, at least at measuring symptomatic infections.

But now, O'Neale said wastewater testing suggests we're probably missing about two thirds of cases. But there's no way of telling who they are. And as well as massively undercounting just how much Covid is spreading through the community, relying on RAT results could present a skewed picture of who is getting infected.

Some communities might struggle to access RAT tests. Others might be less likely to log a positive result, O'Neale said.

"There's going to be different biases in different areas about who is and isn't reporting cases. So you risk missing out on knowing who is bearing the brunt of the infections that are going around."

Auckland University senior lecturer and immunologist Anna Brooks was also firmly "in camp hurry up".

"The infection survey is absolutely critical. It's crazy that it's not in place right now... When our borders opened, the tools should have been in place at that point. So that we could operate in real time and know what is in our country."

Brooks said it was amazing that, two and a half years in, we still have "absolutely no idea" how many symptomless infections slink in the shadows, undetected. A regular infection survey could also track reinfections.

"It's absolutely critical to understand when risks arise and how widespread infections are, because we've got no protections in place, so people can make informed decisions... We've got such little information, so there's a false sense of security that this is behind us, when we're right in the thick of it.

"Yesterday would be great, today is the next best time. Start now."

Otago University epidemiology professor Michael Baker said there was a reason Britain was still doing its weekly infection survey.

"Of all the tools they set up early in the pandemic, this is the one they've kept, because it's been invaluable."

New Zealand has scrubbed up its surveillance data, teasing out Covid-related deaths from the mess of post-positive test mortality numbers and improving hospitalisation data, Baker said. Community case reporting was now the weakest link.

"We've had this big hole in our data, really, from early this year. Once we had widespread Omicron transmission, and we moved away from PCR testing."

A well-designed and ongoing infection prevalence survey could not only fill that gap, but also probe reinfection rates and how many infected people suffer long Covid symptoms, Baker said.

Charting the immune landscape

The second arm of the survey, called seroprevalence, measures immune response.

That would tell you roughly what proportion of Kiwis have had Covid-19 - something we really have no idea about right now. It can also indicate how many infections were asymptomatic.

Remember Swedish state epidemiologist Johan Giesecke's bold April 2020 prediction that Stockholm would reach herd immunity in mid-May? At that time, that would have meant about 60 percent of the population would have had to have been infected with the virus.

An August seroprevalence study of health care workers delivered the bad news that only about 8 percent had actually had Covid-19.

The surveys work by testing blood samples for two different antibodies. Antibodies against the spike protein can be generated by both vaccination and infection. But the presence of antibodies against the nucleocapsid protein, which is inside the virus, shows the person has been infected.

Those are "enormously valuable" landmarks of the immune landscape, Auckland University vaccinologist Helen Petousis-Harris said.

"Seroprevalence should be part of a pandemic plan, like we do for 'flu...It can tell the difference between immunity generated by vaccine and that generated by infection, which would have been enormously useful information."

Australia did its first national seroprevalence study from June-August 2020 and has done one every three months this year, using existing blood donors.

The results show the march of their Omicron outbreak. The first survey, in March 2022, found about one in six (17 percent) Aussies had been infected with the Covid-causing virus SARS-CoV-2. By June that increased to almost half (46 percent) and by September it jumped again, to 65 percent.

Young adults aged 18-29 had the highest rates of historical infection, at least 80 percent.

And four out of 10 kids whose parents thought they had avoided infection, turned out to have infection-induced antibodies, suggesting a lot of silent spread in children.

Baker said while New Zealand's rates might be similar to Australia's, our Omicron wave arrived later and our responses are increasingly diverging. For example, Aussies no longer have to isolate if they test positive.

"So it's going to be more and more important to do our own seroprevalence study," Baker said.

It could also help assess how the burden of disease is falling unequally on Māori and Pacific people, he said.

While not everyone who gets infected produces antibodies, in most cases they're believed to stick around for at least a year after infection, Otago University immunologist associate professor James Ussher said.

So if the survey is conducted early next year, it should still capture infections from New Zealand's Omicron wave.

What it can't do is tell you if those infections are first infections or reinfections. Or if those antibody reminder notes are enough to ward off another infection.

Because seroprevalence studies require blood samples rather than just tests, they're more involved to set up. The cost would have to be weighed against what useful information you could extract, Ussher said.

Information for action

So you find out what percentage of Kiwis have had Covid-19, and how much disease is really out there. But what do you do with that information, now we're not actively managing infections?

Auckland University professor of public health Chris Bullen, who also co-chairs the Lancet Covid-19 Commission's public health taskforce, reckons that ship has sailed.

In the early days, when information was scarce, a seroprevalence survey would have given precious insight into how much of the community was infected, and which groups might be more vulnerable.

But now, with RAT tests and sewage surveillance showing trends of infection, its value has probably diminished, Bullen said.

"When we were talking about it originally, we were really in the dark. But there didn't seem to be much appetite for it in those days. I think that moment in time has gone."

Public Health Agency lead Andrew Old acknowledges the surveys have taken "longer than expected" to put in place, but said the delay has not impacted the government's decisions.

"These are helpful studies, but not critical to our response."

Infection prevalence and seroprevalence surveys are complex, and the project has been impacted by the winter pressure on the health system and health reforms, Old said.

The study would now be completed in two stages, starting in the first quarter of 2023. The second phase, to run in winter 2023, will have a strong Māori and Pasifika component, and will "further enhance understanding of the impact of Covid-19 on specific communities and inform the ongoing public health response", he said.

O'Neale fears that, by the time they're actually done, the surveys will be too watered down to be useful.

"Every time we ask about it, it sounds like it's going to be maybe a little bit smaller and a little bit later...So I'm really worried that what we're going to end up with is one week's worth of swabbing in Whakātane and that's all. And that by itself isn't hugely valuable."

The point of surveillance was "information for action", Baker said. Given the government shows no sign of increasing restrictions in the face of a recent increase in cases, it's unlikely survey results would provoke dramatic policy change.

But he still thought it was worth doing. If an infection survey shows 6000 people are getting infected daily rather than the 3000 case reports, that might be important for politicians prioritising money and effort.

Surveys could also be used to track other missing data, such as reinfections, long Covid, access to antivirals and boosters, and variant sequencing.

"I think it's a vital tool for informing our response going ahead. In many other respects New Zealand does have a world-class surveillance system. This is a really frustrating gap for everyone. I think if it's well-designed, well-run, it can meet multiple objectives and fill really big gaps."

Baker said a good infection survey could also be a first step to better surveillance of other respiratory viruses. Britain is considering expanding its survey into an early warning system for 'flu and RSV.

Brooks said knowledge is power, and we still don't have enough of it. Especially if a new variant emerges that can escape immunity.

"We really don't know what the future holds, so the more tools in our toolbox that we're utilising and gathering data on, the better, so that we have a baseline. We've got to start somewhere. It's not a matter of saying 'Oh, the pandemic's over, what's the point now of checking?'. Um, it's not over."

- This story was first published on Stuff