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Posted: 2022-02-05 18:00:00

Let's set the scene: It's late December, Omicron has taken off like wildfire across large parts of Australia. As a result, queues for PCR tests have blown out, with people waiting in their cars from the early hours of the morning in the hopes of getting in before the close of business. And that's if your local testing centre is even open over the holiday period.

You've got a slight scratch in your throat. Is it COVID-19, or is it just hayfever? You go looking for a rapid antigen test (RAT) but find chemist shelves empty. The grocery store is also out of stock. What do you do?

For many healthy people, comfortable with the knowledge Omicron would only cause a mild illness in the majority of cases, the answer was to lay low and isolate until their symptoms pass.

But now as the dust settles on the latest outbreak, many have been left wondering: did I actually have COVID and if so, is there any way to know for sure?

The hidden caseload

It's widely accepted that at the peak of the Omicron outbreak in Australia, the real number of cases in the community was far higher than official data suggested. This is largely due to the move towards at-home tests — but also the shortages of them — and the significant proportion of people with mild or no symptoms.

While there's no way to know the exact number of unrecorded cases for certain, epidemiologist Tony Blakely from the Melbourne School of Population and Global Health relies on a rule of thumb: for every person that tests positive, he estimates another four infections aren't included in the statistics.

"The case numbers, they're a big underestimate," he says. "That's for several reasons: one, surveillance systems fell over; two, people with mild symptoms just shrug their shoulders and say 'I can't be bothered' and don't notify; and three, asymptomatic people, unless they're a close contact, they have no reason to test and aren't going to be found."

The International Institute of Health Metrics and Evaluation estimates between 80-90 per cent of Omicron cases globally are asymptomatic or with very mild symptoms, based on data from South Africa and the United States.

Professor Blakely believes the percentage in Australia is likely to be lower, possibly due to the previously low levels of natural immunity from past infections, but adds that we are "grossly underestimating" the true number of infected people in the community.

Just this week Queensland Health released the results of a random COVID testing survey, conducted on the Gold Coast, which found up to 90 per cent of people who returned a positive PCR didn't know they had the virus

"A ballpark figure, probably after this wave, about half of us will have been infected," Professor Blakely says.

A woman in a hi-vis vest talks to the driver of a car in a long line of cars
During the Omicron outbreak, wait times for PCR tests exploded. (ABC News: Brendan Esposito)

Epidemiologists use a variety of imperfect tools to estimate the true size of the outbreak while it is happening, including hospitalisation data and the proportion of positive results from PCR tests; the higher the proportion, the more likely there are unrecorded cases out there. But with more people using RATs — and not recording negative results — this method becomes less reliable.

Another way to estimate the real number of infections during an outbreak is serological studies that screen blood samples from a range of sources for antibodies indicating past exposure to the virus.

In the first year of the pandemic, the Kirby Institute in partnership with a handful of other organisations ran three separate serological studies; the first in NSW, then a nationwide study, and finally a survey of metropolitan Melbourne during the state's second wave. The studies analysed samples obtained through blood donation, routine pathology, and screening tests for pregnant women.

"The main purpose of the initial round of surveys was to get a sense of what proportion of infections have been occurring that may have been missed," says John Kaldor, the head of Global Health at the Kirby Institute.

Those tests, conducted long before Omicron had arrived, picked up only low proportions of people in the population with detectable antibodies. If a similar large-scale study was run now, Professor Kaldor says it would be a different story.

"There have been millions of people who've had COVID in Australia, a number that we can at best speculate on," he says. "We don't really have, in any sense, the same degree of accuracy as we had in the past."

In a bid to narrow down estimates on the true size of the outbreak, Professor Kaldor says there are plans to run similar serological surveys in the near future. Data on the true number of infections, he says, are important for informing where control strategies need to be deployed and understanding what pockets of the population may still be most susceptible to infection.

But such studies also have limitations, in that the samples can never be truly representative of the population as a whole. "You can't say the findings correspond exactly to what is happening in the general population," he says.

Identifying past infections

These antibody tests can also be used on individual blood samples, to determine if it's likely someone has previously been infected. The question is then, when and why should you do it?

General practitioners are able to order the tests, which are included on the Medical Benefits Schedule, for their patients, but vice-president of the Australian Medical Association Chris Moy says they should only be used when there's a clear clinical reason.

"People might think you just do tests for fun," he says, "[but] when you do a test, you're trying to correlate something that's happened previously with present-tense symptoms."

He says one example of when it could be useful is if a patient presents with ongoing symptoms consistent with long-COVID but isn't sure if they've had the virus.

"Academic, personal interest is not the same thing as a clinical reason to do it," Dr Moy says. "Most doctors will be wary of using public funds to do a test for interest, when you do a test you have to know what you'll do with the result."

If people are interested in knowing whether they've had the virus, due to curiosity or to receive certain immunity passports for travel, they can also seek out an antibody test through a private provider at their own cost.

But Lawrie Bott, president of the Royal College of Pathologists of Australasia, says this is not encouraged as "previous infection cannot be confirmed or refuted conclusively". 

"Accurate diagnosis by PCR test at the time of COVID-19 infection is far preferable," he says. 

How do antibody tests work?

Because vaccines only target the spike protein of the virus, pathologists are able to differentiate between antibodies caused by the jab compared to those due to natural infection.

Put simply, if a person only has spike antibodies, then they've likely had a vaccine but not an infection. If another protein called nucleocapsid shows up alongside the spike protein, it suggests they've been infected in the past.

"Although crudely, we can say that spike-only antibody equals a recent history of vaccination and spike plus nucleocapsid antibody equals natural infection plus or minus vaccine, it's still an imperfect science," Professor Kaldor warns.

There are also limitations on the time frame for antibody tests. Experts estimate antibodies develop about two weeks after an infection and should last for at least six months, but this can differ from person to person.

"This means that antibody tests performed outside this 'window' may miss evidence of past infection," Dr Bott says. 

"Similarly, elderly or immunocompromised individuals may never develop a detectable antibody response to COVID-19 infection."

Professor Blakely also cautions against an over-reliance on these tests for individuals, explaining that they are most useful at a population level for pandemic planning purposes.

"Do people need to know [if they've had an infection]? Probably not," he says.

"I was RAT negative. I'm 95 per cent confident I actually had COVID. I've been immunised, do I need to know if I've had Omicron with any more accuracy than that? No, not really, I just get on with my life."

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