The Monday after the federal election, my phone dinged and the dreaded message popped up: "I'm so so sorry Bel. I have COVID."
It came from a friend, and just 36 hours prior, we were sitting in her lounge room, eating cheese and watching the polling results roll in.
Because election night was a classic chilly Melbourne autumn evening, her windows were closed and the heater was cranked high.
After marinating in that virus hot box for a good four hours, I was sure that my COVID-free run had come to an end, and I'd soon test positive too.
After returning a questionable RAT result, I took myself off to get a PCR test … which came back negative.
This wasn't the first time I'd been a close contact of someone with the disease and sailed through, seemingly unaffected.
I've had three doses of COVID-19 vaccine, but Omicron seems to get around even those after a few months.
That got me thinking: have I already had COVID-19 and just not known?
Estimates vary for the proportion of infected people who show no symptoms, but Omicron, which has been the dominant variant in Australia this year, seems to elicit more asymptomatic infections than previous variants, so it wasn't outside the realm of possibility.
So I went to my GP, who referred me for a blood test.
How can I tell if I've had COVID-19?
PCR tests and RATs can tell you if you're currently infected with the SARS-CoV-2 virus, which causes COVID-19, but only a blood test can tell you if you've been infected in the past.
My blood sample was sent off to a lab, where it was analysed for antibodies generated by vaccines and an actual infection.
The difference between the two comes down to what our immune system "sees" and reacts to.
Approved COVID-19 vaccines in Australia work by triggering our immune system to make antibodies against the spikes that jut out of the SARS-CoV-2 virus sphere.
An infection, on the other hand, elicits a much broader immune response. It generates antibodies against proteins that make up the virus sphere, called the nucleocapsid, as well as its spiky bits.
If a lab test picks up high-enough levels of nucleocapsid protein antibodies, that's a pretty good sign of prior infection.
It's a different story for people who have been vaccinated with, say, China's Sinovac or Sinopharm vaccines, says Dorothy Machalek, a senior research fellow and epidemiologist at the Kirby Institute who coordinates blood surveys to track SARS-CoV-2 infection in Australia.
These are "whole virus" vaccines and contain killed or inactivated copies of the entire SARS-CoV-2 virus.
This means our body makes antibodies against the sphere and spikes, Dr Machalek says, making it "virtually impossible" to distinguish between an immune response generated by whole-virus vaccination and one by infection.
What can a COVID-19 antibody test tell you?
Antibody blood tests can tell you if you've had COVID-19, but there are some caveats.
COVID-19 antibodies naturally dwindle over time, so if they turn up in your blood test, their levels can't tell you exactly how long ago you were infected — just that you were infected sometime in the past few months.
"We think now [the test] is a useful marker of fairly recent infection, as opposed to whether you've ever been infected," Dr Machalek says.
So a negative result doesn't necessarily mean you've never had COVID-19. You may have had it too long ago for any antibodies to remain.
"If you want to know whether you were infected following the introduction of COVID into Australia in March 2020, there really isn't a way of testing for that.
And if you have a positive result — showing your blood contains COVID-infection-generated antibodies — that doesn't necessarily mean you're protected from reinfection.
That's because not all antibodies are made equal: only some will "neutralise", or stop a viral infection from spreading through our body.
"With some pathogens, such as hepatitis B, if your antibodies are above a certain threshold, you can say you have a level of protection," Dr Machalek says.
"The general [tests] we use for large-scale surveillance studies won't tell you what variant you have either, but you can do additional testing that may give you an indication."
And because antibody blood tests only measure antibodies, they can't provide any information about potential longer-term immune protection, such as types of B cells and T cells, which can kick in and defend us even if our initial stock of antibodies has disappeared.
Why do large-scale blood surveillance?
These kinds of tests aren't just for curious people like me. They're used to track infections on much grander scales.
And there's a chance my sanguine juices, taken during a regular old blood donation at Lifeblood, were included in an ongoing Australia-wide COVID-19 antibody study that Dr Machalek leads.
Earlier this year, the study analysed blood from more than 5,000 donors. It found that by the start of March, at least 17 per cent of Australian adults had been infected with COVID-19 and had enough antibodies to show up on tests.
The team, from the National Centre for Immunisation Research and Surveillance and the Kirby Institute, just finished collecting samples for the next round of results, Dr Machalek says.
They'll hopefully have another report out around the end of July.
Seeing how COVID-19 infections change over time, captured by such "serosurveys", provides insights not necessarily captured by reported data, so is an "important piece of pandemic intelligence", she adds.
"We can use the results from the last serosurvey to plan for what we're seeing now in terms of the surge in cases, and the potential impact that could have on hospitalisations and deaths.
"We know that there's widespread community transmission, but we do need to continue to understand how widespread it is.
My results came back a few days later
So I've not had COVID-19 — at least not this year so far.
That was a real surprise to me.
I have spike protein antibodies in my blood, but very few nucleocapsid protein antibodies.
Maybe I've been incredibly lucky, or perhaps I'm one of the blessed few who is genetically resistant to COVID-19.
My GP had a more pragmatic answer: "That's your vaccines working."
