A serious woman looks pensive

Have I already had COVID-19 and don’t know it? That’s how you’ll find out for sure

The Monday after the federal election, my phone rang and the dreaded message popped up: “I’m so sorry Call. I have COVID.”

It came from a friend and just 36 hours before that we were sitting in her living room eating cheese and watching the polls.

Since election night was a classic, chilly Melbourne autumn night, her windows were closed and the heating was on high.

After marinating for over four hours in that virus hotbox, I was sure my COVID-free run was over and I would test positive soon too.

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After returning a questionable RAT result, I went out to do a PCR test… which came back negative.

This was not the first time I had close contact with someone with the disease and sailed on, seemingly untouched

I’ve had three doses of COVID-19 vaccine, but Omicron seems to be bypassing even those after a few months.

That got me thinking: Have I already had COVID-19 and just didn’t know?

Estimates vary for the proportion of infected people who show no symptoms, but Omicron, which has the dominant variant in Australia this year, seems to provoke more asymptomatic infections than previous variantsso it wasn’t out of the realm of possibility.

So I went to my GP, who referred me for a blood test.

How do I know if I’ve had COVID-19?

PCR tests and RATs can tell you if you are currently infected with the SARS-CoV-2 virus, which causes COVID-19, but only a blood test can tell you if you have been infected in the past.

My blood sample was sent to a lab, where it was analyzed for vaccine-generated antibodies and actual infection.

The difference between the two comes down to what our immune system “sees” and what it responds to.

Approved COVID-19 vaccines in Australia work by activating our immune system to make antibodies against the spikes coming from the SARS-CoV-2 virus sphere.

An infection, on the other hand, triggers a much broader immune response. It generates antibodies against proteins that make up the virus sphere, called the nucleocapsid, as well as its spiny bits.

Your immune system makes a host of antibodies in response to a vaccine — and more if you’re infected.Getty Images: Juan Gaertner/Science Photo Library

If a lab test picks up high enough levels of nucleocapsid protein antibodies, that’s a pretty good sign of a previous infection.

It’s a different story for people vaccinated with, say, China’s Sinovac or Sinopharm vaccines, said Dorothy Machalek, a senior research associate and epidemiologist at the Kirby Institute who coordinates blood tests to detect SARS-CoV-2 infection in Australia.

These are whole virus vaccines and contain killed or inactivated copies of the whole SARS-CoV-2 virus.

This means our bodies make antibodies against the bulb and spikes, Dr. Machalek says, making it “virtually impossible” to distinguish between an immune response generated by vaccination with the whole virus and one from infection.

What Can a COVID-19 Antibody Test Tell You?

Blood tests for antibodies can tell you if you’ve had COVID-19, but there are some caveats.

COVID-19 antibodies naturally decrease over time so if they show 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’re Thinking Now” [the test] is a useful indicator of a fairly recent infection, as opposed to whether you’ve ever been infected,” says Dr Machalek.

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