Are you truly a COVID-19 virgin?

Maybe …or maybe not. Recent studies suggest that many people are unaware of their past COVID-19 infections.

A Canadian study found that, in people with antibodies showing previous infection, roughly half of them thought they were COVID-19 virgins. This study used blood samples from roughly 15,000 Canadian adults surveyed across the country from April 2022 to August 2022. At that point in the pandemic, roughly 54% of adults had antibodies from a natural infection, up from 3% in November 2020 to April 2021.

While the precise numbers in this study are out of date, the key message holds true – many so-called COVID-19 virgins are simply unaware of their past infections.

How could someone have had COVID-19 and not know it? First, they might have had an asymptomatic or mild infection, which is fairly common, even in adults (bear in mind this doesn’t always mean no long-term consequences). Second, they might have assumed they were sick with a common cold, because the symptoms are so similar. Third, they may have tested for COVID-19, but had a negative result despite truly being infected. A single COVID-19 test will miss a lot of infections, especially if the timing was too early; this is true of both rapid antigen tests and PCR tests.

The opposite mixup can also occur – thinking you had COVID-19 when you did not. Among Canadians who suspected that they had previously been infected, but never tested positive, only 56% showed antibodies. This type of mixup is less likely these days, when most North Americans have been infected at least once. Remember, this study was conducted back in 2022, when about half of Canadians had yet to be infected.

The group that knew their COVID-19 history best was those who had evidence, in the form of a positive test. Almost 9 out of 10 Canadian adults who previously tested positive for the virus (by PCR or rapid antigen test) had antibodies from a natural infection.

Here are a couple more fascinating tidbits from recent North American studies:

Younger Canadians and Americans are far more likely to have antibodies from a past COVID-19 infection than their older counterparts.

Roughly 75% of kids and teens in the US had antibodies from a natural infection (data from Feb 2022). Likely, many of these kids/teens are unaware of their past infections.

Rates of COVID-19 virginity are slowly but surely dropping. In Canada, rates of past infection rose from roughly 50% in May 2022, to roughly 80% in Feb 2023, and are still rising. COVID-19 infection rates vary from region to region across Canada and the US.

How do researchers study past infections?

Researchers study past infections by measuring antibodies in blood samples (serum) collected from a random sample of people in the relevant group or region. These are called “seroprevalence” studies. For COVID-19, researchers use special tests designed to detect antibodies to SARS-CoV-2. These tests are specific to natural infections, because they look for antibodies that target the N (nucleocapsid) region of the virus, which vaccines do not contain.

It’s worth noting that seroprevalence studies have some major limitations. The first issue is that antibody levels gradually decline after infection, and are often undetectable a year later. So, these tests often miss infections that happened many months or years ago. The second issue is that antibody (serology) tests, like all biological tests, are imperfect. The sample may not be of great quality, there can be glitches in sample analysis, or the test itself may not have ideal performance. So, while seroprevalence studies can reveal important trends, we should take the precise numbers with a grain of salt.

The Bottom LineIf you’ve tested positive for COVID-19, you can be very confident that you’ve been infected at some point. Otherwise, you can’t be completely sure. You might think you’re a COVID-19 virgin despite having been infected. The reverse is also possible, though less common these days.

Knowing your prior infection status can be an important part of your medical history, especially given the uncertainties about long COVID. This Nerdy Girl is very grateful for home tests, and plans to keep testing her family liberally.

If you want to find out whether you were ever infected – you can get an antibody (serology) blood test from a medical lab. They usually cost around $100. Be sure to use a test that measures N (nucleocapsid) antibodies, not S or spike antibodies and that you time them right. These tests are most reliable one to four months after infection, after which antibody levels may be too low to detect.

Note: Antibody tests that detect evidence of prior infections in the blood are NOT the same as diagnostic tests which tell you if you have COVID-19 right now. Antibody tests (aka serology) look for antibodies against SARS-CoV-2, whereas diagnostic tests (PCR and rapid antigen tests) look for the virus itself.