It’s been two and a half years now since the Covid pandemic emerged from China. Most people world-wide have now likely been infected with the virus at least once, according to many leading epidemiologists. If you think you’ve never had Covid, you’re either lucky or you’re wrong.

Some 58 % of people in the U.S. had contracted Covid-19 through February of this year, according to the Centers for Disease Control and Prevention (CDC). Since then, a persistent wave driven by offshoots of the infectious Omicron variant has kept daily known cases in the U.S. above 100,000 for weeks. Those numbers, however, only count confirmed positive tests. They don’t count people who never got tested.

Julie Wernau, writing in The Wall Street Journal, says geneticists and immunologists are studying factors that might protect people from infection, and learning why some are predisposed to more severe Covid-19 disease. For many, the explanation is likely that they have in fact been infected with the virus at some point without realizing it, said Susan Kline, professor of medicine at the University of Minnesota Medical School. About 40% of confirmed Covid-19 cases are asymptomatic, according to a meta-analysis published in December in the Journal of the American Medical Association.

Many people, like me, have never tested positive for Covid, but they have had “a slight cold”, without a fever, extreme fatigue, or loss of smell and taste – symptoms typically associated with Covid. We may have actually had Covid without knowing it. About 90% of people who get Covid-19 make antibodies that can be detected in their blood, said Sheldon Campbell, a pathologist and lab-medicine doctor at Yale Medicine. But no one is going to do these tests on asymptomatic people.

And then there are those who contract asymptomatic Covid, and yet don’t make antibodies. Most tests can’t distinguish antibodies from infection versus those from vaccination. Because current Covid-19 vaccines target the virus’s spike protein, using a test that looks for antibodies that target a different protein, called the nucleocapsid, can distinguish a prior infection even in those who have been vaccinated. However, the CDC and the FDA discourage antibody testing in many cases, in part because they only show that a person was infected or vaccinated, not how much protection antibodies might provide.

Researchers are currently studying factors that might keep the virus from infecting some people, or that affect how a person responds to the virus. These factors may explain why some patients are immune to the virus and others contract serious disease. Research has suggested that mutations in genes that drive immune response to viruses can affect a person’s ability to fight the disease.

There is some evidence that some people who may have been exposed to certain coronaviruses before the pandemic are equipped with cells that attack SARS-CoV-2 before it can spread, said Steve Jameson at the University of Minnesota Medical School. “Some people come with a little bit of a head start,” he said. People who don’t know whether they have been infected should be careful, he warns, because they might get sick as antibodies wane and new variants arrive.

“There are plenty of people who’ve had the vaccines or even had Covid and then have gotten Covid again,” said Dr. Jameson. “It’s not as if it makes you immortal.”