When I tested positive for COVID-19 over the winter holidays, I spent a good bit of time puzzling over how and where I was exposed. I was infected just as omicron was ripping through New York City, so it wasnt a surprise. But I am also a fairly COVID-cautious health reporter. I wore an N95 in public settings, Id been working from home, and I was newly boosted. How did I end up with a breakthrough infection and not, say, my husband, who goes into work every day? Why was I the one exposed, and not my unvaccinated preschooler who spends his days in the company of germy 3-year-olds?

Of course, health experts have warned since omicron took over as the dominant strain that everyone is likely to get COVID-19 at some point. And a recent Centers for Disease Control and Prevention survey that used blood tests to check for coronavirus antibodies suggests that there have been many more cases than our official counts suggest. Yet all of the data we have at this point suggests there are still millions of Americans who havent been infected.

So what gives? How can it be that two years into a pandemic marked by increasingly contagious variants, so many people remain COVID-free? What separates those of us who have tested positive from those who havent?

Here are a few reasons why some people have never had COVID-19 at this point in the pandemic.

Reason 1: Because vaccines, masking and distancing works

Maybe youre sick of hearing it, maybe not. But doctors, researchers and public health experts have long been harping on the same basic preventive tools for a reason: They work.

There are a few measures that have shown to reduce the spread of infection: getting effectively vaccinated, wearing masks indoors, and maintaining physical distance, said Dr. Marie-Elizabeth Ramas , a New Hampshire-based family physician.

A study published just this week found, for example, that requiring masks in schools may cut infections by nearly 25%. Of course, that data comes at an interesting time, when many of the last states with mask mandates in schools are dropping them.

Ramas note about getting effectively vaccinated is an important one, reflecting that boosters have become an important tool although less than half of people who are fully vaccinated have received one at this point.

So if youve managed to avoid infection so far, theres a good chance its because youve been diligent about masking up, avoiding crowds and getting your shots as soon as you were eligible.

Youre talking to someone who has not had COVID despite it ruling my life for the past three years, and seeing tons of patients who have it. ... A lot of it is just self-management in terms of getting vaccinated and boosted [and] making sure that you have adequate PPE and being fastidious with it, said Dr. Kevin Dieckhaus , chief of infectious diseases at UConn Health in Connecticut.

Reason 2: Because youve remained totally isolated

While both Dieckhaus and Ramas agreed that the vast majority of people have been exposed to the coronavirus over the past two years, both noted that there are some people who are seriously immunocompromised and who have largely hunkered down during the pandemic.

About 7 million Americans are considered immunocompromised, a category that includes (but is not limited to) people who have had cancer, who have had organ transplants, who are undergoing chemotherapy or who are on immunosuppressive medications. While many of these people have found ways to safely head out in public, for others, isolation has been their only real option. And lets not forget that the millions of people with weakened immune systems need this level of protection still.

As one woman whose husband had a heart-lung transplant recently told me for a story on what its like to be immunocompromised in the pandemic : What I wish people who dont know an immune-compromised person would know is that COVID is still here and it is still a real threat to our most vulnerable members of society.