Some people catch COVID over and over while others seem to dodge it entirely, and the reasons come down to a mix of genetics, immune memory, the bacteria living in your nose, how much virus you’re exposed to, and how your immune system was shaped by previous infections. No single factor explains it. Instead, several biological and behavioral variables stack on top of each other, making certain people consistently more vulnerable to reinfection.
Your Genes Shape How Well You Fight the Virus
Your immune system relies on a set of molecules called HLA proteins to identify and attack infected cells. These proteins vary enormously from person to person, and some versions are simply better at recognizing SARS-CoV-2 than others. People who carry a gene variant called HLA-B*15:01 are significantly more likely to have asymptomatic infections, meaning their immune systems neutralize the virus so efficiently they never feel sick. Research published in Nature found that T cells from people with this variant were already reactive to a key piece of the virus even in blood samples collected before the pandemic, likely because of prior exposure to ordinary cold-causing coronaviruses that share similar features.
On the other end of the spectrum, people carrying HLA-B*46:01 appear more vulnerable. This variant has a low predicted ability to bind to SARS-CoV-2 proteins and present them to immune cells, which may leave the body slower to mount a defense. Another variant, HLA-B*07:02, falls somewhere in between: carriers tend to have cross-reactive T cells from past common-cold coronavirus infections that help reduce disease severity. The point is that the genetic hand you were dealt influences how quickly and effectively your immune system spots the virus each time it shows up.
Immune Imprinting Can Work Against You
One of the more counterintuitive reasons for repeat infections involves your immune system’s own memory. When you first encounter SARS-CoV-2, either through infection or vaccination, your body creates antibodies and memory cells tailored to that specific version of the virus. That’s a good thing. The problem comes later, when a new variant arrives that looks somewhat different.
Instead of building a fresh response to the new variant, your immune system tends to reach back into its memory and boost the antibodies it already has, even if those antibodies are a poor match for the current strain. This phenomenon is called immune imprinting. With each successive exposure to a related but different variant, the response can become progressively narrower, focused on features of the original strain rather than the one actually infecting you. The extent of this imprinting depends on the order and type of your exposures (mild infection versus severe illness versus vaccination) and how different the new variant is from what you encountered first.
This helps explain why some people who were infected early in the pandemic and then vaccinated with the original vaccine formula seem to have a harder time fending off newer variants. Their immune systems are essentially looking for a virus that no longer exists in circulation. Researchers have described a related effect called “hybrid immune damping,” where people infected with the original strain before vaccination showed a blunted immune boost when later exposed to Omicron-era variants.
The Bacteria in Your Nose Affect Your Risk
SARS-CoV-2 enters your body through proteins on the surface of cells lining your nose. Two proteins in particular act as the virus’s doorway, and the mix of bacteria naturally living in your nasal passages can dial the production of these entry proteins up or down.
Research in eBioMedicine found that people with high levels of certain common bacteria, including Staphylococcus aureus, had nearly double the odds of elevated expression of these viral entry proteins. People with high levels of two other species had 4.7-fold and 7.4-fold higher odds, respectively. In contrast, people with abundant Dolosigranulum pigrum, a bacterium associated with healthy nasal environments, showed reduced expression of these entry proteins. Your nasal microbiome isn’t fixed by genetics and shifts over time based on your environment, which means two people breathing in the same amount of virus could have very different chances of actually getting infected.
Protection Fades Faster Than You Think
Whether your immunity came from infection, vaccination, or both, it doesn’t last forever. Antibody levels drop most steeply in the first two to three months after an infection, then decline more gradually. Research tracking neutralizing antibodies over time estimated that levels providing roughly 50 percent protection against reinfection last about 990 days (a little under three years) in people who had symptomatic infections. That sounds reassuring, but it applies to the strain that originally caused the infection. When a new variant emerges that partially evades those antibodies, effective protection can shrink to a much shorter window.
The CDC notes that protection from both vaccination and prior infection decreases over time, and that COVID vaccines are updated to target currently circulating strains for this reason. The current recommendation is a 2025-2026 updated vaccine for anyone six months and older. If you’ve recently had COVID, you can delay vaccination by about three months, since the risk of reinfection is lower in the weeks following an infection. But that temporary shield fades, and without updated immunity, your protection gap widens with every new variant wave.
Some People Are Simply Exposed More Often
Biology aside, the simplest explanation for frequent reinfection is frequent exposure. A study of primary healthcare workers in Jiangsu Province found that reinfection rates were notably higher among nurses, people who worked in fever clinics, and those logging more than eight hours per day on the job. Working longer shifts in high-contact settings increases the total viral dose you encounter, and each exposure is essentially another roll of the dice against your current level of immunity.
Regular physical exercise was identified as a protective factor in the same study, reducing reinfection odds by about 14 percent. The mechanism likely involves exercise’s well-documented effects on immune cell circulation and inflammation regulation. People in sedentary jobs with high interpersonal contact face a double disadvantage: more exposure and a less primed immune response.
Subtle Immune Weaknesses Play a Role
Not everyone who gets COVID repeatedly has an obvious immune disorder. Some people have mild, undiagnosed immune deficiencies that make them more prone to all respiratory infections, not just COVID. Conditions like common variable immunodeficiency often go unrecognized for years because symptoms look like “just getting sick a lot.” These individuals produce fewer or less effective antibodies, meaning each infection generates weaker long-term protection.
Even without a formal immune deficiency, natural variation in immune function matters. Some people generate robust, long-lived memory cells after a single infection. Others produce a weaker response that fades quickly. Age, chronic stress, poor sleep, and underlying conditions like diabetes or obesity all blunt immune responses and speed up antibody decline. If you’re someone who feels like you catch everything going around, COVID included, it may reflect a pattern of generally lower immune readiness rather than something specific to this virus.
What This Means in Practice
Repeat COVID infections aren’t random bad luck. They result from a convergence of factors: your genetic immune profile, the imprint left by your first exposures, the bacterial ecosystem in your nose, how quickly your antibodies wane, and how much virus you encounter day to day. Some of these you can’t change. Others you can influence. Staying current with updated vaccines counteracts the narrowing effect of immune imprinting by introducing your immune system to newer viral targets. Regular exercise provides a modest but real protective benefit. And understanding that protection fades helps explain why an infection six or twelve months ago doesn’t guarantee safety now.
People who keep getting COVID aren’t doing anything wrong. Their biology and circumstances create a shorter window of effective immunity, and in a world where new variants continue to circulate, that window closes before the next wave arrives.