In the pantheon of enigmas among COVID's seemingly endless box of mysteries, there are few more perplexing than the phenomenon of long COVID. Also called long-haul COVID, post-acute COVID-19, chronic COVID, or just the long-term effects of COVID, this is a condition in which someone infected with COVID may have coronavirus-related symptoms a month or longer after first contracting the pathogen, according to the Centers for Disease Control (CDC). But does being fully vaccinated against COVID prevent or at least mitigate the possibility of suffering from one or more of the disease's multitude of symptoms for an extended period?

That doesn’t mean that everyone who gets infected with a new strain or a reinfection will show symptoms, serious or otherwise, and vaccination remains the single most effective tool at preventing hospitalizations and deaths. But long COVID is a strange creature that can manifest in patients who didn't show signs of illness when they first contracted the disease.

"Even people who did not have COVID-19 symptoms in the days or weeks after they were infected can have post-COVID conditions. These conditions can have different types and combinations of health problems for different lengths of time," says the CDC. That means a patient may wind up experiencing chronic fatigue (the most commonly reported long COVID symptom), breathing problems, skin issues, gastrointestinal distress, or a hodgepodge of COVID-associated ailments without ever experiencing a more acute version of COVID-19.

The data on long COVID isn't all that robust, especially when it comes to vaccinated people who have breakthrough infections of COVID-19. Some studies suggest that somewhere around one in four COVID patients (not all of whom are necessarily vaccinated) report at least one lasting symptom of the disease 30 days or more after initial diagnosis. But this data is highly inconsistent.

For instance, a more recent analysis by the U.K. government found that "[l]ong COVID symptom prevalence at 12 weeks post SARS-CoV-2 infection is uncertain and estimates vary by study design, ranging from 2.3%–37% in those infected." That's a massive range depending on which study you're looking at. And for good reason: The highest risk factors for long COVID appear to be having a serious COVID-19 case, being female, having high cholesterol or a compromised immune system, or suffering from a pre-COVID respiratory condition, among others. But even these are inconsistent and unpredictable across various age demographics and patients' inner biology. Figuring out who may contract long COVID, and accurately recording whether or not something is actually a symptom of long COVID, is still very much a science in the works.

Which brings us back to the issue of whether or not fully vaccinated people are susceptible to this medical mystery. We're at the point where physicians, academics, and public health experts are still figuring out the best way to record the prevalence of long COVID in those who've received their jabs. Bob Wachter, a physician and chair of UCSF's Department of Medicine, points to Israeli data published in The New England Journal of Medicine showing that 20% of breakthrough COVID cases in the vaccinated led to symptoms that lasted six weeks after infection.

But Farzad Mostashari, the former national coordinator for health information technology at the U.S. Department of Health and Human Services (HHS), isn't quite as convinced that nearly 20% of immunized people could experience long COVID after contracting something like the Delta variant.

Mostashari points out that many people who are asked whether or not they are experiencing symptoms related to a disease they've already had may say they still have symptoms due to "recall bias," the memory of the symptoms they had when they were initially sick. He goes on to suggest a more robust study that would also incorporate random samples of vaccinated people who had never been diagnosed with COVID in the first place to ask them whether or not they're experiencing certain symptoms and when those symptoms began. A follow-up blood test could confirm a case of COVID, and then it's just a matter of shoring up the timeline to see whether or not it's a recent infection or long-haul COVID.

But there just still isn't enough information to date to definitively proclaim or predict who may be affected by this twist on COVID-19. The only certainty is that COVID vaccines—and masking and distancing precautions in regions with surging Delta variant cases—is the most effective known method of preventing the coronavirus’s spread.