It's a question few people know how to answer, even after three pandemic years and more than 100 million Covid cases in the U.S.: When someone gets infected today, what is their risk of developing long Covid?

"Even the medical community is unclear on all of this. The data is just emerging so rapidly and the estimates are varied," said Dr. Rainu Kaushal, chair of the department of population health sciences at Weill Cornell Medicine.

In the absence of definitive data on long Covid risk, seven researchers investigating the condition's prevalence in the U.S. offered NBC News their best guesses. Most said it's fair to assume that the current risk for vaccinated people is 10% or less, and some thought the odds were smaller — 5% or lower.

Even reinfections bring a risk of long Covid, they added.

"You may get long Covid the second time around, the third time around. That reinfection is absolutely consequential, and you’re pretty much doing Russian roulette again," said Dr. Ziyad Al-Aly, chief of research and development at the Veterans Affairs St. Louis Health Care System and a clinical epidemiologist at Washington University in St. Louis.

Since the start of the pandemic, nearly 38 million adults in the U.S. have reported post-Covid symptoms that lasted three months or longer, according to an NBC News analysis of data from a household survey conducted by the Census Bureau. The survey results showed that as of February, around 11% of adults who'd ever had Covid were experiencing long Covid.

Those figures lump vaccinated and unvaccinated cases together, though the risk profiles differ: The researchers interviewed estimated that for unvaccinated people, the risk of long Covid is around 15% to 20%.

The U.S. is still recording more than 225,000 new Covid cases each week, according to the Centers for Disease Control and Prevention, so long Covid is an ever-present threat.

"Out of 100 people you see, 95 or 96 people are going to be just fine. But you could be one of those unlucky four, and I personally don’t want to take my chances," Al-Aly said.

The risk of long Covid seems to be shrinking

Long Covid is most often defined as symptoms lasting at least three months after a coronavirus infection, though the symptoms are wide-ranging and vary in severity and duration. Many patients report fatigue, brain fog, shortness of breath, heart palpitations, and muscle, joint or chest pain.

Overall, the rate of long Covid has been decreasing since the Census Bureau started including questions about it in its household survey in June 2022. At that time, 19% of respondents who'd had Covid reported current symptoms that had lasted three months or longer.

But Sharon Saydah, a senior epidemiologist at the CDC who leads the agency’s post-Covid conditions team, said the survey is "not a good way to judge individual risk," given that it includes new long Covid cases and those that have lasted since earlier in the pandemic.

Experts offered a few likely explanations for the downward trend in long Covid cases: the uptake of vaccines and treatment, and the nature of the omicron variant.

A spate of studies have shown that even getting a single Covid shot decreases a person’s chances of long-term symptoms, and that the risk declines further with each additional shot. The antiviral Paxlovid also seems to lower the odds of long Covid, according to a study that’s awaiting peer review.

"We now have more things that can help reduce your risk," said Dr. Michael Gottlieb, an emergency medicine physician and researcher at Rush University Medical Center.

Studies have also shown a decline in long Covid rates after the rise of the omicron variant, though researchers disagree as to whether the variant itself is associated with fewer long-term health issues than its predecessors.

"The existing data does suggest that the risk of long Covid in the post-omicron era may be less, but it's confounded by the rise in vaccination rates occurring essentially over the same time period," said Dr. Stuart Katz, a principal investigator with the National Institutes of Health's RECOVER initiative, which is studying the long-term effects of Covid.

Factors that influence a person's risk of long Covid include their age, sex, health history and the severity of their illness. According to a review in the journal Nature, women and people with Type 2 diabetes or ADHD may have an elevated risk.

Kaushal said she is seeing persistent heart and kidney problems in older men who had severe Covid, as well as sleep disorders and shortness of breath in younger women.

New long Covid cases may be getting less severe, on average

There is some hope that new cases of long Covid are milder than ones that developed earlier in the pandemic, due to the same factors that are causing rates to decline.

"The incidence of it, the severity of it and the duration of it seems to be decreasing," Kaushal said.

Dr. Sun Yoo, director of the Covid Ambulatory Monitoring Program at UCLA Health, said she still sees debilitating cases of long Covid, but "in general overall, I am seeing less of it and less severe versions of it."

Katz noted, however, that the sheer number of omicron infections may partly explain why doctors are seeing more patients with milder symptoms.

And some vaccinated people still face persistent, life-altering health issues after getting Covid.

"I see these patients — some of them literally cannot even read an email. They have brain fog that is so profound, it substantially impairs their cognitive abilities," Al-Aly said.

Studies give a range of long Covid rates

Quantifying the risk of long Covid is difficult for many reasons, and Saydah said it's also tough to compare the rates to other post-viral illnesses, since other viruses haven’t caused this level of widespread disease over a short time.

Studies of long Covid have defined the condition in distinct ways and evaluated different types of patients.

"Everybody’s kind of come up with different approaches," said Dr. Roy Perlis, who directs the Center for Quantitative Health at Massachusetts General Hospital. "There’s the health records approach. There’s the survey approach. ... All of those tell us different things. None of them are perfect."

One of the largest studies on the topic, from Al-Aly’s team in St. Louis, suggested that the overall risk of long Covid was 4% to 7% during the first year of the pandemic.

"My hunch is now that it’s lower than that," he said.

That study examined the health records of more than 181,000 patients at the VA who got Covid through March 2021, then compared that group to nearly 4.4 million veterans who did not test positive.

But relying on health records can lead to underestimates, since such studies only include patients who have sought medical care and been tested for Covid.

To account for that, Yoo and her team at UCLA directly observed more than 1,000 Covid patients who were at high risk of severe illness or hospitalized during the first year of the pandemic. Their study found that 30% had lingering symptoms up to 90 days after their diagnosis or hospital discharge.

She hypothesized that the overall risk for vaccinated people today could be around 5% to 10%.

Kaushal, too, said her "gut sense is that we're closer to 5%."

Saydah said those estimates "seem reasonable" given CDC data, which put the rate of long Covid at 20% from March 2020 to November 2021. Now, she explained, "studies are showing that vaccination reduces your risk anywhere from 20% to 50%."

The RECOVER initiative's observational study has enrolled more than 18,000 people, so it could eventually give clearer estimates of the risk of long Covid, according to Katz. Interim findings expected later this year may also shed light on the condition's most common symptoms.

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