CHICAGO – Wendy French of northwest suburban Lake in the Hills used to run 10 miles a day several times a week before she caught COVID-19 in September, which left her fatigued and suffering from a variety of symptoms for months after the virus was supposedly gone.

The previously healthy 45-year-old stopped running and even began dreading typical household chores such as doing laundry, because it required standing up for so long that she grew tired.

But after French got her first dose of the Pfizer vaccine in mid-April, she described feeling healthy for the first time in more than seven months. The second dose in May brought greater improvement.

"I felt really good the next day after the first one," she said. "I had more energy than I've had in weeks."

It's a phenomenon that has surprised — and elated — medical experts: A growing number of COVID-19 "long-haulers," those with lingering long-term symptoms linked to the virus, are reporting sudden improvement after getting a COVID-19 vaccine.

Initial research has found that 30 to 40% of these patients describe some symptom relief post-vaccination, the latest medical mystery posed by the virus.

It's still unclear why some survivors don't seem to get better weeks or even months after infection. Now scientists and physicians are trying to understand why many of these patients seem to feel better after getting vaccinated, improvements that range from a mild decline in symptoms to a life-changing return to their pre-COVID-19 health.

Theories include the possibility that the vaccine might be stopping a harmful immune response in long-haulers or that the shot could be resetting their immune systems. Some clinicians and scientists have theorized that long-haulers suffer from residual amounts of virus remaining in their bodies, and vaccination might help their immune systems fight off what's left.

Equally puzzling is why the shots seem to help some long-haul patients recuperate while others report no symptom improvement after getting immunized.

"We don't have a lot else to offer people with long-COVID," said Dr. Daniel Griffin, an infectious disease physician at Columbia University in New York. "So the thing we encourage across the board is get those vaccine doses, see if it makes a difference."

French was initially hesitant to get vaccinated, fearing her condition might deteriorate further after the shot. She recalled waiting at a Walgreens pharmacy before her first appointment, still ruminating about the decision.

"What if I feel worse again?" she thought at the time.

But she had read accounts online from other COVID-19 long-haulers whose symptoms dissipated after getting the shot, which gave her the courage to get vaccinated as well.

She felt better after the first dose, then her energy seemed to fade but returned once again after the second dose, with improvement ever since.

In addition to fatigue, she had also suffered from vertigo-like symptoms, which disappeared after getting the shots. The mother of two has begun running again, and she hopes her recovered health is permanent.

She recalled her husband's recent reaction: "Thank God, you came back."

A few other post-COVID-19 ailments persist, such as the sensation of a lump in her throat when swallowing or a strange taste of certain foods, namely bananas, which suddenly have an odd chemical flavor.

Yet French said these conditions are minor compared to the fatigue, adding that "everything is mostly better."

"It worked for me," she said. "I can get back to my old life again."

Social media sites offer a glimpse at the shock — and joy —of so many long-haul COVID-19 patients who finally found relief after getting vaccinated.

"There IS hope!" one man posted on the Facebook page of Survivor Corps, an international group of more than 167,000 members who share their experiences with the virus. "I was totally asymptomatic when I had COVID, but the after­effects had pretty much ruined me, ruined my life, I was a shadow of myself. I didn't think I was going to make it or ever get any semblance of my life back. But I DID, now I'm hoping, praying, wishing that we all do/can!"

"I had my second vaccine a month ago and I can feel the clouds lifting," a woman responded. "I don't want to jinx it by saying I'm cured, but I feel I'm heading in the right direction."

A survey on the Survivor Corps Facebook site found that roughly 40% of participants experienced some health improvement post-vaccination. The poll, which was tallied in May, includes nuanced questions about which vaccine the respondent took and the degree of improvement, as well as which symptoms seemed to have lessened or disappeared. Other respondents indicated no change post-vaccination, and some reported feeling worse.

Scientist Natalie Lambert has been studying the experience of long-haul COVID-19 patients since the start of the pandemic and has been compiling data and narratives from the Survivor Corps site.

She points to the results of the recent poll as a hopeful sign that vaccination might give other long-term patients relief as well as offer clues to explain underlying causes of long-haul COVID-19.

"It's not only a miracle that people are starting to feel better," said Lambert, associate research professor at Indiana University School of Medicine. "It helps us to understand, how is this disease, how is it causing the damage that results in someone becoming a long-hauler?"

While just 30 to 40% of long-haul COVID-19 patients found some symptom relief after they received a vaccination, Columbia University's Griffin said, he acknowledges that everyone doesn't recover. He still urged all long-haulers to get the shot and see if it helps.

"I uniformly recommend it," he said.