Vaccination could offer a glimmer of hope for people who are still enduring symptoms weeks or months after recovering from COVID-19.
Somewhere between about 10% and 30% of people who catch COVID-19 end up with long-term symptoms referred to colloquially as long-COVID or long-haul COVID. Although most of the people with lingering problems had a bad bout with the disease, some barely had any symptoms at all.
A new theory is emerging, though still preliminary, that getting a COVID-19 vaccine could help some of these long-suffering people. 
In a survey of nearly 600 people who self-reported lingering symptoms after COVID-19, 47% saw no difference after vaccination, 39% improved after getting a vaccine and 14% felt worse. The survey was conducted by Survivor Corps, a grassroots group of people with long-term COVID-19. 
Their most common lingering symptom was fatigue, which was reported by nearly everyone and lasted 100 or more days. Other frequent long-term symptoms included shortness of breath or difficulty breathing, difficulty concentrating, inability to exercise or be active, diarrhea, headache and loss of smell and/or taste.
It's biologically plausible that vaccination could help address some of these symptoms, said Akiko Iwasaki, a professor of epidemiology and immunobiology at Yale, who developed the theory. 
It may be that their bodies still harbor particles or pieces of the virus that causes that causes COVID-19. The vaccine could theoretically stimulate their immune system to hunt down these remaining bits and clear them away so they can no longer cause inflammation.
"We know that the vaccine elicits very robust antibodies that can bind to the virus and viral remnants and clear it," she said. 
Iwasaki said it would be terrific if this works, because that would make vaccination a "cure" for many people with lingering symptoms. "Get rid of the source and that's it."
But that's probably not the whole answer.
It might also be that in some people, a COVID-19 infection pushed their immune system into overdrive. For them, a vaccine might provide only temporary relief, toning down this overresponse while the vaccine is circulating in the body, but not addressing the underlying problem.
But even in this case, knowing that would be important, Iwasaki said, because it would suggest that treatment to turn down an immune over-reaction, could be helpful.
"We really need to study their immune system," she said.