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Mayo Clinic is reporting an additional benefit of vaccination beyond protection from COVID-19 infection — a lower likelihood of severe symptoms if people end up with long COVID.

The study offers some of the first proof about a long-held assumption that vaccination reduces the severity of coronavirus infections, which then in turn forecasts whether people will suffer severe post-COVID symptoms.

Long COVID patients in the Mayo study were much less likely to have abdominal symptoms if they had been vaccinated, which is a big deal, said Dr. Greg Vanichkachorn, medical director of Mayo's COVID Activity Rehabilitation Program.

"It sounds silly. Abdominal pain? That's the improvement?" he said. "It can be quite devastating ... I have some people who can't eat like two or three different foods because of food sensitivity, even years after their infections."

The study in the Journal of Investigative Medicine comes amid low but persistent COVID levels in Minnesota, where hospitalizations have increased since Independence Day. The 140 COVID-19 hospitalizations in Minnesota on Tuesday were up from 41 on July 3, but far below the state's pandemic record of 1,864 in late 2020.

COVID levels remain low, but are back to where they were three months ago. Where they go from here is unclear. The latest sampling at 36 wastewater treatment plants across Minnesota found slight increases in viral material in some regions but no change in others. An update on Twin Cities wastewater levels is due Friday, after little change last week.

Most COVID cases right now involve mild symptoms that are treated clinically, but the 18 patients in HealthPartners' hospitals are having respiratory issues associated with the disease, said Dr. Mark Sannes, an infectious disease specialist with the Bloomington-based health care system.

"Because we are now only testing patients with symptoms, the hospital numbers likely reflect individuals who are admitted and have respiratory symptoms from COVID," he said. "They could still be admitted for other things like appendicitis or kidney stones, but they are having some symptom that is prompting us to test them."

Almost six in 10 Minnesota adults have had COVID-19 since the pandemic began in early 2020, and about one in four infected adults ended up with long COVID, according to survey results posted by the Centers for Disease Control and Prevention. Long COVID is loosely defined by U.S. health authorities as symptoms lingering four or more weeks after infection.

Minnesota's overall COVID rate in that survey is likely an undercount based on studies checking random blood samples for antibodies against the coronavirus, but the long COVID rate is the best estimate available now. The Minnesota Department of Health is completing two telephone surveys to come up with its own state estimates.

Vanichkachorn said the Mayo study results support the merits of COVID-19 vaccination for protection from infection and long COVID. Federal officials are expected to authorize a new booster dose that will protect against the dominant XBB strains of the coronavirus circulating in the United States.

"This gives us more reason why we think vaccination is really important," he said.

The study involved 477 patients Mayo treated for long COVID between May 2021 and July 2022. Just over half had been vaccinated, and they were less likely to report "loss of smell, chest pain, dizziness, numbness, shortness of breath, tremors and weakness," according to a Mayo statement. Rates of fatigue, muscle pain and irregular heartbeat were similar between the vaccinated and unvaccinated groups.

The study found the vaccine was highly likely to protect people with long COVID from abdominal pains, but it came with limitations that prevented researchers from proving its effect on other symptoms. The vaccinated patients also were more likely to seek rapid treatment for COVID-19, and that haste might have played a role in the reduced severity of their long COVID later, Vanichkachorn said.

The study also focused on patients who had COVID when the severe delta coronavirus variant was dominant in late 2021. Subsequent variants have produced lower long COVID rates, regardless of vaccination.