Since the beginning of the pandemic, doctors have found that people who become very ill with COVID-19 often experience kidney problems, not just the lung impairments that are the hallmark of the illness.

Now, a large study suggests that kidney issues can last for months after patients recover from the initial infection and may lead to a serious lifelong reduction of kidney function in some patients.

The study, published Wednesday in the Journal of the American Society of Nephrology, found that the sicker COVID patients were initially, the more likely they were to experience lingering kidney damage.

But even people with less severe initial infections could be vulnerable.

"You see really, across the board, a higher risk of a bunch of important kidney-associated events," said Dr. F. Perry Wilson, a nephrologist and associate professor of medicine at Yale, who was not involved in the study. "And what was particularly striking to me was that these persisted."

Kidneys play a vital role in the body, clearing toxins and excess fluid from the blood, helping maintain a healthy blood pressure, and keeping a balance of electrolytes and other important substances. When the kidneys are not working properly or efficiently, fluids build up, leading to swelling, high blood pressure, weakened bones and other problems.

The heart, lungs, central nervous system and immune system can become impaired. In end-stage kidney disease, dialysis or an organ transplant may become necessary. The condition can be fatal.

The new study, based on records of patients in the Department of Veterans Affairs health system, analyzed data from 89,216 people who tested positive for the coronavirus between March 1, 2020, and March 15, 2021, as well as data from 1,637,467 people who were not COVID patients.

Between one and six months after becoming infected, COVID survivors were about 35% more likely than non-COVID patients to have kidney damage or substantial declines in kidney function, said Dr. Ziyad Al-Aly, chief of the research and development service at the VA St. Louis Health Care System and senior author of the study.

"People who have survived the first 30 days of COVID are at risk of developing kidney disease," said Al-Aly, a nephrologist and assistant professor of medicine at Washington University School of Medicine.

Because many people with reduced kidney function do not experience pain or other symptoms, "what's really important is that people realize that the risk is there and that physicians caring for post-COVID patients really pay attention to kidney function and disease," he said.

The two sets of patients in the study differed, in that members of one group had all been infected with COVID and members of the other group may have had a variety of other health conditions. Experts cautioned that there were limitations to the comparisons.

The researchers tried to minimize the differences with detailed analyses that adjusted for a long list of demographic characteristics, preexisting health conditions, medication usage and whether people were in nursing homes.

Another limitation is that patients in the VA study were largely male and white, with a median age of 68, so it is unclear how generalizable the results are.

One strength of the research, experts said, is that it involves over 1.7 million patients with detailed electronic medical records, making it the largest study so far on COVID-related kidney problems.

While the results most likely would not apply to all COVID patients, they show that for those in the study, "there's a pretty notable impact on kidney health in survivors of COVID-19 over the long term, particularly those who were very sick during their acute illness," said Dr. C. John Sperati, a nephrologist and associate professor of medicine at Johns Hopkins, who was not involved in the study.

Other researchers have found similar patterns, "so this is not the only study suggesting that these events are transpiring after COVID-19 infection," he added.

He and other experts said that if even a small percentage of the millions of COVID survivors in the United States developed lasting kidney problems, the impact on health care would be great.

To assess kidney function, the research team evaluated levels of creatinine, a waste product that kidneys are supposed to clear from the body, as well as a measure of how well the kidneys filter the blood called the estimated glomerular filtration rate.

Healthy adults gradually lose kidney function over time, about 1% or less a year, starting in their 30s or 40s, Wilson said. Serious illnesses and infections can cause more profound or permanent loss of function that may lead to chronic kidney disease or end-stage kidney disease.

The new study found that 4,757 COVID survivors had lost at least 30% of kidney function in the year after their infection, Al-Aly said.

That is equivalent to roughly "30 years of kidney function decline," Wilson said.

COVID patients were 25% more likely to reach that level of decline than people who had not had the illness, the study found.

Smaller numbers of COVID survivors had steeper declines. But COVID patients were 44% more likely than non-COVID patients to lose at least 40% of kidney function and 62% more likely to lose at least 50%.

End-stage kidney disease, which occurs when at least 85% of kidney function is lost, was detected in 220 COVID patients, Al-Aly said. COVID survivors were nearly three times as likely to receive the diagnosis as patients without COVID, the study found.

Al-Aly and his colleagues also looked at a type of sudden renal failure called acute kidney injury, which other studies have found in up to half of hospitalized COVID patients. The condition can heal without causing long-term loss of kidney function.

But the VA study found that months after their infection, 2,812 COVID survivors suffered acute kidney injury, nearly twice the rate in non-COVID patients, Al-Aly said.

Wilson said the new data supported results of a study of 1,612 patients that he and colleagues conducted that found that COVID patients with acute kidney injury had significantly worse kidney function in the months after leaving the hospital than people with acute kidney injuries from other medical conditions.

In the new study, researchers did not directly compare COVID survivors with people infected with other viruses, like the flu, making it hard to know "are you really any sicker than if you just had another bad infection," Sperati said.

In a previous study by Al-Aly's team, however, which looked at many post-COVID health issues, including kidney problems, people hospitalized with COVID-19 were at significantly greater risk of developing long-term health problems in virtually every medical category, including cardiovascular, metabolic and gastrointestinal conditions, than were people hospitalized with the flu.

Every type of kidney impairment measured in the new study was much more common in COVID patients who were sicker initially — those in intensive care or who experienced acute kidney injury in the hospital.

People who were less ill during their COVID hospitalization were less likely to have lingering kidney problems but still considerably more likely than non-COVID patients.

"People who are at highest risk are the people who really had it bad to start with," Al-Aly said. "But really, no one is spared the risk."

The study also found that even COVID patients who never needed hospitalization had slightly higher risk of kidney trouble than the general VA patient population. But the risk seemed so small, Sperati said, that "I don't know that I would hang my hat on" those results.

Wilson noted that some COVID patients who did not need hospitalization were nonetheless quite ill, needing to stay in bed for days. He said it's possible that those were the ones who developed long-term kidney dysfunction, rather than people at the mildest end of the COVID spectrum.

Doctors are unsure why COVID can cause kidney damage. Kidneys might be especially sensitive to surges of inflammation or immune system activation, or blood-clotting problems often seen in COVID patients may disturb kidney function, experts said.

This article originally appeared in The New York Times.