Some people who experience cognitive issues after long COVID continue to struggle with brain function for at least two years, a new study shows.

Researchers in the United Kingdom found that people who reported having long COVID symptoms for at least 12 weeks after being infected with the coronavirus showed reduced performance in tests for, among other things, memory, reasoning and motor control, for up to two years after the infection. Their findings were published in the Lancet journal eClinicalMedicine.

The researchers used an online platform to test thousands of people to better understand how COVID affects brain function and how long those symptoms persist.

It allowed them "to quantify how big the effect of COVID is and who was most affected," said Nathan Cheetham, a senior postdoctoral data scientist at King's College London, who led the study.

In the United States alone, millions of people have reported symptoms of long COVID. Long COVID symptoms can vary widely and include fatigue, respiratory and heart problems, digestive problems and neurological issues such as brain fog. Research shows a majority of people experiencing long COVID symptoms have reported brain fog — a collection of symptoms, including impaired attention, concentration, memory and processing speed. These symptoms can linger for weeks, months or years.

"There has been some contention about whether the cognitive effects of an earlier COVID infection do actually persist," said Anna S. Nordvig, a neurologist at Weill Cornell Medicine who founded a clinic for long-haulers with brain fog. In her own practice, however, she has "absolutely seen these symptoms persist — unfortunately, over three years in some cases, and many times after just a mild COVID infection," she said.

Anecdotally, cognitive difficulties are the most common symptoms seen in long COVID patients, said Greg Vanichkachorn, medical director of the Covid Activity Rehabilitation Program at Mayo Clinic.

"We tend to see a lot of folks get better with physical things like their fatigue and their endurance," he said. "But the cognitive deficits, those are the ones that seem to last the longest. Sometimes they never go away, and a lot of people have had to permanently adapt in order to continue functioning."

People who have experienced brain fog firsthand previously told the Washington Post that it had kept them from activities such as driving, biking and public speaking. Some said they had to alter work schedules or stop working entirely. Nearly all of them also said it had forced them to rely on a notebook — keeping to-do lists that include the most basic tasks such as remembering to eat.

The researchers at King's College London assessed 3,335 voluntary participants in 2021 using an online platform that used 12 tasks to test their working memory, attention, reasoning and motor control. For instance, some of the tasks asked them to remember words or objects, measuring memory function — the area where the researchers saw the most consistent effect.

Participants who had been experiencing long COVID symptoms for 12 weeks or more were found to have significantly larger cognitive deficits — comparable to a 10-year increase in age.

In 2022, 1,768 participants who completed the first round of testing then completed a second round, showing no significant improvements or declines in their test scores over a nine-month period, according to the findings.

Some participants who were vaccinated and then contracted the coronavirus during the course of the study showed no evidence of cognitive impairment, but it was a small number of people, and it's unclear whether that was because of vaccination, the variant or other factors.

There were some limitations to the study. The researchers relied on voluntary participation. More than 80 percent of the participants in the study were women, more than 95 percent were white, and the majority were ages 50 to 64. Also, most lived in more affluent areas.

"What about all the other people who are not in those groups who are out there?" Vanichkachorn said. "We know they must be suffering from long COVID, but how does their cognitive impairment affect their lives?"

The researchers did not conduct pre-COVID cognitive testing, so it's not known whether some participants may already have been experiencing cognitive deficits. Instead, researchers used a control group to compare cognitive skills of healthy individuals to those previously infected with the virus.

Finally, there was no monitoring of the cognitive testing because it was completed online.

Still, experts say, the study is important in understanding the cognitive effects of COVID.

One of the most important takeaways from the study, experts say, is that the longer the duration of symptoms — the 12-week cutoff — the more likely participants were to experience cognitive deficits. This finding shows the need for early intervention, possible treatment and tracking of patients to make sure their cognition does not worsen.

"What this tells us is that we have to follow folks long term," said Dennis Kolson, a neurologist in the Penn Neuro Covid Clinic at the University of Pennsylvania. "We have to follow them to determine what happens three, four, five years down the road."