By almost any measure, living through a once-in-a-century pandemic emergency that killed 7 million people — including 1.1 million in the U.S. alone — was a harrowing experience.

Now, there's new evidence suggesting our brains bear the scars of that ordeal.

Living through the pandemic aged our brains faster — even among people who never became sick with COVID-19, according to a recent study.

"Our study suggests that the experience of living through the COVID-19 pandemic was associated with slightly faster brain aging, even in people who were never infected with the virus," the study's lead author, Ali-Reza Mohammadi-Nejad of the University of Nottingham in England, wrote in an email to The Times.

"This effect was subtle but measurable," he added.

The study, published this month in the journal Nature Communications, wasn't designed to pinpoint the exact cause of the accelerated brain aging.

"But we believe the cumulative stressors of the pandemic — such as prolonged isolation, disrupted routines, reduced physical and cognitive activity, and economic uncertainty — likely contributed to the observed brain changes," Mohammadi-Nejad said.

Those factors are all known to influence brain health over time. As the study notes, "it remains unclear whether these brain aging effects may be at least partially reversible."

Dr. Peter Chin-Hong, a UC San Francisco infectious-disease expert who wasn't affiliated with the study, said its findings demonstrate "that even if you don't get infected, all the repercussions of the pandemic — like social isolation and stress — can have an impact on brain health."

"We know that other things that we do have an impact on brain health, like exercise and diet and sleep," Chin-Hong said. "So it makes sense that something as global and as profound as a pandemic would have an impact on brain health."

Those effects don't necessarily include immediate decreased cognitive function, however. Although researchers documented accelerated aging in generally all the brain scans they studied, only those who contracted COVID-19 showed "small but measurable declines in cognitive performance — mainly in processing speed and mental flexibility," according to Mohammadi-Nejad.

"This suggests that infection may introduce additional biological effects, such as inflammation or vascular issues, that are more directly linked to cognitive symptoms," he said.

Past studies have noted cognitive effects after COVID-19 illness, especially in severe cases, he said. But what's new is that evidence of cognitive decline was found even in this study's population "of generally healthy volunteers, most of whom had mild cases."

"It shows that even mild infection can leave subtle traces in the brain, although the effect size is relatively small," Mohammadi-Nejad said.

The effect was especially pronounced among older people, the study found, "suggesting a complex model of cognitive decline due to more pronounced accelerated brain aging from infection-related factors in older people."

The study was based on an analysis of brain scans from the UK Biobank, a large biomedical database in the United Kingdom that contains health information from volunteers taken both before and after the onset of the pandemic.

The study focused on a group of 996 participants and compared brain scans taken before and after the pandemic began. The group comprised primarily middle-aged and older people, ranging in age from 47 to 79. Only those who were generally healthy were included in the study, meaning they didn't have chronic illness such as heart disease, diabetes, dementia, kidney disease or major depression.

In this group, scientists found the accelerated brain aging was seen more strongly overall in older people and men. People who were socioeconomically disadvantaged — ranking lower on indicators such as income, education, employment and health access — also had more pronounced brain aging, Mohammadi-Nejad said, "likely due to increased exposure to pandemic-related stress and fewer resources to buffer its effects."

This isn't the first study to suggest brain health was altered by the experience of living through the pandemic.

A study published last year in the journal Proceedings of the National Academy of Sciences, by scientists at the University of Washington, suggested that COVID-19 lockdown measures "resulted in unusually accelerated brain maturation in adolescents" and was greater in magnitude in females than males.

The authors of that study noted that "females are at a higher risk for developing anxiety and mood disorders than males during typical adolescent development."

But among adults, past research has suggested "men may be more susceptible to certain forms of brain aging," Mohammadi-Nejad said. Other research, the study noted, has found "greater male susceptibility to cortical atrophy and neuroinflammation under stress, which aligns with our findings of heightened pandemic-related brain aging in males."

There are a number of limitations to the study. The people participating in the study who had COVID-19 had mostly mild cases — and they were generally healthier than the population overall, since those with chronic illnesses were excluded.

Among study participants who contracted COVID-19, fewer than 4% required hospitalization. The vast majority had mild illness; and all participants tested negative for COVID-19 within two to three weeks.

Other factors also could contribute to the documented brain aging, including "reduced physical activity, poorer diets and increased alcohol consumption" during the pandemic, the study said.

Many unanswered questions remain. If further studies do prove the pandemic-related factors caused accelerated brain aging, rather than simply being associated with it, how long will those effects last? And, "once you have it, can you do anything to make it better?" Chin-Hong asked.

The study doesn't answer the question of whether the accelerated brain aging is reversible.

But it is well known that there are things that are good for general brain health, Mohammadi-Nejad said: physical activity, mental stimulation, social interaction, healthy sleep and good nutrition among them.

"Public policies that reduce social isolation and ensure continued access to physical, cognitive, and emotional well-being during major disruptions could help mitigate future effects on brain health," he said.

For some, the study may raise the question of whether the pandemic-era response measures employed in Britain were worth the cost.

But answering that question today — years after COVID ceased to be a novel public health threat — is complicated.

"Our study wasn't designed to evaluate public health policies or determine what should or shouldn't have been done. What we show is that the pandemic experience, independent of infection, was associated with changes in brain health," Mohammadi-Nejad said.

COVID-19 today is very different than the dark early period of the pandemic, when the disease devastated many families, prematurely killing grandparents and leaving children to grow up without their dad or mom. The death rate was much higher in the initial phase of the emergency, with hospitals in some areas overwhelmed with staggering numbers of critically ill people.

The risk of getting long COVID, and suffering debilitating symptoms such as brain fog or persistent fatigue, was also once much higher than it is today.

Early in the pandemic, "it was a more serious time for COVID," Chin-Hong said. "It was a time when you didn't want to get infected at all.... Like, who would want to get Alpha or Delta, you know?" he added, referring to the variants that preceded Omicron.

Today's version of COVID is "less invasive — even independent of the fact that we have more immunity," Chin-Hong said. The latest subvariants of the coronavirus don't "get inside the body as much as the earlier variants."

Chin-Hong said he's glad there was a time early in the pandemic where "we tried to avoid COVID as much as possible," noting that infection itself has a chance of affecting the brain.

That said, it's also clear that health issues such loneliness have a clear effect on brain health. Dr. Vivek Murthy, the former U.S. surgeon general, issued an advisory on what he called a national epidemic of loneliness and isolation in 2023, and warned that loneliness is associated with a greater risk of dementia, depression, anxiety and premature death.

About half of U.S. adults experienced loneliness even before the pandemic, according to Murthy's report, which urged the public to fight loneliness and isolation by taking steps to strengthen their relationships. The British government in 2018 found loneliness to be such a public health concern that it created a new position: minister of loneliness.

The latest study underscores the idea that things such as exercise, sleep, diet, social connection and stress reduction are important for our brain health, according to Chin-Hong.

Resolving stress and a lack of social connection "is probably as important as ... focusing on things like blood pressure and the things we traditionally think about," he said.

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