Some covid-19 patients say that after their immediate illness subsided, they suffered lingering symptoms for months, including fatigue, insomnia, brain fog and respiratory problems.

This has become informally known as “long-haul” covid, or simply “long covid.” And while the precise cause is still being researched, a study out of England released Wednesday suggests these symptoms may be startlingly common.

This research, part of an ongoing survey backed by the British government, found that 2 million people living in England reported experiencing “long covid” symptoms, with women and lower-income people particularly susceptible.

“Our findings do paint a concerning picture of the longer-term health consequences of COVID-19, which need to be accounted for in policy and planning,” Paul Elliott, an Imperial College epidemiologist who led the study, said in a statement.

The apparent prevalence of long covid will raise questions about ongoing treatment and vaccination strategy — not just in England but in the rest of the world, where reports of long-term symptoms are also reported to be widespread but poorly understood.

The British findings were collected by Imperial College as part of its virus-tracking REACT 2 study, based on self-reported data from 508,707 adults between September and February. The study is a preprint and has not yet been peer-reviewed.

Nearly 27,000 of the respondents, roughly 6 percent, said they experienced at least one of 29 symptoms linked with covid-19 for three months or longer. Of those who said they had been infected by the coronavirus, that figure jumped to more than a third.

The two main categories of symptoms were ongoing respiratory issues and those related to fatigue.

The long-term problems were more common among women, especially as ages rose. People who smoked, were overweight or obese, lived in deprived areas or had been admitted to a hospital were also found to have higher risk of long covid.

Given the high number of cases of the coronavirus recorded in England — just over 4 million at present, according to the British government — the researchers said that it was possible that 2 million people or more had experienced symptoms of long covid across the country.

That number is a considerable jump from others. An estimate from the British government’s Office of National Statistics released just this month had put the number of people across all of Britain, including England, Wales, Scotland and Northern Ireland, at 1 million.

“Long covid is substantially more common than we originally thought,” David Strain, a senior clinical lecturer at the University of Exeter, said in published comments on the new study, adding that it appeared to be having “a substantial impact on people of all ages with lasting consequences.”

Other countries have pointed to large numbers of people experiencing long-term covid symptoms.

Another study from Norway published this week found that more than half of home-isolated young adults with mild covid-19 had suffered significant symptoms six months after their initial infection.

The most prevalent symptoms in that study, conducted by researchers at the University of Bergen, were difficulty concentrating and respiratory problems.

A workshop at the National Institutes of Health held in December suggested that between 10 and 30 percent of the people infected with the virus in the United States would suffer some kind of long-term symptoms.

NIH in February launched a $1.15 billion initiative over four years to study the causes and treatment of long-haul covid.

There has been anecdotal evidence in the United States that some people suffering from symptoms of long covid improved after being vaccinated, but so far there have not been conclusive studies on the matter.

In response to the latest data from the REACT 2 study, British officials pledged more action to understand and treat long-term covid-19 symptoms. British Health Secretary Matt Hancock said that the government had set aside $70 million for research on the subject.

“Long covid can have a lasting and debilitating impact on the lives of those affected,” Hancock said Thursday. “Studies like this help us to rapidly build our understanding of the impact of the condition, and we are using these findings and other new research to develop support and treatments.”