People who catch influenza could have prolonged symptoms that are similar to those in some patients with long Covid, a study suggests.
But lasting symptoms occur to a lesser extent among those who have had a bout of flu, academics said.
The study examined how many patients experienced long-lasting symptoms after having Covid-19 and other contributing factors, such as whether different age groups or genders are affected more.
Researchers, led by academics from the University of Oxford and the National Institute for Health Research, also set out to see if they could draw any comparisons with flu.
“We found in a cohort of patients with influenza that the same symptoms also tend to occur, but they tend to occur at lower rates,” said the institute's academic clinical fellow, Dr Max Taquet, who led the analysis.
The team estimated that symptoms linked to long Covid were about 50 per cent more common among those who had a Covid-19 infection than those who had flu.
The study, which analysed health records of more than 273,000 people who had Covid-19 in the US, and 114,000 who had flu, outlined nine long Covid symptoms and the proportion of people who were suffering between 90 and 180 days after initial infection.
They were: abnormal breathing, 8 per cent; abdominal symptoms, 8 per cent; anxiety or depression, 15 per cent; chest or throat pain, 6 per cent; cognitive problems such as "brain fog", 4 per cent; fatigue, 6 per cent; headache, 5 per cent; muscle pain, 1.5 per cent; and other pain, 7 per cent
All nine symptoms were more frequently reported after Covid-19 than after influenza.
The analysis found that about 30 per cent of people who had flu experienced some sort of symptoms three to six months later, compared to about 42 per cent of those who had Covid-19.
When looking solely at Covid-19, the researchers found that different groups were affected by long-lasting symptoms in different ways.
For instance, older people and men had more breathing difficulties and cognitive problems, whereas young people and women had more headaches, abdominal symptoms and anxiety or depression.
Patients admitted to hospital were more likely to suffer cognitive problems and fatigue compared to people who did not need to be admitted, and people who did not need hospital care were more likely to have headaches than those who needed to be admitted.
When taking all factors into account, the research team estimated that 37 per cent of people who had a Covid-19 infection had at least one long Covid symptom three to six months after infection.
But the authors stressed there were “important caveats” that meant the results might not be generalised.
People included in the study with both flu and Covid could have been sicker than those in the general population because they had sought medical help for their symptoms.
“Over one third of patients were diagnosed with at least one of the long Covid symptoms between three and six months after their Covid-19 illness," Dr Taquet said.
“The severity of the illness, age and sex affected the incidence and profile of long Covid symptoms. Similar symptoms were seen in people after influenza but they occur and co-occur less commonly.”
Health officials have warned that this year’s flu season could be particularly troublesome as immunity to influenza viruses waned during the pandemic.
The government is hoping that more people than ever will have their flu shot, with over-50s and clinically vulnerable people being called on to get a vaccine.
“There are various post-viral conditions recognised and many of us who have experienced flu know how you don’t always feel completely better as quickly as you’ve been hoping or expecting to,” said Prof Paul Harrison, the study leader, from the department of psychiatry at the University of Oxford.
The study has been published in the journal PLOS Medicine.
The Office for National Statistics estimates that 970,000 British people are suffering ongoing symptoms after a Covid-19 infection.
The figures, based on self-reported symptoms, also suggest 384,000 people are still living with symptoms a year after infection.
Other statistics office estimates, based on data from participants of the Coronavirus Infection Survey who had a lab-confirmed case of Covid-19, suggest that between 3 and 12 per cent of people have symptoms 12 weeks after infection.