Tens of thousands of people may still experience lasting symptoms more than a year after being infected with COVID-19, according to a new U.K. study鈥攂ut the incidence rates of long COVID have also fallen with each new variant.

The study, published Tuesday in the , looked at more than 270,000 adults across the U.K. and found that those who were infected with the original strain of SARS-CoV-2 were much more likely to experience both more serious and more long-lasting cases of long COVID compared to those infected with newer variants.

Symptoms which persist more than 12 weeks beyond the initial COVID-19 infection is the general definition for long COVID, a chronic condition which can present with varying levels of intensity.

The question of how many COVID-19 cases will result in long COVID, and just what the scope of this burden is on patients, is something that has hung over the pandemic.

Researchers found that one in 13 participants who contracted COVID-19 reported were still experiencing symptoms 12 weeks later, and one in twenty were still experiencing symptoms a year after their initial infection.

According to this new study, while there is a lasting burden for thousands of patients, the chance of getting long COVID has gone down with each variant as we move away from the original strain that emerged in late 2019 and spread across the world in 2020, referred to in the study as the COVID-19 鈥渨ild type.鈥

"While the landscape has changed considerably since the early peak of the COVID-19 pandemic, this analysis shows that a proportion of adults are still experiencing lasting symptoms,鈥 Christina Atchison, principal clinical academic fellow within the Imperial College London鈥檚 School of Public Health and first author on the study,

鈥淚mportantly, we find that compared to wild type virus, those infected when Omicron was dominant were far less likely to report symptoms lasting beyond 12 weeks. This may reflect the changing levels of immunity in the population from previous exposure to the virus and vaccination.鈥

The study looked at which variant was dominant at the time of specific cases, and then looked at the chance of patients developing long COVID, and found that the chance of a case turning into long COVID got successively lower with each new variant. The study included Alpha, Delta and the first Omicron variant.

Those infected when Omicron was the dominant variant were 88 per cent less likely to report long COVID than those infected with the original strain of the virus.

This new study is the latest analysis to come from the REal-time Assessment of Community Transmission (REACT) study, which is an ongoing research program being run by Imperial College London which investigates various aspects of the virus.

For this new study, researchers looked at follow-up survey responses from 276,840 adults who had taken part in earlier REACT studies. Using these survey responses, researchers examined the self-reported health, quality of life and symptom profiles for people with long COVID compared to those who recovered from COVID-19 without lasting symptoms and those who never contracted COVID-19.

The study found that self-reported mental health and quality of life were worse among participants with long COVID compared to those who either never had COVID-19 or who recovered without long-lasting symptoms.

鈥淥ur latest findings from the REACT study offer further insights into the underlying factors which are associated with prolonged symptoms after initial COVID-19,鈥 Paul Elliott, chair in epidemiology and public health medicine from the School of Public Health at Imperial College London, said in the release.

"We find that the variant of SARS-CoV-2 people are infected with, the initial severity of their symptoms, and whether they have pre-existing health conditions all have an impact on whether they will develop lasting symptoms.鈥

Most COVID-19 cases had symptom resolution within two weeks, with one in ten people reporting symptoms for more than four weeks, which is under the threshold for long COVID according to the World Health Organization.

The most common symptoms reported by the 7.5 per cent of patients who reported having symptoms persisting beyond 12 weeks in the study were mild fatigue, difficulty thinking and join pains. Patients with long COVID were also nine times more likely than other patients to report a loss or change of sense of smell or taste and seven times more likely to report shortness of breath.

These long COVID patients reported a reduction in their ability to carry out daily tasks. Around 31 per cent of patients who had symptoms at 12 weeks no longer had those symptoms at 52 weeks, which means that 69 per cent of long COVID cases went on to persist for more than a year.

Women were slightly more likely to report symptoms persisting past 12 weeks.

Researchers noted that earlier estimates from the REACT-2 study had suggested that 21.6 per cent of adults who had contracted COVID-19 at some point had experienced one or more lasting symptoms 12 weeks after their initial illness鈥攁n incidence rate that would suggest one in five COVID-19 cases would result in long COVID.

However, researchers pointed out that this earlier estimate was made without a negative control group, citing other studies which found different incidence rates. A recent nationwide Scotland study, researchers noted, found that eight per cent of symptomatic COVID-19 cases would still experience symptoms 6-12 months later.

鈥淲hile we have gained valuable insights into the groups at risk of lasting symptoms, we are undertaking detailed interviews to further understand the variation in people鈥檚 experiences and the impact on their everyday lives,鈥 Helen Ward, a professor with the School of Public Health at Imperial College London, said in the release.

"We are also planning further follow-up of people involved in the REACT studies to assess the broader longer-term impact of the pandemic on health and well-being.鈥