The Omicron variant of the SARS-CoV-2 virus is less likely to cause long COVID than the Delta strain, according to a study published in The Lancet journal.

Long COVID is defined as having new or ongoing symptoms four weeks or more after the start of the disease, the researchers said.

Symptoms include fatigue, shortness of breath, loss of concentration, and joint pain, which can adversely affect day-to-day activities, and in some cases can be severely limiting, they said.

The researchers found that the odds of experiencing long COVID were between 20-50 per cent less during the Omicron period versus the Delta period, depending on age and time since vaccination.

"The Omicron variant appears substantially less likely to cause Long-COVID than previous variants but still 1 in 23 people who catch COVID-19 go on to have symptoms for more than four weeks," said study lead author Claire Steves from King's College London, UK.

The study identified 56,003 UK adult cases first testing positive between December 20, 2021, and March 9, 2022, when Omicron was the dominant strain.

Researchers compared these cases to 41,361 cases first testing positive between June 1, 2021, and November 27, 2021, when the Delta variant was dominant.

The analysis shows 4.4 per cent of Omicron cases were long COVID, compared to 10.8 per cent of Delta cases.

However, the absolute number of people experiencing long COVID was in fact higher in the Omicron period, the researchers said.

This was because of the vast number of people infected with Omicron from December 2021 to February 2022, they said.

The UK Office of National Statistics estimated the number of people with long COVID actually increased from 1.3 million in January 2022 to 2 million as of May 1, 2022.

"Given the numbers of people affected it is important that we continue to support them at work, at home and within the NHS," Steves added.