Almost one third of people report at least one ongoing symptom between six and 12 months after a coronavirus infection, a survey of 152,000 people in Denmark has found.

The study includes one of the largest groups yet of people who were not hospitalized with COVID-19, and followed them for longer than other major studies, the researchers from Denmark's State Serum Institute (SSI) said.

The yet-to-be peer-reviewed study suggested that the most commonly reported long-term symptoms were changes in sense of smell and taste, as well as fatigue.

Conducted between September 2020 and April 2021, well before the recent Omicron variant surge, the questionnaire-based study compared the responses of 61,002 people who had tested positive for the coronavirus six, nine or 12 months before with those of 91,878 people who had tested negative.

In total, 29.6 percent of the respondents who had tested positive reported at least one ongoing physical symptom six to 12 months after infection, compared to 13 percent in the control group.

Just over half (53.1 percent) of those with positive tests said they had experienced either mental or physical exhaustion, sleep problems or cognitive problems within the six to 12 months after infection. That compared to 11.5 percent in the control group.

New diagnoses of anxiety and depression were also more common among those with a history of SARS-CoV-2 infection, the study said.

Anders Peter Hviid, the author of the study and also an epidemiology professor at SSI, said the results were another sign that the long tail of COVID-19 should be considered by policymakers.

"It's something you should take into account when you are weighing up the risks and benefits of... the interventions you are making, and vaccinations," he said in a phone interview with Reuters, stressing that more studies are needed.

The World Health Organization (WHO) calls the syndrome post-COVID-19 condition, or "long COVID," and defines it as ongoing symptoms as it occurs in individuals three months after the initial infection and lasts for at least two months. Common symptoms include fatigue, shortness of breath, cognitive dysfunction and others that have an impact on everyday functioning.

The WHO estimates that between 10 percent and 20 percent of people are affected after they recover from their initial illness and says more work is needed on the longer-term prognosis.

David Strain, a lecturer at the University of Exeter Medical School in the UK, who was not involved in the study, called the report "really concerning."

"If Omicron is causing 'long COVID' at the same rate as these earlier variants, we could be looking at a major crisis over the next 12 months given the number of people who have been exposed to this virus," he said.

(With input from Reuters)