(NEXSTAR) – As medical experts and patients suffering from so-called “long-COVID” try to figure out why symptoms can linger for months, or even beyond a year, a new study sheds light on how common they may actually be.

The University of Oxford-led research, published Tuesday in PLOS Medicine, looked at the likelihood of long-haul symptoms and compared it to that of influenza.

After looking at the anonymized health records of 273,618 COVID-19 survivors over a 6-month period, the researchers found that 37% of patients had one or more long-COVID symptoms beyond the 3-month mark – 1.5 times more likely than with influenza.

The study also found that some of the long-COVID patients reported not suffering from a symptom in the first three months.

“Research of different kinds is urgently needed to understand why not everyone recovers rapidly and fully from COVID-19,” said Oxford Professor Paul Harrison, who headed the study. “We need to identify the mechanisms underlying the diverse symptoms that can affect survivors. This information will be essential if the long-term health consequences of COVID-19 are to be prevented or treated effectively.’”

What symptoms were included

The study tracked nine different symptoms, showing the percentage of people who experienced one at some point during the entire 6-month period versus in the 3-6 month period:

1. Abnormal breathing (18.71% in the 1- to 180-day period; 7.94% in the 90-to-180-day period)
2. Fatigue/malaise (12.82%; 5.87%)
3. Chest/throat pain (12.60%; 5.71%)
4. Headache (8.67%; 4.63%)
5. Other pain (11.60%; 7.19%)
6. Abdominal symptoms (15.58%; 8.29%)
7. Myalgia (3.24%; 1.54%)
8. Cognitive symptoms (7.88%; 3.95%)
9. Anxiety/depression (22.82%; 15.49%)

What else they found

The study also found difference between patients and their likelihood of becoming a long-hauler after the acute phase of COVID-19.

Long-term symptoms were found more frequently in people who had been hospitalized, the study found, and slightly more common among women.

In terms of what symptoms people reported, there were also common themes.

Breathing and cognitive difficulties were more common among men and older people, while young people and women were more likely to feel headaches, abdominal symptoms and anxiety/depression.

While the study doesn’t explain what causes the long-COVID symptoms, the researchers say they hope it changes expectations for recovery time and treatment after a positive test.