Researchers at the Queen Mary University of London have suggested the existence of "long colds," which might be as prevalent as long COVID-19. According to scientists, individuals can experience prolonged symptoms after respiratory infections, even if the tests for COVID-19 come back negative.

Some of the most common symptoms of a long cold were coughing stomach pain, and diarrhea lasting more than four weeks after infection.

The research, published in The Lancet’s EClinicalMedicine journal, suggests some – though not all – people may suffer long-term effects from colds, flu, or pneumonia.

"Our findings suggest that there may be long-lasting health impacts from other respiratory infections that are going unrecognized, although we do not yet have evidence that these symptoms have a similar duration to long COVID-19," the researchers said.

For the study, experts looked at data from 10,203 people taking part in the Covidence U.K. study into coronavirus in the population.

At the time of this analysis, some 1,343 had suffered a COVID-19 infection and 472 had a respiratory infection that tested negative for COVID-19.

The results showed that 22% of people with COVID-19 suffered prolonged symptoms after infection, as did 22% of those who had an infection that was not COVID-19, professor Adrian Martineau, from Queen Mary University of London, who worked on the study, told the PA news agency.

He said there was a "similar risk of prolonged symptoms" irrespective of whether it was caused by COVID-19 or non-COVID-19 infection.

Professor Martineau said people with COVID-19 in the study were more likely to suffer taste and smell problems – known symptoms of coronavirus – and light-headedness or dizziness than those without.

They also suffered heart palpitations, sweating, and hair loss.

Meanwhile, those in the non-COVID-19 group were more likely to have a cough or a hoarse voice than people with COVID-19, something that was surprising given a cough is often associated with COVID-19.

Both groups suffered breathlessness and fatigue.

The study also suggested that the more serious an illness, the greater the chance of having long-term symptoms.

However, the experts said more studies are needed on why some people suffer while others do not.

Lead author Giulia Vivaldi from the Queen Mary University of London said: "Our findings shine a light not only on the impact of long COVID-19 on people’s lives but also other respiratory infections. A lack of awareness – or even the lack of a common term – prevents both reporting and diagnosis of these conditions. As research into long-term COVID-19 continues, we need to take the opportunity to investigate and consider the lasting effects of other acute respiratory infections. These ‘long’ infections are so difficult to diagnose and treat primarily because of a lack of diagnostic tests and there are so many possible symptoms. There have been more than 200 (symptoms) investigated for long COVID-19 alone."

The study looked at people’s symptoms after their initial infection using standardized questionnaires.

The longest time from initial infection to reports of ongoing symptoms was 37 weeks for non-COVID-19 infections and 64 weeks for COVID-19.

The non-COVID-19 group all tested negative for COVID-19, while those in the COVID-19 group either tested positive or had classic coronavirus symptoms but developed the infection at a time when testing was not routinely available.

To ensure the results were robust, professor Martineau said further analysis of the data had been carried out.

This was of those who tested positive and those who did not and the findings were the same as for the larger group of people in the study.

"Our findings may chime with the experience of people who have struggled with prolonged symptoms after having a respiratory infection despite testing negative for COVID-19 on a nose or throat swab," Martineau said.

Dr. David Strain, clinical senior lecturer and honorary consultant at the University of Exeter, welcomed the study, adding the researchers had "demonstrated, at least in the short term, the persistence of symptoms can be troubling not just after COVID-19 but after many other infections."

He said this was not a new phenomenon, citing that the Spanish flu epidemic in 1918-1920 left many people with post-viral illness.

He said the research would focus on the "urgent need for further research into post-viral syndromes," what causes them, and the need for treatments.

Data from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington’s School of Medicine suggests around 145 million people around the world had long COVID-19 during the pandemic and that millions continue to suffer from its symptoms.