At least 10% of people infected with SARS-CoV-2, the virus that causes COVID, have symptoms that last more than four weeks after the infection. With more than 770 million infections to date, this translates to tens of millions of people living with the long-term consequences of COVID, known as “long COVID”.
More than 200 symptoms of long COVID have been studied, with some of the most common being fatigue, breathlessness and cognitive difficulties, such as memory problems or “brain fog”. The condition can be debilitating – many people have to reduce their working hours or are unable to work entirely.
But COVID may not be alone in causing long-lasting symptoms.
In a new paper, my colleagues and I report the findings of a study comparing long-term symptoms reported by people who experienced different types of acute respiratory infection. We asked more than 10,000 people to report on 16 symptoms commonly found in long COVID, such as fatigue, breathlessness, aches and pains, and dizziness. We then compared how common these symptoms were among three groups: people who had reported COVID, those who had reported another acute respiratory infection (but had tested negative for COVID), and those who had not reported either infection.
We focused on long-term symptoms by only including people who had been infected more than four weeks earlier. We also took into account people’s general health before they were infected, and whether they had any existing respiratory conditions.
Read more: Long COVID patients are much more likely to have multiple organ abnormalities
Our study showed that all the symptoms considered were more common in people with previous COVID than in people with no infections, regardless of whether they reported long COVID. But this finding wasn’t unique to COVID. Almost all the symptoms we looked at were also more common in people with non-COVID respiratory infections than in those with no infection.
In other words, our findings hinted towards the existence of a “long cold”: long-lasting health effects from other respiratory infections, such as colds, flu, or pneumonia, that are currently going unrecognised.
Some of the most common symptoms of the long cold include coughing, stomach pain, and diarrhoea. These symptoms were reported an average of 11 weeks after the infection. While a severe initial infection seems to increase the risk of long-term symptoms, our research does not yet tell us why some people suffer extended symptoms while others do not.
Important differences
Importantly, we have no evidence that symptoms of the long cold have the same severity or duration as long COVID. In fact, we saw some important differences in the symptoms reported in the two groups, with those recovering from COVID more likely to experience light-headedness or dizziness and problems with taste and smell.
These findings shine a light not only on the impact of long COVID on people’s lives, but also other respiratory infections.
A lack of awareness, or even the lack of a common term, such as “long cold” or “long flu”, prevents both reporting and diagnosis of these conditions. And people who do report their long cold may still struggle to get a diagnosis, owing to the wide range of symptoms and lack of diagnostic tests.
Long-lasting symptoms after respiratory infections are not a new phenomenon. Studies in survivors of two previous coronavirus outbreaks – the severe acute respiratory syndrome (Sars) pandemic and the Middle East respiratory syndrome (Mers) outbreak – have found long-term impacts on lung function, quality of life and mental health. And some people hospitalised with influenza A have experienced respiratory and psychological problems at least two years after being discharged from hospital.
But most of the research so far has focused on people with severe disease, often severe enough to be hospitalised. Little is known about the long-term effects respiratory infections might have among people whose acute disease episode is less severe.
Long COVID has bucked this trend, being studied in people with all levels of severity of the initial infection. This is in large part due to strong patient advocacy, showing that it can affect even those with mild initial symptoms.
In demanding recognition of their condition, people with long COVID have cast a much-needed spotlight on post-infection syndromes more generally. Now is the time to improve our understanding, diagnosis and treatment of these conditions. Let’s not wait for another pandemic.