It has been dubbed “Flurona” – the unlucky combination of flu and coronavirus infections in the same individual at the same time. In the past week, several people with such co-infections have been identified – a pregnant woman in Petah Tikva city, Israel, and two children in California and Texas, in the United States – but in fact, scientists have been documenting such cases in the medical literature for some time.

“Especially in people with underlying factors, the occurrence of such a concomitant infection can aggravate the complications caused by COVID-19.”

When someone is infected with two or more disease-causing organisms at once, it is called a co-infection. These are thought to be relatively common. For instance, many people who are infected with HIV also develop tuberculosis (TB), whereas in previous influenza pandemics, bacterial co-infections have been a major cause of death (during the 2009 influenza pandemic, a quarter of severe or fatal cases of influenza involved a bacterial co-infection). So, it is not necessarily surprising that people with COVID-19 sometimes have an additional infection at the same time – in this case influenza.

So just how prevalent is this condition, what are the symptoms, and does catching both viruses at once put you at any greater risk than if you are infected with either one individually?

How common is “Flurona”?

One recent review led by Masoud Dadashi at Alborz University of Medical Sciences in Karaj, Iran, identified 15 case reports (scientific publications describing individual, or small groups of patients) of influenza-COVID-19 co-infections, and a further 11 studies which tried to estimate the prevalence of such cases. These studies heralded from the USA, China, Iran, Japan, Spain, France, Italy, Switzerland and Brazil, illustrating that such co-infections are relatively widespread. Although there isn’t enough good quality data to draw firm conclusions, based on these studies, Dadashi and his colleagues estimated that overall, some 1.2% of COVID-19 patients might also have influenza.

However, it is very difficult to say for sure, because doctors wouldn’t necessarily know someone had both viruses unless they specifically tested for them – and such testing is unlikely to happen unless someone is hospitalised. Even then, they won’t always be tested for other infections.

The possibility of “flurona” was identified early on in the pandemic by doctors treating COVID-19 patients in Wuhan, China. Here, just over half of 307 hospitalised COVID-19 patients simultaneously tested positive for an influenza virus – although this could have been due to the time of year, a localised flu outbreak, or some other factor – and such patients were identified as being at a higher risk of poor health outcomes.

What is clear is that COVID-19-influenza co-infections can occur, and have been identified in many different countries so far. Such co-infections are also likely to become more common in northern hemisphere countries as winter progresses, due to rapidly escalating number of Omicron cases, and the onset of the flu season – predicted to be especially bad this year due to increased social mixing following two years of strict infection control measures in many countries.

What are the symptoms?

Both viruses cause respiratory infections, and are transmitted through the tiny droplets we breathe, cough or sneeze out of our mouths and noses, and through surfaces contaminated with these droplets. They also have a tendency to infect many of the same cells – chiefly, those lining the upper and lower respiratory tract – and can cause a broad spectrum of illness, from relatively mild or asymptomatic infections, through to severe disease or death.

Based on the studies reviewed by Dadashi and colleagues, the most common symptoms experienced by Flurona patients are fever (89.4% of cases), cough (79.3%), shortness of breath (24.1%), muscle aches and pain (20.7%) and difficult or laboured breathing (20.7%). Such symptoms are also common among those infected with either virus alone, making it difficult to identify someone who has been co-infected.

How dangerous is it?

In theory, existing damage to cells lining the respiratory tract could make it easier for SARS-CoV-2 to gain traction in the body. “Especially in people with underlying factors, the occurrence of such a concomitant infection can aggravate the complications caused by COVID-19,” Dadashi and his colleagues wrote.

In one study, researchers led by Lei Bai at Wuhan University, China, deliberately co-infected cells or live mice with influenza A virus and SARS-CoV-2, and found that doing so boosted viral entry into cells and elevated the viral load, resulting in more severe lung damage in infected mice.

However, the extent to which this occurs in real-life conditions is unclear. A separate review by Louise Lansbury and colleagues at the University of Nottingham, UK, which analysed various types of co-infection in COVID-19 patients, was unable to determine whether those infected with an additional virus had a worse prognosis. The three Flurona patients identified in recent media reports are said to be doing well.

Could other types of co-infection occur?

According to Lansbury’s review, patients with COVID-19 are more likely to be co-infected with a bacterium than another virus. By combining the data from 30 individual studies, including a total of 3,834 patients, they estimated that some seven percent of hospitalised COVID-19 patients had a bacterial co-infection – the most common being Mycoplasma pneumoniae, which can cause pneumonia. Such infections were more common among people who were admitted to intensive care units.

Three percent of hospitalised patients were co-infected with a virus, with respiratory syncytial virus (RSV) – a common cause of respiratory infections in young children – and influenza A (a type of flu virus), the most common.

How to avoid Flurona

Both SARS-CoV-2 and influenza are spread by being in close contact with an infected person, and either breathing the same air, or touching the same surfaces as them. This means the same strategies can be taken to avoid them: frequent handwashing, wearing a facemask, social distancing and ensuring rooms are well-ventilated. The risk of both infections is also greatly reduced through vaccination, so this is highly recommended.

TOPICS: COVID-19Influenza

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