As many of us get closer to our first COVID-19 winter, we have many things to feel optimistic about: vaccine results are on the horizon, mortality rates for those infected have fallen and established therapeutic strategies, such as treatment with the antiviral drug remdesivir, seem to be reducing illness. Unfortunately, winter also brings with it a whole swath of respiratory viruses, with influenza leading the charge. So should we be concerned about our chances of catching both influenza and SARS-CoV-2 viruses at the same time, known as a co-infection?
Recent news of a confirmed case of a flu-COVID co-infection in the Bay Area, California, is a stark reminder of the challenges that still lie before us in tackling this pandemic. The Solano County Department of Health and Social Services broke the news of an unfortunate person who suffered from both an influenza infection and COVID-19, but who now appears to be fully recovered. This is by no means the first case of such a co-infection, and it won’t be the last.
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At this stage of the pandemic, we are aware of many patients that, through sheer bad luck, have experienced a flu-COVID co-infection. The first reports of such patients appeared in a preprint for The Lancet on March 25. It was reported that nine patients from a total of 1,054 from Wuhan and neighbouring regions tested positive for both SARS-CoV-2 and influenza viruses (five influenza A and four influenza B). Of these nine patients, only one had not recovered from COVID-19 by the time of publication.
Since then there have been at least 83 published reports from across the world of patients infected with both influenza and COVID-19. Interestingly, in many of these studies the researchers found that the influenza infection did not make the COVID-19 patients’ medical outcomes any better or worse. But the number of co-infected patients is still quite low because it is hard to know whether patients have a flu-COVID co-infection. This is because once a COVID-19 positive test comes back, doctors rarely seek further tests for other viral infections. Instead, they proceed to treat the COVID-19 symptoms, which can look remarkably similar to influenza symptoms.
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How the viruses co-exist
The influenza virus is known for its ability to recombine and essentially pick up additional features from other influenza viruses during a co-infection, which can result in animal-to-human transmission, as the world witnessed during the 2009 swine flu pandemic. However, SARS-CoV-2 is a coronavirus, a completely different virus family to influenza viruses, and this potential for recombination doesn’t exist. But the potential danger of an immune system overload due to the co-infection of both these viruses is still a possibility.
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The importance of the flu vaccine in the 2020-21 winter season cannot be stressed more, both to reduce the chance of flu-COVID co-infections in the general public and also to reduce the number of hospitalisations due to influenza while medical staff are focused on rising COVID-19 cases.
It is still unclear exactly how people will react to a flu-COVID co-infection, and although the California patient fully recovered, others may not be so lucky.
David Courtney receives funding from the European Research Council.