As winter rages, and viruses continue to spread, you might find yourself wondering: Can I catch two at once? What about three, or four, or (gulp) more? And, perhaps most importantly, what happens if I do?
It’s a question that was posed a lot back in 2022 when fears of a so-called “tripledemic” of COVID-19, flu, and respiratory syncytial virus (RSV) were rife.
“Most of what we know about virus infection, virus pathogenesis, [and] virus epidemiology is based on the one virus-one disease approach,” Pablo Murcia, a virologist at the MRC-University of Glasgow Centre for Virus Research, told The Scientist at the time. “And that’s not real.”
Can you be infected with multiple viruses at once?
Unfortunately, yes – you can be infected with more than one virus at a time. However, we know relatively little about how these viruses interact, Murcia added.
In one 2019 study, Murcia and colleagues analyzed respiratory illness cases in Scotland between 2005 and 2013, finding that of all patients who had at least one virus, 11 percent also had another – some unlucky individuals harbored up to five.
What happens if you are?
If you do catch more than one virus at once, it is possible that the symptoms of one could compound the symptoms of the other, leaving you feeling worse off than if you simply had one virus. Although, not necessarily.
“The best data about coinfection come from studies of more serious viruses, such as HIV and hepatitis,” wrote Dr Richard Klasco in a 2018 piece for The New York Times. “These studies show that coinfection can worsen, ameliorate or have no impact on the course of an illness. The outcome depends on the viruses involved.”
For example, coinfection with HIV and hepatitis C virus can result in a worse outcome, Klasco adds.
Referring to respiratory co-infections, Dr Armando Paez, chief of the Infectious Disease Division, Baystate Health, said: “Hospitalization is often required for those suffering from co-infections due to their more serious cases of illness.”
But it’s not all bad news. As Klasco mentioned, not all co-infections will have a negative outcome. A dual infection with influenza A and B, he says, doesn’t seem to worsen symptoms. The same is probably true of the common cold:
“While you can get two colds at once, you probably won’t feel any worse than you would with one. The difference that you might experience is being sick for longer than you might otherwise expect.”
Sometimes, viruses can block one another in what is known as viral interference. This is what, according to some experts, likely happened during the pandemic: COVID may have taken precedence over flu and RSV, effectively blocking them.
Similarly, in the autumn of 2009 during the swine flu pandemic, rhinovirus, which is responsible for the common cold, began to dominate in some parts of Europe, before swine flu took over again. That same year, the pandemic delayed the onset of RSV by up to two and a half months.
This sort of interference can arise in a number of ways. For example, different viruses can target the same receptors on host cells, meaning that the first to gain entry can prevent other viruses from getting access. Viruses can also compete for resources once inside host cells, resulting in a "survival of the fittest" type situation.
The best-understood mechanism of interference involves interferons – defensive molecules produced by the cells of vertebrates when they sense a virus is present. Interferons trigger the expression of genes that can prevent more than one virus from entering a cell or stop viruses that are present from replicating or exiting the cell.
Although viral interference can provide short-term immunity against other viruses, using these and other methods, it’s by no means a given and it’s still very much possible to catch multiple viruses at the same time.
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