Catching a cold when you already have the flu sounds like a nightmare scenario. But fortunately, this doesn't happen very often, a new study finds.
Indeed, the researchers found that having the flu actually reduces a person's chances of developing an infection with a common cold virus.
"What we found is that during certain seasons when you have high levels of circulation of influenza, you are less likely to catch a cold caused by a rhinovirus [the main cause of colds]," said study lead author Dr. Pablo Murcia, a senior lecturer at the MRC-Centre for Virus Research at the University of Glasgow in Scotland, said in a video about the study. This finding was true at both the population level (meaning across the population as a whole) and the individual level (meaning within an individual person).
Researchers typically study cold and flu viruses separately, "but we've shown here that we need to also be studying these viruses together like it's an ecosystem," Murcia said in a statement. "If we understand how viruses interact and how certain viral infections may favor or inhibit each other, then maybe we can develop better ways to target viruses."
The study is published today (Dec. 16) in the journal Proceedings of the National Academy of Sciences.
More flu, less colds
In the new study, the researchers analyzed information from more than 36,000 individuals in Scotland who provided more than 44,000 throat and nose swabs for testing for respiratory illnesses over a nine-year period. These samples were tested for 11 types of respiratory viruses, such as rhinoviruses, influenza A and B viruses, respiratory syncytial virus and adenoviruses.
In this population, 35% tested positive for at least one virus, and 8% tested positive for co-infection with at least two viruses.
Interestingly, a computer analysis of the data showed that when flu activity picked up in the winter, infections with rhinoviruses decreased.
"One really striking pattern in our data is the decline in cases of the respiratory virus rhinovirus ... occurring during winter, around the time that flu activity increases," said study first author Sema Nickbakhsh, a research associate at the Centre for Virus Research.
What's more, when the researchers looked at individual patients, they found that people infected with influenza A were 70% less likely to also be infected with rhinovirus, compared with patients infected with other types of viruses.
The new study cannot determine the reason for the inhibitory effect between flu viruses and rhinovirus. But the researchers have a theory — these viruses may be in competition with each other in their quest to replicate and cause you misery.
"We believe respiratory viruses may be competing for resources in the respiratory tract," Nickbakhsh said. It may be that these viruses compete for specific cells to infect, or that a person's immune response to one virus makes it harder for the other virus to also cause infection, she said.
And there could be other factors at play, such as people staying at home when they are sick, which may reduce the chances of catching another virus.
More studies are needed to better understand the biological mechanisms underlying these virus-virus interactions, the authors said.
Originally published on Live Science.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.