People who catch either influenza or a common cold-causing rhinovirus are less likely to be infected by the other pathogen, according to a study which sheds light on how different viruses co-infecting a patient may interact. The study, published in the journal PNAS, noted that common cold infections appear to be less frequent in the influenza season, and vice versa.
The researchers, including those from the University of Glasgow in the UK, collected samples from 44,230 cases of acute respiratory illness -- in 36,157 patients -- and tested them for 11 types of respiratory viruses over nine years.
They found that 35 per cent of the samples tested positive for a virus and, of those, 8 per cent were co-infected with more than one type. According to the researchers, the most striking interaction was between influenza A viruses and rhinoviruses -- a type that causes the common cold.
Using computer modelling of the data, they found that the inhibitory interactions between influenza and rhinoviruses appeared to occur within individual people, as well as at a population level.
Patients with influenza A were approximately 70 per cent less likely to also be infected with rhinovirus, than were patients infected with the other virus types, the study said.
"One really striking pattern in our data is the decline in cases of the respiratory virus rhinovirus, which is typically a mild common cold causing virus, occurring during winter, around the time that flu activity increases," said study co-author Sema Nickbakhsh, from the University of Glasgow.
The researchers suspect that the respiratory viruses may be competing for resources in the respiratory tract.
"There are various possibilities we're investigating, such as these viruses are competing for cells to infect in the body, or the immune response to one virus makes it harder for another unrelated virus to infect the same person," Nickbakhsh said.
Citing an example, they said, viruses from the same species like different strains of influenza could be expected to compete or generate an overlapping immune response in the body.
"Traditionally people have studied viruses in isolation - you study only flu or rhinovirus - but we've shown here that we need to also be studying these viruses together like it's an ecosystem," said study co-author Pablo Murcia from the University of Glasgow.
The researchers are currently trying to understand how respiratory viruses, including influenza and rhinovirus, interact with each other. "If we understand how viruses interact and how certain viral infections may favour or inhibit each other, then maybe we can develop better ways to target viruses," Murcia said.
According to the researchers, studying the interactions between viruses could help to explain why different viruses circulate in different seasons, or why they affect different age groups.
They said it may also help in understanding why certain types of viruses infect different parts of the respiratory tract, like the nose or the lungs. However, the researchers cautioned that the study came with limitations.
The correlations observed cannot show what is causing these interactions, and the samples were only taken from people with symptoms of a respiratory infection. So, they said, it may not capture how the viruses behave in people who don't develop symptoms.
"A key thing to note with this research is that we're looking at average risks over a very large number of patients who have sought healthcare - that's not to say that occasionally unlucky individuals can't be infected with influenza and a cold virus at the same time," Nickbakhsh said.