Cold and flu unlikely to attack you at the same time, study says

People were less likely to catch either influenza or a common cold-causing rhinovirus if they were already infected with the other virus, according to a new study from the Medical Research Council-University of Glasgow Centre for Virus Research has found.

Understanding how these distinct viruses inhibit each other could improve forecasts of respiratory disease outbreaks and help control their spread, the researchers say.

Common cold infections appear to be less frequent in the influenza season and vice versa. The new study, published in the journal Proceedings of the National Academy of Sciences, is the first to provide strong evidence for this interaction at both the population and individual level.

Samples from 44,230 cases of acute respiratory illness, in 36,157 patients, were tested for 11 types of respiratory viruses over nine years. Using this data, the researchers found that 35 percent tested positive for a virus. Of those, 8 percent were infected with more than one type of virus.

The most striking interaction they found was between influenza A viruses and rhinoviruses. Inhibitory interactions between influenza and rhinoviruses appeared to occur within individual people as well as at a population level.

Patients with influenza A were about 70 percent less likely to also be infected with rhinovirus than were patients infected with the other virus types.

"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 Dr. Sema Nickbakhsh, the first author on the paper.

She said respiratory viruses may be competing for resources in the respiratory tract, in the same way animals in the wild compete for food.

"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," she said.

Viruses usually studied in isolation

Viruses from the same species – for example different strains of influenza – could be expected to compete or generate an overlapping immune response in the body. But the researchers say what makes these findings interesting is the interaction is between completely different types of viruses.

Dr Pablo Murcia, who led the research, said up until now, viruses have been studied in isolation – only flu, or only rhinovirus.

"We’ve shown here that we need to also be studying these viruses together like it’s an ecosystem," he said. “Studying interactions between viruses could help to explain why different viruses circulate in different seasons or why they affect different age groups, and within the body why certain types of viruses infect different parts of the respiratory tract, like the nose or the lungs.”

An example of how these viruses spread at the population level is when a person infected with one virus stays home and consequently does not catch another virus.

“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 health care," said Dr. Nickbakhsh. "That’s not to say that occasionally unlucky individuals can’t be infected with influenza and a cold virus at the same time.”

Source: University of Glasgow: gla.ac.uk