Cold infections may be less frequent in people with the flu

People were less likely to catch either influenza or a common cold-causing rhinovirus if they were already infected with the other virus, a new study by scientists from the Medical Research Council-University of Glasgow Centre for Virus Research has found. Understanding how these distinct viruses inhibit each other could help public health planning to improve forecasting models that predict respiratory disease outbreaks and strategies for controlling disease spread, say the scientists. It has been observed before that 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 study with enough samples 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 in NHS Greater Glasgow and Clyde. Using this data, the researchers found that 35% tested positive for a virus and, of those, 8% were co-infected with more than one type of virus. The most striking interaction they found was between influenza A viruses and rhinoviruses, a type of virus that can cause the common cold.