London School of Hygiene & Tropical Medicine

Health experts have observed that children with asthma tend to have the worst symptoms at the same times each year - when school starts in the autumn and after extended breaks. Researchers previously speculated that environmental factors such as air quality in schools might be to blame, but the new study confirms that the primary driver of seasonal waves of worsening asthma symptoms, which can lead to hospitalisations, is the prevalence of common colds.

Lead author Dr Rosalind Eggo, who conducted the primary analyses while at The University of Texas at Austin (UT Austin) and now works at the London School of Hygiene & Tropical Medicine, said: "This study investigates the role of common colds in triggering severe asthma attacks (exacerbations). We tried to understand the role of colds in asthma attacks on a very large scale - we looked at eight large cities in Texas, and seven years of data.

"We use a mathematical model for common cold infections to simulate the transmission of those viruses between people. We find that transmission is higher when children are in school, and returning to school after long holidays leads to peaks in infection. Consequently, the risk to asthmatics is higher at these times, so we hope our work will spur development of preventive strategies aimed at these times. "

Earlier studies into the cause of asthma attacks involved swabbing individual patients to detect viruses. In the new study, the team led by researchers at UT Austin, investigated population-wide patterns of how common colds circulate among adults and children throughout the year to learn about the role of the viruses. They built a computer model that incorporated possible drivers of asthma attacks and compared the output of the model to a large set of real-world health data: the timing and locations of about 66,000 asthma hospitalisations from cities across Texas.

By testing each driver independently, the researchers could determine the relative impact of each and find the weighted combination of factors that best fit the data. They determined that the spread of cold viruses, which is heavily influenced by the school calendar, is the primary driver of asthma attacks.

Senior author Lauren Meyers, Professor of Integrative Biology and Statistics and Data Sciences at UT, Austin, said: "The school calendar predicts common cold transmission, and the common cold predicts asthma exacerbations. This study provides a quantitative relationship between those things."

The authors speculate on the mechanism behind this relationship; when children are out of school, they tend to spend less time with other children and are exposed to fewer viruses. As a result, their viral immunity decreases. When they return to school, they are exposed to viruses at much higher rates, and this is also the time when they are most susceptible.

The researchers also found that for adults, unlike children, the primary driver of asthma attacks is prevalence of the flu virus.

Finally, the team developed more accurate rates of transmission of cold viruses than have been produced by previous studies. That information may help shed light on how common colds spread, and how we can protect people who are most vulnerable to them.

This research was funded by the National Institute of General Medical Sciences.

Publication

• Rosalind M. Eggoa, James G. Scottc, Alison P. Galvanie, Lauren Ancel Meyersa, Respiratory virus transmission dynamics determine timing of asthma exacerbation peaks: Evidence from a population-level model, Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1518677113