The spread of cold viruses is the primary driver of asthma exacerbations among schoolchildren and is heavily influenced by the school calendar, according to a new study.
"The school calendar predicts common cold transmission, and the common cold predicts asthma exacerbations. And this study provides a quantitative relationship between those things," said senior author Lauren Meyers, professor of integrative biology and statistics and data sciences at The University of Texas at Austin.
The researchers published their results in the February 23, 2016 issue of Proceedings of the National Academy of Sciences.
Meyers and colleagues took a population level approach to investigate common cold circulation and its consequences for asthma patients. They created a model of common cold transmission with different contact patterns for adults and children, which was modified by school vacations. They jointly fit this cold transmission model into a model of biological and environmental exacerbation triggers to estimate effects on hospitalization risk in eight large Texas cities.
Their computer model incorporated the possible drivers of asthma exacerbations and compared the output of the model to a large set of real-world health data, specifically the timing and locations of about 66,000 asthma hospitalizations during a seven-year period.
For children, they found that daily viral prevalence was the strongest predictor of asthma hospitalizations, with transmission reduced by 45% during school closures. They detected a transient period of nonspecific immunity between infections lasting 19 days.
For adults, hospitalizations were more variable, with influenza driving wintertime peaks.
Particulate matter and ozone were not important predictors, perhaps because of the large geographic area of the populations, the researchers suggested.
“The school calendar clearly and predictably drives seasonal variation in common cold prevalence, which results in the ‘back-to-school’ asthma exacerbation pattern seen in children and indirectly contributes to exacerbation risk in adults,” they stated.
Previously, researchers had speculated that environmental factors, such as air quality in schools, might be to blame for the observation that children with asthma tend to have the worst symptoms at the same times each year — when school starts in the fall and after extended breaks such as Spring Break. Patient swabbing studies have linked respiratory viral infections to these exacerbations.
The new study confirms that the primary driver of seasonal waves of worsening asthma symptoms is the prevalence of common colds.
The researchers speculated on the mechanism behind the relationship: Children who are out of school tend to spend less time with other children and therefore are exposed to fewer viruses, and their viral immunity decreases. When they return to school, they are exposed to viruses at much higher rates, and at a time when they are most susceptible to infection.
The study provides a framework for anticipating the seasonal dynamics of common colds and the associated risks for people with asthma. "This work can improve public health strategies to keep asthmatic children healthy. For example, at the riskiest times of year, doctors could encourage patient adherence to preventative medications, and schools could take measures to reduce cold transmission," said Meyers.
Eggo RM, Scott JG, Galvanie AP, Meyersa LA. Respiratory virus transmission dynamics determine timing of asthma exacerbation peaks: Evidence from a population-level model. Proc Natl Acad Sci USA. 2016; 113: 2194–2199. doi: 10.1073/pnas.1518677113