A new study reported that children with asthma are more often infected with cold viruses soon after they return to school after a seasonal or holiday break. Worsening asthma symptoms result in millions of missed school and work days and healthcare costs of about $50 billion in the United States each year.

The study, “Respiratory virus transmission dynamics determine timing of asthma exacerbation peaks: Evidence from a population-level model,” was published in the journal Proceedings of the National Academy of Sciences (PNAS).

Healthcare professionals observed that asthmatic children have worsening disease symptoms at the same time every year, usually when they return to school after summer and spring holidays. They had long suspected that factors like air quality in schools could be responsible for the symptoms, but the new study indicates that the primary cause of seasonal reoccurrence of asthma symptoms is the prevalence of common colds, which often leads to hospitalization.

“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,” the study’s senior author, Lauren Meyers, professor of integrative biology and statistics and data sciences at The University of Texas at Austin, said in a news release.

The team investigated population-wide patterns of common colds circulation among adults and children, and disease exacerbations. They computationally modeled possible drivers of asthma exacerbations and compared the computations to a large set of real-world health data. Researchers used data of about 66,000 asthma hospitalizations cases from cities across Texas, and over a seven-year period. The spread of cold viruses was related to the school calendar, which was found to be the primary driver of asthma exacerbations.

“The school calendar predicts common cold transmission, and the common cold predicts asthma exacerbations,” Meyers said. “And this study provides a quantitative relationship between those things.”

The authors speculated about the underlying mechanism, and suggested it could be the children’s decreased immunity after school holidays, as they were likely exposed to fewer viruses during such periods. The research team established more accurate rates of transmission of cold viruses than previous studies, and such information might help shed light on how people can be protected who are most vulnerable to common colds.