During the 2018-2019 influenza season in Washington state, the burden of medically attended acute respiratory illness (ARI) was dominated by influenza A(H3N2) and A(H1N1) across all age groups, with respiratory syncytial virus (RSV) being the most common pathogen among children aged 1 to 4 years and coronaviruses being the most common pathogen in adults aged 65 years and older, according to study results published in Clinical Infectious Diseases.

The authors used surveillance data from the US Influenza Vaccine Effectiveness Network to estimate the burden of ARI in ambulatory care settings for 12 respiratory viruses in Washington state between November 18, 2018, and April 20, 2019.

In the 1860 people enrolled in the study, 2059 respiratory viruses were detected. Influenza A(H3N2) was the most frequently detected virus occurring in 441 enrollees (21 cases per 1000 population), followed by influenza A(H1N1) occurring in 348 enrollees (18 cases per 1000 population). The third most common virus detected was coronavirus (229E, NL63, OC43, and HKU1) occurring in 297 enrollees (13 cases per 1000 population).

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Coinfection occurred in 9.6% (n=179) of enrollees and were most common among children aged 1 to 2 years. Coinfection was most common for human parechovirus, human bocavirus, adenovirus, and influenza C virus. Coinfection was least common for influenza A viruses and RSV subtype B.

When the analysis was stratified by age group, RSV was the most common cause of medically attended ARI among children “with a combined incidence RSV A and B of over 100 cases per 1000 in children aged 1 to 2 years and nearly 60 cases per 1000 in children aged 2 to 4 years,” noted the authors. Rhinovirus was the second most common pathogen associated with medically attended ARI in this age group with an incidence of 33 cases per 1000. Among those participants aged ≥65 years, coronavirus caused the highest incidence of medically attended ARI (18 cases per 1000).

Limitations of this study included participants who were seeking only ambulatory medical care, those who had a prepaid healthcare/insurance provider, covering only a single influenza season, and excluding patients with ARI without a cough and children aged less than 1 years.

Disclosure: Two study authors declared affiliations with the pharmaceutical industry. Please see the original reference for a full list of their disclosures.


Jackson ML, Starita L, Kiniry E, et al; on behalf of the Seattle Flu Study Investigators. Incidence of medically attended acute respiratory illnesses due to respiratory viruses across the life course during the 2018/19 influenza season. Clin Infect Dis. Published online February 16, 2021. doi:10.1093/cid/ciab131