Incidence of medically attended acute respiratory illnesses due to respiratory viruses across the life course during the 2018/19 influenza season

BACKGROUND: While multiple respiratory viruses circulate in humans, few studies have compared the incidence of different viruses across the life course.

We estimated the incidence of outpatient illness due to 12 different viruses during November 2018 through April 2019 in a fully enumerated population.

METHODS: We conducted active surveillance for ambulatory care visits for acute respiratory illness (ARI) among members of Kaiser Permanente Washington (KPWA).

Enrolled patients provided respiratory swab specimens which were tested for 12 respiratory viruses using RT-PCR.

We estimated the cumulative incidence of infection due to each virus overall and by age group.

RESULTS: The KPWA population under surveillance included 202,562 individuals, of whom 2,767 (1.4%) were enrolled in the study.

Influenza A(H3N2) was the most commonly detected virus, with an overall incidence 21 medically attended illnesses per 1,000 population; the next most common viruses were influenza A(H1N1) (18 per 1,000), coronaviruses (13 per 1,000), respiratory syncytial virus (RSV, 13 per 1,000), and rhinovirus (9 per 1,000).

RSV was the most common cause of medically attended ARI among children aged 1-4 years; coronaviruses were the most common among adults aged ≥65 years.

CONCLUSIONS: Consistent with other studies focused on single viruses, we found that influenza and RSV were major causes of acute respiratory illness in persons of all ages.

In comparison, coronaviruses and rhinovirus were also important pathogens.

Prior to the emergence of SARS-CoV-2, coronaviruses were the second-most common cause of medically attended ARI during the 2018/19 influenza season.