Monthly parainfluenza virus incidence rates were nearly 6 times higher in September 2021 vs the second highest reported monthly incidence rates among children 4 years and younger in Finland in the last decade, following lifting of COVID-19–related restrictions.
Finland saw record parainfluenza virus cases among children in fall 2021, likely attributed to lifted social restrictions from the COVID-19 pandemic, according to a study published in Influenza and Other Respiratory Viruses.
To come to this finding, the study authors conducted a nationwide register-based retrospective epidemiologic surveillance study, using data on parainfluenza virus findings from the National Infectious Diseases Register.
Between January 2012 and December 2021, 3887 parainfluenza virus incidences were detected. Data were stratified by age groups 0 to 4 years, 5 to 9 years, and 10 to 14 years, with a majority (81%) of cases occurring in children 4 years and younger. Monthly and weekly incidences were calculated per 100,000 children in each age group.
Monthly and weekly rates were relatively stable and within seasonal rates until March 2020 when cases dropped to nearly zero until the beginning of 2021.
For children 4 years and younger, monthly incidence rates were approximately 6 times higher in September 2021 (120 per 100,000 children) compared with the second highest reported monthly incidence rates (21 per 100,000 children) in the last decade. Past high monthly incidence rates were recorded in winter 2014-2015, spring 2017, and winter 2018-2019.
In the last decade, parainfluenza virus incidence was overall lower in the older age groups, although they also saw a measurable increase in fall 2021. The highest recorded weekly incidence (35 per 100,000 children) was reported in week 41 of 2021.
“The peak in 2021 was rapid, and the incidence turned immediately to rapid decline, which has been seen in previous incidence peak as well,” the authors noted.
Although incidence rates increased, the timing of these peaks also changed.
The typical parainfluenza virus epidemic peak is in spring, with some increases in late winter. However, the 2021 epidemic season began in early fall.
“As the last restrictions were abandoned in fall 2021, we saw an increasing trend in emergency department visits and hospitalizations due to respiratory symptoms in the youngest children (aged 0-4 years),” the authors explained. “Timing of this early epidemic was prior to traditional [respiratory syncytial virus] season and rhinovirus detections remained in normal levels.”
Social restrictions, including school closures, reduced the spread of common respiratory pathogens among children in Finland. After the reopening of schools and other restrictions were lifted, rhinovirus began to spread as it normally did in the country, with pediatric emergency department visit rates increasing to normal levels by summer 2020.
However, influenza and respiratory syncytial virus cases were still low in winter 2020-2021.
The authors noted the need for additional information, such as hospital data, to develop a better understanding of the clinical perspective of this record high.
“Our findings highlight the importance of active pathogen surveillance during a pandemic,” the authors said. “A majority of the testing resources have been used to test COVID-19, but we should also recognize other pathogens.”
Kuitunen I, Artama M, Haapanen M, Renko M. Record high parainfluenza season in children after relaxation of COVID-19 restrictions in fall 2021—a nationwide register study in Finland. Influenza Other Respi Viruses. Published online March 22, 2022. doi:10.1111/irv.12983