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While respiratory syncytial virus (RSV) did not appear to be more severe during its resurgence in 2021, hospital admissions increased for kids, possibly as a result of delayed first infection or no recent reinfection due to COVID-related restrictions, a population-based cohort study from Denmark suggested.

Among over 300,000 kids younger than 5 years, the mean number of RSV-associated admissions increased from an average of 1,477 during the 2016-2017 to 2019-2020 RSV seasons to 3,000 during the 2021-2022 season (risk ratio [RR] 2.0, 95% CI 1.9-2.1), reported Ulrikka Nygaard, PhD, of Copenhagen University Hospital, and co-authors in .

The largest increases in hospital admissions were seen among those:

• Younger than 3 months (RR 2.2, 95% CI 2.1-2.4)
• Ages 18-23 months (RR 2.3, 95% CI 1.9-2.6)
• Ages 24-35 months (RR 3.7, 95% CI 3.2-4.4)
• Ages 36-47 months (RR 4.9, 95% CI 3.7-6.6)
• Ages 48-59 months (RR 5.5, 95% CI 3.7-8.3)

Furthermore, use of mechanical ventilation was needed for 54 patients during the most recent season, up from a range of 15 to 28 per year during the 2016-2017 to 2019-2020 seasons (RR 2.3, 95% CI 1.6-3.3).

Compared with the four pre-COVID RSV seasons, risk of mechanical ventilation was higher in infants younger than 3 months (RR 2.2, 95% CI 1.3-3.7) and in those ages 2 to 5 years (RR 4.6, 95% CI 1.7-12.6) during the 2021-2022 season.

In Denmark, "the RSV resurgence was 7 months later than normal RSV seasons, and occurred after the relaxation of COVID-19 restrictions, which had included prolonged periods of daycare and school closures," Nygaard and team noted. A previous study showed that full reopening of schools across multiple countries was associated with a 23 times higher risk of RSV infections, albeit with a wide confidence interval (95% CI 1.09-495.84).

"The increased risk of hospital admissions in 2021-22 coincided with a decreased population immunity after the extended period of low RSV exposure following the strict public health interventions implemented to target COVID-19," the authors continued. "Compared with the pre-COVID RSV epidemics, the highest increase in RSV-associated hospital admissions and mechanical ventilation in 2021-22 occurred in young infants (aged <3 months), possibly due to reduced levels of maternally derived RSV antibodies following diminished RSV exposure among pregnant women, and in children aged 2-5 years, probably reflecting postponed primary RSV infection or waning immunity due to no recent reinfection."

Across all RSV seasons included in the study, respiratory failure due to bronchiolitis led to use of mechanical ventilation in 69 of 87 children younger than 3 months or those with severe comorbidities.

Notably, among 46 children with no risk factors for severe RSV, 40 received mechanical ventilation due to additional complications, including neurological (35%), cardiac (2%), and pulmonary complications (50%).

"These rare complications are important to be aware of, since new promising RSV-preventive interventions for healthy infants are currently being investigated," Nygaard and colleagues wrote. "If such interventions postpone time to primary RSV infection, in the future, more older children might be admitted to hospital due to atypical complications, other than classic bronchiolitis."

For this study, the researchers used data on RSV-associated hospital admissions from the Danish National Patient Registry and demographic and clinical data on children who received mechanical ventilation through prospective real-time data collection in 2021-2022 and retrospective data collection for the 2016-2017 to 2019-2020 seasons from all eight pediatric and neonatal intensive care units in Denmark.

They included 310,423 children younger than 5 years of age. Overall, comorbidities included chronic neurological diseases, prematurity, congenital heart disease, chronic lung disease, metabolic diseases, airway abnormalities, and cancer.

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