In a recent report published in Eurosurveillance, researchers presented the initial respiratory syncytial virus (RSV) surveillance findings between August and December 2022.
In March 2020, Danish authorities enforced several measures and interventions for coronavirus disease 2019 (COVID-19) mitigation. In addition to curtailing the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) spread, the interventions reduced the transmission of other respiratory infections, such as RSV infections and influenza. During the summer of 2021, following the gradual upliftment of COVID-19 preventive measures, an unusually high number of RSV infections was observed out-of-season among Danish residents. The unusual RSV epidemic was followed by another epidemic during the autumn and winter of 2022/23.
About the report
In the present report, researchers reported prefatory findings of respiratory syncytial virus surveillance in Denmark.
An individual with a first positive RSV-positive report in the 2022/23 period was considered RSV-positive, irrespective of clinical findings. Cases of RSV infections were linked to the Danish national patient registry to extract data from hospitalized individuals. RSV-associated hospitalizations were described as involving individuals hospitalized for ≥12 hours; the specimens tested RSV-positive RSV within four days before or during hospitalization. For surveillance, an RSV season lasted between week 21 of one year and week 20 of the next year.
By week 48, 8,461 RSV cases were reported with median patient age of 16.0 months, among whom 4,348 (51%) were male. From week 31 (mid-August) onward, RSV infections gradually increased. From week 34 (late August) onward, RSV case counts steeply increased, with a peak in week 45 and a 7.0-day incidence rate of 0.2 cases among every 1,000 individuals.
For the pre-pandemic seasons between 2017 and 2018 and between 2018 and 2019/20, the mean rate of RSV incidence was 0.1 cases among every 1,000 individuals, indicating that the rate of RSV incidence for the current epidemic was 2.5-fold greater compared to that during pre-pandemic seasons. However, the peak incidence during the 2022/23 season was lesser than during the summer RSV epidemic in 2020/21.
By week 48, 3,417 individuals were hospitalized, their median age was 14.0 months, and 1,789 (52%) of them were male. Incidence rates for RSV-associated hospitalizations peaked during week 45 and were comparable to those observed during the summer RSV epidemic in 2021. The mean 7.0-day peak incidence of RSV-associated hospitalizations for the three pre-pandemic seasons was 0.04 cases among every 1,000 individuals, and that during the ongoing 2022/23 season was 0.1 cases among every 1,000 individuals. The findings indicated that the 7.0-day incidence rate of RSV-associated hospitalizations had doubled during the pre-SARS-CoV-2 pandemic seasons.
Particularly, RSV-associated hospitalizations were greater among infants aged below six months and children aged below two years. However, a noteworthy increase in incidence rates of RSV-associated hospitalizations was observed among individuals aged above 45 years, particularly among individuals aged equal to or greater than 80 years. Ten percent (n=107 out of 1,026 individuals) aged above 45 years had been admitted to the ICU (intensive care unit) during their hospitalization.
Annual RSV testing rates increased in 2021, with even higher testing rates in 2022. The 7.0-day RSV testing rates during the ongoing epidemic seem to be comparable to those observed during the seasonal peaks in previous years, with peak 7.0-day RSV testing rates of 0.5, 0.63and 0.6 among every 1,000 individuals during the pre-pandemic seasons between 2017 and 2018, between 2018 and 2019, and between 2019 and 2020. In week 47, the ongoing (between 2022 and 2023) season’s peak RSV testing rate was 0.6 among every 1,000 individuals.
During the ongoing season, RSV-B's prime subtype in circulation was RSV-B, identified in 1,927 (98%) of the tested samples. RSV-A was identified in only 34 cases (2.0%), and only six individuals (0.3%) were infected with RSV-A, B. RSV-A was dominant in Denmark during the season between 2021 and 2022, but the trajectory of RSV infections terminated with an RSV-B tail.
The RSV epidemic during the summer season of 2021 showed atypical intensity and timing and was most likely facilitated by an immunological debt due to the lack of RSV circulation while COVID-19 preventive measures were being enforced, and the RSV epidemic coincided with the gradual upliftment of restrictions.
In addition, factors such as the subtype of circulating RSV and cross-protective immunity could have also affected the epidemic dynamics. RSV infections were observed largely among young infants and children, and adults over 45 years, especially those aged >80 years. The finding indicated a considerable burden of RSV among elders, which requires further investigation.
Overall, the findings highlighted the two out-of-season RSV epidemics which occurred in Denmark with unusual intensity. The findings raise concerns about normal epidemic patterns, and the ongoing epidemic observed to be closer to the winter season indicates a gradual approach towards regular seasonality of RSV infections, similar to that during the pre-pandemic period may be observed.
However, the two large atypical RSV epidemics underscore the need for continued annual and real-time RSV surveillance efforts, with analysis of epidemiological and laboratory data, as advocated by the European CDC (Centre for Disease Control and Prevention). Double or triple epidemics with concurrent influenza, RSV infections, and COVID-19 may occur, during which elders may require the highest degree of care based on the high RSV incidence observed among the older population.