COVID-19 public health measures altered respiratory syncytial virus (RSV) epidemiology. We examined age-stratified trends in RSV-related disease in Australian children in 2020 compared with previous years.

BACKGROUND AND OBJECTIVES

COVID-19 public health measures have altered respiratory syncytial virus (RSV) epidemiology, resulting in an unseasonal summer epidemic in Australia in 2020. We aimed to determine if the shifted RSV epidemic was more severe compared with previous years or if age-specific changes were associated with the resurgence.

METHODS

Through this multicenter study, we analyzed routinely collected datasets from the Sydney Children’s Hospitals Network in southeast Australia. We examined overall trends in RSV-related disease in children aged <16 years in 2020 compared with 2014–2019. We compared observed and expected counts for RSV infections, RSV-related hospitalizations, and emergency department visits for acute respiratory illness.

RESULTS

In 2020, there was a shift in the peak of RSV-related disease from autumn and winter to early summer. Compared with previous years, the overall frequency of RSV infections increased in children aged 2 to 4 years (83.91%; 95% confidence interval [CI], 34.21% to 192.07%; P < .01). There was also an increase in the peak case counts of RSV infections and RSV-coded hospitalizations for some age groups. There was an overall reduction of RSV-coded hospitalizations (−31.80%; 95% CI, −41.13% to −18.96%; P < .01) and ICU admissions (−44.63%; 95% CI, −60.76% to −5.96%; P < .05) in 2020 compared with previous years.

CONCLUSIONS

Our observations provide evidence that the shifted 2020 RSV season was no more severe than previous years. Increased RSV infections in children aged 2 to 4 years may be explained by a buildup in age-specific population susceptibility and increased testing in older children.

What’s Known on This Subject

COVID-19 public health measures disrupted the transmission of endemic respiratory viruses and were associated with suppression of the 2020 southern hemisphere respiratory syncytial virus (RSV) season. An unseasonal resurgence of RSV infections was observed after the relaxation of interventions.

What This Study Adds

We examined age-stratified trends in RSV-related disease in southeast Australia and observed an unseasonal epidemic in 2020 that appeared no more severe than previous years. Increased RSV infections were found in children aged 2 to 4 years.

Mitigation of the COVID-19 pandemic has necessitated extraordinary changes to community behavior at a population level and affected the transmission of endemic respiratory pathogens.1  Respiratory syncytial virus (RSV), a leading cause of acute lower respiratory infection in young children with a typically consistent annual seasonality, is a compelling example.2,3 

Suppression of the characteristic RSV season in temperate Australia during the autumn and winter of 2020 has been demonstrated in reports of reduced RSV-related disease from several Australian states, including New South Wales (NSW), Western Australia, and Victoria.1,38  Our previous report from NSW, the most populous state in Australia, demonstrated a 93.4% reduction in RSV detections concurrent with an 85.9% reduction in bronchiolitis hospitalizations and an 89.1% reduction in intensive care admissions due to bronchiolitis from April to June 2020, representing a remarkable reduction of RSV-related disease during the usual autumn-winter epidemic period.3 

This RSV suppression persisted until September 2020 (spring) when an interseasonal resurgence of RSV detections in Western Australia and NSW was observed.6,9  In Victoria, where restrictions were prolonged because of a COVID-19 outbreak from June to September 2020, a later resurgence of RSV-related disease was observed.7,8  Suppression of RSV-related disease followed by an atypical, interseasonal resurgence has also been described in the northern hemisphere.1012  These observational data highlight both the potential effectiveness of pandemic interventions in reducing the burden of RSV disease and the potential for unpredictable future epidemics when interventions are relaxed.13 

Existing Australian reports have described 2020 RSV epidemiology using passive laboratory data reported in aggregate or by age-specific syndromes, such as bronchiolitis.1,35 ,8,9  Although these data demonstrate a clear suppression and resurgence of RSV-related disease in Australian children, the severity and age characteristics associated with the shifted epidemic are unclear. For this analysis, we combined data sources from 2 major pediatric centers to examine trends in RSV-related disease in 2020 compared with prepandemic years (2014–2019) in the context of a single, major lockdown in NSW. We aimed to determine if the shifted RSV epidemic was larger or more severe compared with previous years or if age-specific changes were associated with the resurgence.

In this multicenter study, we analyzed datasets routinely collected from the Sydney Children’s Hospitals Network (SCHN) in the state of NSW in southeast Australia. SCHN is the largest provider of tertiary pediatric services in Australia and comprises 2 major pediatric hospitals: The Children’s Hospital at Westmead (CHW) and Sydney Children’s Hospital at Randwick.

We included SCHN electronic medical records from January 2014 (from January 2015 for laboratory data) to December 2020 for all children younger than 16 years from the following datasets: (1) the laboratory respiratory pathogen polymerase chain reaction test datasets from each site laboratory; (2) the SCHN inpatient admissions dataset coded by the International Classification of Diseases, Tenth Revision, Australian Modification (ICD-10-AM); and (3) the SCHN emergency department (ED) visits dataset coded by the Systemised Nomenclature of Medicine Clinical Terms.

For the laboratory dataset, we included all children who underwent an RSV polymerase chain reaction test at either study site. This dataset included a mixture of inpatients, ED patients, and outpatients. Repeat detections with the same result in an individual within a 14-day period were excluded. For the hospital inpatient admissions dataset, we included all RSV-coded hospitalizations (ICD-10-AM J21.0, acute bronchiolitis due to RSV; J20.5, acute bronchitis due to RSV; and J12.1, pneumonia due to RSV; and B97.4, RSV classified elsewhere) and all unspecified bronchiolitis–coded hospitalizations (J21.9) based on principal and additional diagnosis fields. For the ED visits dataset, we included all visits mapping to an RSV or unspecified acute respiratory illness (ARI) code (asthma, acute upper respiratory infection, acute lower respiratory infection, bronchiolitis, cough, pneumonia, and wheezing). No exclusions were applied to the hospital inpatient admissions or ED visits datasets.

We interpreted the results of the analysis in the context of 2020 NSW COVID-19 epidemiology, public health interventions, school holidays, pediatric health service use, and NSW Health–reported laboratory data (COVID-19 notifications and RSV detections). We produced an annotated epidemiological curve from publicly available data sources.6,1417  COVID-19 notifications and RSV detections were obtained from routinely collected, publicly reported data sources on March 13 and May 23, 2021, respectively.6,17 

We performed a time series analysis to forecast total counts and their 95% confidence intervals (CIs) in 2020 based on the annual counts from 2014 to 2019. Linear or quadratic models were used depending on the pattern of change in the counts before 2020 and model fit indices, including the mean absolute percent error, Akaike information criterion or Schwartz’s Bayesian criterion. No significant autocorrelation in the time series of annual counts was detected. Model parameters were estimated using the maximum likelihood method. We compared the observed and forecasted counts in 2020 by calculating the percent difference between these 2 numbers using the following equation:
observedfrequencyexpectedfrequencyexpectedfrequency×100%.

We also plotted counts by week to visualize the trend for each dataset. We stratified the analysis by the age groups 0 to 5 months, 6 to 11 months, 12 to 23 months, 2 to 4 years, and 5 to 15 years and by ICU admission. The number of RSV-related hospitalizations and RSV-related ICU admissions provided a marker for disease severity to be compared with previous years. We also examined the median age of patients in each dataset, the proportion of positive RSV test results by site and age, the frequency of RSV tests by site and age, and the frequency of RSV subtype infections.

This study was approved by the SCHN Human Research Ethics Committee (2020/ETH01432).

Figure 1 shows the frequency of COVID-19 notifications, RSV detections, and key events during the 2020 COVID-19 pandemic in NSW. Across all ages, 4766 COVID-19 notifications and 22 997 RSV detections were reported by NSW Health in 2020.6,17  A sharp decrease of both COVID-19 notifications and RSV detections correlated with statewide public health measures, including a lockdown in the autumn months (March to May). RSV detections remained at low levels until the spring (September) and then increased steadily, peaking in early summer at epidemiological week 51 (December), then declining sharply.

FIGURE 1

Timeline of COVID-19 notifications, RSV detections, and key events in NSW, 2020. Epidemiological weeks end on Saturday for COVID-19 notifications or Sunday for RSV detections. NSW Health–reported RSV detections include data from 14 NSW laboratories (public and private) and detections from one of the study site hospitals (Sydney Children’s Hospital at Randwick). RSV detection data were not reported from all laboratories for all weeks. Key events related to COVID-19 epidemiology, public health interventions, and NSW school holidays were derived from publicly available sources.14,15,17 

FIGURE 1

Timeline of COVID-19 notifications, RSV detections, and key events in NSW, 2020. Epidemiological weeks end on Saturday for COVID-19 notifications or Sunday for RSV detections. NSW Health–reported RSV detections include data from 14 NSW laboratories (public and private) and detections from one of the study site hospitals (Sydney Children’s Hospital at Randwick). RSV detection data were not reported from all laboratories for all weeks. Key events related to COVID-19 epidemiology, public health interventions, and NSW school holidays were derived from publicly available sources.14,15,17 

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A quarantine period of 14 days for international arrivals was legislated from March 17, 2020, followed by school closures and stay-at-home orders from late March.14  Restrictions were relaxed from mid-May, and schools returned to face-to-face learning; however, enhanced infection prevention measures, including physical distancing, hygiene promotion, and targeted restrictions on gathering and movement, remained in place throughout the year.14  Masks were not mandated or widely used in the NSW community in 2020.14  A sharp decline of pediatric health service use was observed during the lockdown period.18  Presentations related to chronic conditions returned to baseline levels by June; however, a reduction in presentations related to acute infectious conditions persisted until October.18 

The neighboring state of Victoria experienced a substantial COVID-19 outbreak from June to September 2020.8  Seeding of COVID-19 into NSW occurred with some associated community transmission, but a lockdown was avoided.6,14  A COVID-19 outbreak in Sydney’s Northern Beaches district in December 2020 prompted a local lockdown that extended into 2021 and resulted in additional restrictions for the Greater Sydney region, including the introduction of mask mandates.14 

NSW early childhood education centers (ECECs) remained open for the duration of 2020, with enhanced infection prevention measures recommended and reduced ECEC attendance reported.3,14,16,19  Enhanced infection prevention measures at schools and ECECs emphasized the exclusion of unwell staff and children; enhanced personal hygiene, including the promotion of and access to hand hygiene facilities; enhanced environmental cleaning; and reduced mixing through separating cohorts of children.16,20 

In 2020, there was a shift in the timing of the peak of laboratory-confirmed RSV infections and an increase in peak case counts of infections for some age groups (Fig 2A). From 2015 to 2019 the peak of infections occurred during autumn and winter, and in 2020, the peak of infections occurred in early summer (December), reflecting routinely reported RSV detections from wider NSW.6  In 2020, we observed an increase in the frequency of infections detected in children aged 2 to 4 years compared with the expected value based on observed numbers in previous years (83.91%; 95% CI, 34.21% to 192.07%; P < .01) (Table 1). The median age of children with an RSV infection in 2020 was higher in 2020 than previous years (14 vs 7 months) (Table 2). RSV test percent positivity in 2020 was higher for children aged 2 to 4 years and lower for all other age groups compared with previous years (Table 3).

FIGURE 2

Weekly age-stratified epidemiological curves for RSV-related disease data streams, 2014–2020. A, Laboratory-confirmed RSV infections. B, RSV-coded hospitalizations (ICD-10-AM J21.0, J20.5, J12.1, and B97.4) and unspecified bronchiolitis–coded hospitalizations (J21.9). C, ARI ED visits. Epidemiological week ends on Saturday. Visits were mapped to an RSV-specific code or unspecified ARI code (asthma, acute upper respiratory infection, acute lower respiratory infection, bronchiolitis, cough, pneumonia, and wheezing). aLaboratory data for 2015–2019; 55 individuals in the CHW dataset had both RSV-A and RSV-B detected, and these were counted once.

FIGURE 2

Weekly age-stratified epidemiological curves for RSV-related disease data streams, 2014–2020. A, Laboratory-confirmed RSV infections. B, RSV-coded hospitalizations (ICD-10-AM J21.0, J20.5, J12.1, and B97.4) and unspecified bronchiolitis–coded hospitalizations (J21.9). C, ARI ED visits. Epidemiological week ends on Saturday. Visits were mapped to an RSV-specific code or unspecified ARI code (asthma, acute upper respiratory infection, acute lower respiratory infection, bronchiolitis, cough, pneumonia, and wheezing). aLaboratory data for 2015–2019; 55 individuals in the CHW dataset had both RSV-A and RSV-B detected, and these were counted once.

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TABLE 1

Comparison of RSV-Related Disease in 2020 With the Average of 2014–2019 by Age

Data Source and Age Group2014–2019,a Average Annual Count (SD)2020, nDifference From the Expected, % (95% CI)
Laboratory-confirmed RSV infectionsa,b   Linear model estimates 
 0–5 mo 453.4 (43.15) 343 −22.08 (−48.58 to 60.73) 
 6–11 mo 196 (25.37) 174 −20.37 (−48.51 to 75.61) 
 12–23 mo 200.8 (28.36) 284 23.53 (−20.43 to 176.06) 
 2–4 y 131 (23.27) 311 83.91 (34.21 to 192.07)** 
 5–15 y 67.2 (19.83) 63 −37.87 (−56.67 to 9.78) 
 Total 1048.4 (113.47) 1175 1.37 (−29.95 to 83.35) 
RSV-coded hospitalizations (ICD-10-AM J21.0, J20.5, J12.1, B97.4)   Linear model estimates 
 0–5 mo 454.67 (32.25) 283 −42.67 (−52.68 to −27.31)** 
 6–11 mo 175.83 (33.27) 121 −47.33 (−59.71 to −23.98)* 
 12–23 mo 170 (11.22) 163 −9.34 (−26.54 to 18.38) 
 2–4 y 100 (14.87) 136 12.4 (−16.25 to 70.81) 
 5–15 y 44.83 (12.89) 44 −35.04 (−48.16 to −13.03)* 
 Total 946.83 (89.31) 747 −31.8 (−41.13 to −18.96)** 
Unspecified bronchiolitis–coded hospitalizations (ICD-10-AM J21.9)   Quadratic model estimates 
 0–5 mo 182.17 (47.85) 173 −52.49 (−61.67 to −37.51)** 
 6–11 mo 204.5 (30.43) 130 −59.08 (−65 to −50.76)** 
 12–23 mo 127.67 (14.91) 102 −33.16 (−58.75 to 76.07) 
 2–4 y 6.17 (0.75) — — 
 5–15 y 0.67 (0.82) — — 
 Total 521.17 (84.53) 407 −51.74 (−56.65 to −45.59)*** 
ARI-coded ED visitsc   Quadratic model estimates 
 0–5 mo 1583.67 (134.38) 1100 −44.52 (−58.21 to −17.49)* 
 6–11 mo 1845.5 (99.09) 1380 −36.13 (−47.73 to −17.89)* 
 12–23 mo 2910.67 (180.95) 2732 −23.27 (−31.58 to −12.66)** 
 2–4 y 3789.83 (306.47) 3400 −29.25 (−44.13 to −3.55)* 
 5–15 y 2656.67 (365.68) 2207 −40.78 (−61.57 to 29.04) 
 Total 12 786.33 (1003.2) 10 819 −33.37 (−46.33 to −12.15)* 
RSV-coded hospitalizations with ICU (ICD-10-AM J21.0, J20.5, J12.1, B97.4)   Linear model estimates 
 0–5 mo 131.67 (18.76) 68 −41.61 (−63.86 to 51.92) 
 6–11 mo 34.33 (4.63) 19 −35.23 (−58.14 to 43.05) 
 12–23 mo 25.33 (4.55) −47.47 (−59.55 to −25.11)* 
 2–4 y 17.17 (6.11) 12 −66.39 (−80.73 to 31.61)d 
 5–15 y 12.67 (5.43) −77.34 (−83.76 to −62.56)** 
Data Source and Age Group2014–2019,a Average Annual Count (SD)2020, nDifference From the Expected, % (95% CI)
Laboratory-confirmed RSV infectionsa,b   Linear model estimates 
 0–5 mo 453.4 (43.15) 343 −22.08 (−48.58 to 60.73) 
 6–11 mo 196 (25.37) 174 −20.37 (−48.51 to 75.61) 
 12–23 mo 200.8 (28.36) 284 23.53 (−20.43 to 176.06) 
 2–4 y 131 (23.27) 311 83.91 (34.21 to 192.07)** 
 5–15 y 67.2 (19.83) 63 −37.87 (−56.67 to 9.78) 
 Total 1048.4 (113.47) 1175 1.37 (−29.95 to 83.35) 
RSV-coded hospitalizations (ICD-10-AM J21.0, J20.5, J12.1, B97.4)   Linear model estimates 
 0–5 mo 454.67 (32.25) 283 −42.67 (−52.68 to −27.31)** 
 6–11 mo 175.83 (33.27) 121 −47.33 (−59.71 to −23.98)* 
 12–23 mo 170 (11.22) 163 −9.34 (−26.54 to 18.38) 
 2–4 y 100 (14.87) 136 12.4 (−16.25 to 70.81) 
 5–15 y 44.83 (12.89) 44 −35.04 (−48.16 to −13.03)* 
 Total 946.83 (89.31) 747 −31.8 (−41.13 to −18.96)** 
Unspecified bronchiolitis–coded hospitalizations (ICD-10-AM J21.9)   Quadratic model estimates 
 0–5 mo 182.17 (47.85) 173 −52.49 (−61.67 to −37.51)** 
 6–11 mo 204.5 (30.43) 130 −59.08 (−65 to −50.76)** 
 12–23 mo 127.67 (14.91) 102 −33.16 (−58.75 to 76.07) 
 2–4 y 6.17 (0.75) — — 
 5–15 y 0.67 (0.82) — — 
 Total 521.17 (84.53) 407 −51.74 (−56.65 to −45.59)*** 
ARI-coded ED visitsc   Quadratic model estimates 
 0–5 mo 1583.67 (134.38) 1100 −44.52 (−58.21 to −17.49)* 
 6–11 mo 1845.5 (99.09) 1380 −36.13 (−47.73 to −17.89)* 
 12–23 mo 2910.67 (180.95) 2732 −23.27 (−31.58 to −12.66)** 
 2–4 y 3789.83 (306.47) 3400 −29.25 (−44.13 to −3.55)* 
 5–15 y 2656.67 (365.68) 2207 −40.78 (−61.57 to 29.04) 
 Total 12 786.33 (1003.2) 10 819 −33.37 (−46.33 to −12.15)* 
RSV-coded hospitalizations with ICU (ICD-10-AM J21.0, J20.5, J12.1, B97.4)   Linear model estimates 
 0–5 mo 131.67 (18.76) 68 −41.61 (−63.86 to 51.92) 
 6–11 mo 34.33 (4.63) 19 −35.23 (−58.14 to 43.05) 
 12–23 mo 25.33 (4.55) −47.47 (−59.55 to −25.11)* 
 2–4 y 17.17 (6.11) 12 −66.39 (−80.73 to 31.61)d 
 5–15 y 12.67 (5.43) −77.34 (−83.76 to −62.56)** 
a

Laboratory data for 2015–2019.

b

Five individuals in CHW dataset had both RSV-A and RSV-B detected, and these were counted once.

c

Visits mapped to an RSV-specific code or unspecified ARI code (asthma, acute upper respiratory infection, acute lower respiratory infection, bronchiolitis, cough, pneumonia, and wheezing).

d

Estimates were derived from a quadratic model with a better model fit (Akaike information criterion, Schwartz’s Bayesian criterion, mean absolute percent error) compared with the linear model.

*

P < .05.

**

P < .01.

***

P < .001.

TABLE 2

Comparison of the Median Age of RSV-Related Disease in 2020 With 2014–2019

Data SourceMedian Age, Months (IQR)Pa
2014–20192020
Laboratory-confirmed RSV infectionsb 7 (2–18) 14 (4–29) <.0001 
RSV-coded hospitalizations (ICD-10-AM J21.0, J20.5, J12.1, B97.4) 6 (2–16) 9 (3–23) <.0001 
RSV bronchiolitis–coded hospitalizations (ICD-10-AM J21.0) 4 (2–8) 4 (2–7) .27 
RSV pneumonia–coded hospitalizations (ICD-10-AM J12.1) 24 (16–38) 26 (18–39) .07 
RSV classified elsewhere–coded hospitalizations (ICD-10-AM B97.4) 19 (7–42) 22 (12–42) .07 
RSV bronchitis–coded hospitalizations (J20.5) 13 (3–24) 32.5 (6–59) .49 
Unspecified bronchiolitis–coded hospitalizations (ICD-10-AM J21.9) 8 (4–12) 7 (3–12) .02 
ARI SNOMED CT–coded ED visitsc 24 (11–52) 25 (12–52) <.0001 
Data SourceMedian Age, Months (IQR)Pa
2014–20192020
Laboratory-confirmed RSV infectionsb 7 (2–18) 14 (4–29) <.0001 
RSV-coded hospitalizations (ICD-10-AM J21.0, J20.5, J12.1, B97.4) 6 (2–16) 9 (3–23) <.0001 
RSV bronchiolitis–coded hospitalizations (ICD-10-AM J21.0) 4 (2–8) 4 (2–7) .27 
RSV pneumonia–coded hospitalizations (ICD-10-AM J12.1) 24 (16–38) 26 (18–39) .07 
RSV classified elsewhere–coded hospitalizations (ICD-10-AM B97.4) 19 (7–42) 22 (12–42) .07 
RSV bronchitis–coded hospitalizations (J20.5) 13 (3–24) 32.5 (6–59) .49 
Unspecified bronchiolitis–coded hospitalizations (ICD-10-AM J21.9) 8 (4–12) 7 (3–12) .02 
ARI SNOMED CT–coded ED visitsc 24 (11–52) 25 (12–52) <.0001 

Numbers <6 are suppressed from presentation. IQR, interquartile range; SNOMED CT, Systemised Nomenclature of Medicine Clinical Terms.

a

Median age compared by using Wilcoxon rank sum test.

b

Laboratory data for 2015–2019; 55 individuals in the CHW dataset had both RSV-A and RSV-B detected, and these were counted once.

c

Visits mapped to an RSV-specific code or unspecified ARI code (asthma, acute upper respiratory infection, acute lower respiratory infection, bronchiolitis, cough, pneumonia, and wheezing).

TABLE 3

Comparison of RSV Test Percent Positivity in 2020 With 2015–2019

Age Group2015–2019,an (%)2020, n (%)Pb
0–5 mo 2267 (23.82) 343 (19.54) <.0001 
6–11 mo 980 (20.20) 174 (14.29) <.0001 
12–23 mo 1004 (16.32) 284 (13.43) .0016 
2–4 y 655 (9.71) 311 (11.74) .0041 
5–15 y 336 (4.13) 63 (2.08) <.0001 
Total 5242 (14.81) 1175 (10.92) <.0001 
Age Group2015–2019,an (%)2020, n (%)Pb
0–5 mo 2267 (23.82) 343 (19.54) <.0001 
6–11 mo 980 (20.20) 174 (14.29) <.0001 
12–23 mo 1004 (16.32) 284 (13.43) .0016 
2–4 y 655 (9.71) 311 (11.74) .0041 
5–15 y 336 (4.13) 63 (2.08) <.0001 
Total 5242 (14.81) 1175 (10.92) <.0001 
a

Calculated by pooling all records between 2015 and 2019.

b

Calculated using Fisher’s exact test for each age group; 55 individuals in the CHW dataset had both RSV-A and RSV-B detected, and these were counted once.

The proportion of children who tested positive for RSV followed similar trends at both hospitals from 2015 to 2020 (Supplemental Fig 3). There was an increase in the frequency of testing at CHW in 2020 compared with previous years (Table 4, Supplemental Fig 4). RSV-A was the predominant circulating subtype in 2020, representing 90.89% of all RSV infections (Supplemental Table 5).

TABLE 4

Comparison of RSV Testing Frequency in 2020 With 2015–2019

Age GroupAverage Annual Number of RSV Tests (SD)Difference From the Expected, % (95% CI)
2015–20192020
CHW    
 0–5 mo 1392.2 (175.99) 1406 −17.6 (−30.54 to 1.28) 
 6–11 mo 699.2 (95.21) 1032 17.46 (10.02 to 25.98)** 
 12–23 mo 837.6 (151.01) 1710 52.69 (35.99 to 74.07)*** 
 2–4 y 925.8 (242.93) 2252 66.81 (21.33 to 166.84)* 
 5–15 y 1058.8 (343) 2465 48.76 (3.73 to 162.9)* 
 Total 4913.6 (975.41) 8865 32.08 (11.64 to 61.68)* 
Sydney Children’s Hospital at Randwick 
 0–5 mo 511.6 (60.24) 349 −31.9 (−58.12 to 82.06) 
 6–11 mo 271.2 (42.93) 186 −41.56 (−63.15 to 41.12) 
 12–23 mo 392.6 (80.85) 405 −23.19 (−44.77 to 26.05) 
 2–4 y 423 (121.96) 396 −36.45 (−58.29 to 33.38) 
 5–15 y 566.8 (130.54) 564 −29.28 (−46.31 to 3.59) 
 Total 2165.2 (394.31) 1900 −31.62 (−52.27 to 20.49) 
Age GroupAverage Annual Number of RSV Tests (SD)Difference From the Expected, % (95% CI)
2015–20192020
CHW    
 0–5 mo 1392.2 (175.99) 1406 −17.6 (−30.54 to 1.28) 
 6–11 mo 699.2 (95.21) 1032 17.46 (10.02 to 25.98)** 
 12–23 mo 837.6 (151.01) 1710 52.69 (35.99 to 74.07)*** 
 2–4 y 925.8 (242.93) 2252 66.81 (21.33 to 166.84)* 
 5–15 y 1058.8 (343) 2465 48.76 (3.73 to 162.9)* 
 Total 4913.6 (975.41) 8865 32.08 (11.64 to 61.68)* 
Sydney Children’s Hospital at Randwick 
 0–5 mo 511.6 (60.24) 349 −31.9 (−58.12 to 82.06) 
 6–11 mo 271.2 (42.93) 186 −41.56 (−63.15 to 41.12) 
 12–23 mo 392.6 (80.85) 405 −23.19 (−44.77 to 26.05) 
 2–4 y 423 (121.96) 396 −36.45 (−58.29 to 33.38) 
 5–15 y 566.8 (130.54) 564 −29.28 (−46.31 to 3.59) 
 Total 2165.2 (394.31) 1900 −31.62 (−52.27 to 20.49) 
*

P < .05.

**

P < .01.

***

P < .001.

In 2020, there was a shift in the timing of the peak of RSV-coded hospitalizations that corresponded with the shift in RSV infections (Fig 2B). An overall decrease in the frequency of hospitalizations was observed in 2020 compared with the expected value (−31.80%; 95% CI, −41.13% to −18.96%; P < .01) (Table 1). An increase in the peak case counts of hospitalizations was also seen in 2020 compared with previous years, most prominently in children aged 2 to 4 years. Hospitalizations frequency for age groups 12 months to 5 years fell within expected ranges based on the 2015–2019 trend. For children aged 0 to 5 months, 6 to 11 months, and 5 to 15 years, there was a decrease in the frequency of hospitalizations in 2020 compared with the expected value. The median age of children with RSV-coded hospitalizations was higher in 2020 than previous years (9 vs 6 months) (Table 2).

There was an overall decrease in the frequency of RSV-coded ICU admissions in 2020 compared with the expected value (−44.63%; 95% CI, −60.76% to −5.96%; P < .05) (Table 1). The total proportion of hospitalized children admitted to the ICU in 2020 was lower at 15.13% than the average of 23.60% for previous years (−28.59%; 95% CI, −52.82% to 46.75%).

For ICD-10-AM–coded unspecified bronchiolitis hospitalizations, there was a shift in the timing of the peak in 2020, with no change in peak case counts (Fig 2B). An overall decrease of hospitalizations was observed in 2020 compared with the expected value (−51.74%; 95% CI, −56.65 to −45.59%; P < .001) (Table 1).

In 2020, there was a shift in the timing of the peak of ARI ED visits for age groups <5 years (Fig 2C). The resurgence in ARI ED visits precedes the resurgence of RSV-related disease. There was an overall decrease in ARI ED visits in 2020 compared with the expected value (−33.37%; 95% CI, −46.33 to −12.15%; P < .05) (Table 1). The median age of children with ARI ED visits was higher in 2020 than previous years (25 vs 24 months) (Table 2).

Our analysis of RSV-related disease in southeast Australia reflects the clear shift in the 2020 RSV season highlighted in other Australian reports.6,7,9  Additionally, we observed an overall decrease in severe RSV in 2020 compared with previous years, as evidenced by reductions in RSV-related hospitalizations and ICU admissions. We observed a paradoxical increase in the frequency of RSV infections in children aged 2 to 4 years compared with previous years.

Our study provides evidence that the delayed RSV season associated with COVID-19 public health measures did not result in a more severe RSV epidemic. We examined RSV-related hospitalizations as a surrogate for severe disease and found similar peak case counts and a reduced frequency of hospitalizations in children aged <12 months compared with previous years. We also found an overall reduction of RSV-coded ICU admissions in 2020 and no difference in the proportion of children requiring intensive care. Recently published studies from Europe and Australia provide further evidence that the delayed RSV epidemic was no more severe than previous seasons based on an examination of clinical outcomes, including length of stay and intensive care requirement.12,21  Conversely, a brief report from the United States suggested an increased intensive care requirement.11  Ongoing research is needed to understand the impact of disrupted RSV transmission on disease severity in different settings and over time.

We observed substantial increases in peak case counts of RSV infections concentrated in older age groups and an increase in the median age of children with RSV (14 vs 7 months) in 2020 compared with previous years, reflecting the trends described in other Australian studies which analyzed laboratory data.7,9  When analysis was restricted to RSV-coded hospitalizations, we observed a more modest increase in the median age of children (9 vs 6 months) and smaller increases in peak case counts, predominantly in children aged 2 to 4 years. A buildup in age-specific population susceptibility, including an immune-naive cohort of older children carried forward into late 2020, may explain changes in age distribution.13  Children aged >2 years have a reduced propensity for severe RSV disease22 ; hence, increased RSV infections in this group did not translate to a significant increase in hospitalizations. Increased peak case counts of RSV-related disease may reflect an increase in the size of the epidemic despite overall reductions in severe disease.

A lowered threshold for respiratory virus testing in the context of the COVID-19 pandemic has been reported1,21  and likely contributed to the increases in RSV-related disease and older age distribution. We found evidence of increased testing at one study site in 2020 and reason that increased diagnosis of RSV disease occurred, particularly in older children who may have been under-tested in the past. The increase in the proportion of positive RSV test results in children aged 2 to 4 years in 2020 compared with previous years, however, suggests a true increase of RSV disease in this age group.

The resurgence of RSV disease was likely driven by a buildup in population susceptibility coupled with relaxed community behaviors. The lockdown in autumn (March to May) was associated with a rapid decline in all pediatric health service use and in RSV-related disease.3,18  This trend continued in the winter months after lockdown, as reflected by a persistent reduction of pediatric presentations related to acute infectious conditions and of RSV-related disease.6,18  Enhanced infection prevention, particularly in schools and ECECs,3,20,23  may have contributed to the interruption and to maintaining the suppression of RSV during the usual peak period. Survey data from March 2020 indicated a high uptake of COVID-19–related hygiene and avoidance strategies in the Australian community.24  However, there is a general lack of data pertaining to school and ECEC settings and no follow-up data to understand to what extent behaviors changed throughout the course of the year. Such information could have provided an indication of the contribution of individual behavior change to the suppression and resurgence of RSV. Mask uptake in the NSW community was limited until the introduction of mandates in early 202114  and so did not contribute. Beyond COVID-19, the promotion of enhanced infection prevention in school and ECEC settings should be explored as low-cost, sustainable interventions to reduce the transmission of all respiratory viruses.23 

In contrast to RSV, the suppression of influenza virus was maintained throughout 2020, indicating distinct differences in the transmission dynamics of these 2 respiratory viruses.6  Influenza virus circulation may be more sensitive to COVID-19–related public health measures, in particular global travel restrictions, than RSV.1  We observed a striking predominance of RSV-A in 2020 that was also reported from the state of Victoria.7  This predominance in RSV-A reflects the recently described reduction in RSV genetic diversity across Australia in 2020 and the presence of 2 genetically distinct RSV-A clades, which likely continued to circulate at low levels in the population during COVID-19 restrictions.25  Operational, but reduced-capacity ECECs represent one possible reservoir of RSV that may have been sustained during the lockdown. Older children, particularly pre- and early-school-aged children (3–6 years) are believed to play an important role in driving RSV epidemics,26  and these age groups warrant further consideration.

Strengths of our study include the use of multiple, large, and robust datasets stratified by age and severity to compare 2020 RSV epidemiology with that of the previous 6 seasons. A limitation of our analysis is the incomplete capture of the delayed RSV epidemic curve; however, examination of SCHN data sources demonstrated that RSV-related disease declined rapidly and returned to baseline levels by early February. Consequently, we captured a high proportion of the RSV epidemic cases, including the epidemic peak, and therefore consider that our study provides sufficient data to elucidate differences in age distribution and severity.

Another limitation of our study is our reliance on RSV-specific codes, which have been shown to underestimate the true frequency of RSV disease.27  Conversely, unspecified bronchiolitis and ARI-related disease may reflect the circulation of other respiratory pathogens. High rhinovirus activity in 20206  may explain the early rise in ARI ED visits for conditions of bronchiolitis, wheezing, and asthma.2830  As with other similar studies, we were unable to fully account for the presence of potential confounding factors, such as changes to models of care, health care–seeking behavior, and respiratory virus testing practices, during the COVID-19 pandemic.

In conclusion, COVID-19 public health measures were clearly associated with a shift of the 2020 RSV season in NSW; however, the delayed epidemic was no more severe than previous years. A buildup in population susceptibility and increased testing may account for the increase of RSV infections in children aged 2 to 4 years. COVID-19–related public health measures will likely continue to have an impact on the circulation of RSV and result in unpredictable future epidemics, further emphasizing the need for robust RSV surveillance to support policy decisions and research related to RSV prophylaxis, future vaccines, nonpharmaceutical prevention strategies, and health system preparedness.

The authors thank Jane Shrapnel and Jake Davis from the SCHN Management Support and Analysis Unit and clinicians and laboratory staff at CHW, Sydney Children’s Hospital at Randwick, and NSW Health Pathology (South Eastern Area Laboratory Service).

Drs Britton, Lingam, and Homaira conceptualized and designed the study, oversaw data analysis and interpretation, and reviewed and revised the manuscript; Dr Hu conceptualized and designed the study, collected the data, carried out the data analysis, and reviewed and revised the manuscript; Dr Saravanos conceptualized and designed the study, collected the data, performed the data analysis, drafted the initial manuscript, and reviewed and revised the manuscript; Drs Rawlinson and Kesson provided laboratory data, supported interpretation of the laboratory data, and critically reviewed and revised the manuscript for important intellectual content; Drs Wood, Jaffe, Bartlett, and Muscatello supported the interpretation of the data and critically reviewed and revised the manuscript for important intellectual content; and all authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.

FUNDING: Dr Britton was supported by a National Health and Medical Research Council early career fellowship (GNT1145817), Dr Hu was supported by a National Health and Medical Research Council research fellowship (GNT1158646), Drs Lingam and Hu were supported by the Financial Markets Foundation for Children, and Dr Saravanos was supported by The University of Sydney Postgraduate Award. The funders and sponsors did not participate in the work.

     
  • ARI

    acute respiratory illness

  •  
  • CHW

    The Children’s Hospital at Westmead

  •  
  • CI

    confidence interval

  •  
  • ECEC

    early childhood education center

  •  
  • ED

    emergency department

  •  
  • ICD-10-AM

    International Classification of Diseases, Tenth Revision, Australian Modification

  •  
  • NSW

    New South Wales

  •  
  • RSV

    respiratory syncytial virus

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Competing Interests

FINANCIAL DISCLOSURES: The authors have indicated they have no financial relationships relevant to this article to disclose.

POTENTIAL CONFLICT OF INTEREST: The authors have indicated they have no potential conflicts of interest to disclose.