Respiratory syncytial virus in pediatric influenza‐like illness cases in Lombardy, Northern Italy, during seven consecutive winter seasons (from 2014–2015 to 2020–2021)

Abstract Introduction Respiratory syncytial virus (RSV) is the major cause of lower respiratory tract illness in young children and can also cause influenza‐like illness (ILI). Here we investigated the epidemiological features of RSV infection in pediatric ILI cases in Lombardy (a region in Northern Italy accounting nearly 10 million inhabitants) from 2014–2015 to 2020–2021 winter seasons. Material and Methods Data for this study were retrieved and statistically analyzed from the database of virological influenza surveillance of the regional reference laboratory for Lombardy within the Italian influenza surveillance network (InfluNet). Results RSV accounted for nearly 19% of pediatric ILI with a risk of infection nearly two‐fold greater than that of individuals ≥15 years. RSV positivity rate increased to 28% considering 0–5 years old children. Although in children ≤5 years the risk of infection from influenza viruses resulted nearly two‐fold higher than the risk of RSV infection, the age group 4–6 months and 7–12 months showed a five‐fold greater risk of infection from RSV than from influenza. Children ≤5 years of age with pre‐existing underlying health conditions had a nearly five‐fold greater risk of getting RSV infection than otherwise healthy 0–5 years old children. RSV was identified in ILI cases <15 years of age in all considered winter seasons except in the 2020–2021 season. Discussion Sentinel surveillance of ILI allowed us to identify groups at higher risk of RSV and influenza infection and to define the start, duration, timing, and intensity of the RSV and influenza community circulation. This surveillance approach can be implemented to assess the RSV circulation and impact in a real‐time manner.


| INTRODUCTION
Respiratory syncytial virus (RSV) has been recognized as the second most common cause of death in infants. It has been estimated that RSV is responsible for 34 million new episodes of lower respiratory tract infection (LRTI) worldwide, 1 accounting in 2005 for 199 000 deaths among children under 5 years, 99% of which occurred in low-resource settings. However, also in high income countries, such as the UK, the average rate of hospital admissions due to RSV bronchiolitis has increased by an average of 1.8% per year since 2004 2 with RSV bronchiolitis accounting for 12% of admissions to pediatric intensive care unit. 2 RSV is recognized as the pathogen most frequently involved in acute viral bronchiolitis and pneumonia in young children; in fact, in children under 2 years of age it causes from 40% to 90% of bronchiolitis hospitalizations and up to 50% of pneumonia admissions. [3][4][5] In this age group, the highest frequency of RSV-related severe manifestations and mortality are generally observed, with a peak in children under 3 months of life. 6 Moreover, severe outcomes are frequently associated with pre-existing underlying health conditions, such as chronic lung diseases or congenital heart diseases. 3,7 The reported RSV hospitalization rates across the European countries ranged from 2.5 to 11 per 1000 children within the first 4 years of life, and from 19 to 22 per 1000 children among those younger than 12 months of age. 8 These data highlight the significant health, financial and social impact of RSV in both high-and low-income countries.
RSV is an RNA virus of the Pneumoviridae family 9 that primarily spreads via respiratory droplets when a person coughs or sneezes, and through direct contact with contaminated surfaces. 10 RSV is one of the most contagious human pathogens, with over 80% of children experiencing RSV infections by 2 years of age. 11 Reinfection occurs throughout life and can occur more than once in the same season. 12 RSV seasonality is highly dependent on geographic location and climate. In temperate regions an annual seasonal pattern is predictably limited to 3-5 months during winter and autumn. 13,14 Numerous explanations for this have been proposed, including the possibility that inclement climate modifies human behavior, reducing outdoor activities and increasing indoor crowding enhancing exposure and transmission of RSV [15][16][17] or that the low temperatures present during winter prolong the stability of RSV in fomites. 14 Besides hospitalisations and severe manifestations, [18][19][20][21][22] infections can also lead to mild symptoms causing influenza-like illness (ILI), clinically undistinguishable from other common respiratory infections. 20,[23][24][25][26] Since RSV infections occur primarily during autumn and winter-thus overlapping the seasonal circulation of influenza viruses 27 -the virological surveillance of ILI can be a useful tool to monitor RSV circulation-in addition to influenza viruses-in the general population so as to define community viral transmission. The primary objective of this study was to analyze the epidemiological characteristics of RSV in pediatric (<15 years of age) ILI cases observed in the framework of the epidemiological and virological surveillance of influenza-like illness (ILI) in Lombardy (Northern Italy) during seven consecutive winter seasons, namely from 2014-2015 to 2020-2021. The secondary objectives of this study were (i) to describe the epidemiology of RSV infection in children ≤5 years of age with ILI during seven consecutive seasons (from 2014-2015 to 2020-2021), focusing on the following age groups: 0-3 months, 4-6 months, 7-12 months, and 13-24 months and (ii) to compare the epidemiological characteristics of RSV and influenza virus infections in children ≤5 years of age with ILI over the study period.

| METHODS
Data for this report were retrieved from the database of virological influenza surveillance of the regional reference laboratory for Lombardy (Northern Italy), operating within the Italian influenza surveillance network (InfluNet). 28

| Epidemiological and virological surveillance
The Italian influenza surveillance network (InfluNet) 28 combines epidemiological and virological surveillance of influenza to provide information to monitor influenza activity in Italy. The network aims to track influenza epidemics, detecting their start, monitoring their spatial-temporal spread, identifying populations at risk and circulating influenza viruses, and estimating the impact on the community and healthcare structures.
InfluNet relies on the voluntary participation of sentinel physicians (both pediatricians and general practitioners) who survey 2% of the general population, ensuring the representativeness of all age groups (0-4 years, 5-14 years, 15-64 years, and ≥65 years), with homogenous geographical distribution. 28 To increase surveillance sensitivity, the percentage of population under surveillance was increased from 2% to 4% in the 2020-2021 season.
All sentinel physicians are involved in epidemiological surveillance and in charge to report weekly on the number of outpatients seeking care in their ambulatory facilities for ILI occurrence. 28 The "zero reporting" strategy -based on the report of the absence of ILI cases under syndromic surveillance -is adopted. This is crucial for ensuring that sentinel physicians have not merely forgotten to report cases. A number of sentinel physicians is also in charge of collecting respiratory samples (nasopharyngeal swabs, NPS) for virological surveillance. 28 Virological surveillance has an observation period of 28 weeks and extends from week 46 to week 17 of the following year, according to the InfluNet operational protocol. 29 Sampling must be performed in the acute phase of illness and within 7 days from symptoms' onset.

| Case definition
The ILI case definition according to the European Center for Disease Prevention and Control (ECDC) 30 is: an abrupt onset of fever (>38 C) or feverishness, one or more respiratory symptoms (cough, sore throat and/or shortness of breath) and one or more systemic symptoms (myalgia, headache and/or malaise). For children, the case definition of ILI also includes: irritability, loss of appetite and persistent, inconsolable crying. Influenza in toddler is usually associated with vomit and diarrhea and sometimes with fever. Among pre-schooling children influenza infection can cause bloodshot eyes and conjunctivitis with fever and among children aged 1-5 years, laryngo-tracheitis and bronchitis/bronchiolitis should be considered.

| Surveillance data
The following epidemiological surveillance data were collected through the InfluNet database (www.iss.it/site/rmi/influnet): • Aggregate consultations for ILI by week and by age group reported by sentinel primary health care providers in Lombardy.
• Case-based data on: • influenza vaccination status • administration of antiviral influenza drugs.
The following virological influenza surveillance data are collected through the database of the regional reference laboratory: • types and subtypes of influenza viruses collected from sentinel sources • detection of RSV.
This analysis considered data from seven consecutive winter seasons, namely, from 2014-2015 to 2020-2021. For continuous variables, such as age distribution, the unpaired t test was performed and the inter-quartile range (IQR) was computed as difference of first and third quartile.

| Statistical analysis
We estimated influenza and RSV seasonal characteristics, including season onset (or start), duration, peak and offset (or end) applying the RS10 method, which defines the start of epidemic season as the first 2 consecutive weeks when virus detection exceeds 10% of viruspositivity rate. 31 A p value <0.05 was considered significant (two-tailed test).

| Epidemiological and virological results of influenza-like illness surveillance
During the seven winter seasons considered (from 2014-2015 to 2020-2021), the cumulative incidence of ILI cases in the general population ranged from 5% in 2020-2021 to 16.2% in 2017-2018, with a mean value of 10.3% ( Figure 1). In all seasons the highest cumulative incidence of ILI cases was reported in the 0-4 years age group (mean: 22.7%, range: 7.7%-43.6%), followed by the 5-14 years age group (mean: 13.4%, range: 3.2%-21.0%), then decreasing in the adult (15-64 years) age group (mean: 10.1%, range: 5.5%-15.6%), and in the ≥65 years age group (mean: 4.6%, range: 2.0%-7.4%) ( Figure 1). RSV epidemic onset ranged from week 46 to week 5, with a peak of RSV detection from week 52 to week 10, and epidemic offset ranged from week 5 to week 12 (Table 1)  Note: For each season, the week of onset (or start), peak, offset (or end), and epidemic width (duration) of ILI and RSV/IV epidemic are reported. The season onset, peak, offset, and epidemic width were estimated applying the RS10 method, which define the start of RSV and influenza epidemic season as the first two consecutive weeks when virus detection exceeds 10% of RSV/IV-positivity rate. The onset of ILI corresponds to the week when ILI incidence exceeds the threshold value of 2‰. n.d.: not determined. was 27.8% (103/370), 12.3% (28/227) in the 6-10 years age group, and 7.2% (11/153) in the 11-14 years age group (

| Epidemiology of respiratory syncytial virus in children ≤5 years of age with ILI
A total of 370 ILI cases ≤5 years were tested for RSV, information on gender, age and presence of comorbidities are detailed in Table 4.   (Table 4).
Among ILI cases ≤5 years of age and with comorbidities, the RSV

ACKNOWLEDGEMENTS
The authors would like to thank the general practitioners and pediatricians involved in ILI surveillance network InfluNet in Lombardy region.
This study is performed as a side project of the regional reference laboratory for Lombardy operating within the Italian influenza surveillance network (InfluNet). Influenza surveillance activities were funded by DG Welfare, Regione Lombardia, Italy.
Un unconditional support for the statistical elaboration of data was provided by Sanofi Pasteur that had no role in the study design, data collection, data analysis, interpretation of the data, writing the manuscript and in the decision to submit this paper for publication.

CONFLICT OF INTEREST
None declared.
T A B L E 5 Risk of RSV infection-expressed as odds ratios with corresponding 95% CI-in ILI cases by age group from 2014-2015 to 2020-2021 winter seasons in Lombardy

INSTITUTIONAL REVIEW BOARD STATEMENT
The study was conducted according to the guidelines of the

INFORMED CONSENT STATEMENT
Informed consent for virus detection and data publication was not required since data and samples from outpatients with ILI were collected and analyzed anonymously within the National Influenza Surveillance Program.

PEER REVIEW
The peer review history for this article is available at https://publons. com/publon/10.1111/irv.12940.