Seasonality of Respiratory Syncytial Virus in Belgium

Presenter: M. Raes (e-mail: [e-mail])

Introduction: Infections caused by respiratory syncytial virus (RSV) occur in most children by 2 years of age. Although symptoms are often absent or mild, RSV is the most frequent cause of bronchiolitis and pneumonia in infants and young children and an important source of morbidity, mortality, and financial burden worldwide. Prophylaxis using Palivizumab is recommended for infants with risk factors for severe RSV infection (e.g., lung disease or prematurity), with a maximum of 5 monthly doses during the RSV season. However, the epidemiology of RSV differs widely across latitudes and meteorological conditions. In temperate climates, RSV activity is maximal during winter, with biannual cycles in some countries, but not in others. As the monthly administration of Palivizumab needs to be timed according to local circulation patterns of the virus, we investigated the seasonal pattern of RSV in Belgium.

Materials and Methods: Data on inpatient and ambulatory RSV detection tests performed in children under the age of 3 years from July 2004 to June 2014 were collected from 9 hospitals spread throughout Belgium, covering approximately 25% of pediatric beds in the country. RSV epidemics were identified based on the criteria of the National Respiratory and Enteric Virus Surveillance System and using the ISO standard to define weeks. The start of an epidemic was defined as the first of 2 consecutive weeks in which at least 10% of the tested samples tested positive for RSV and a minimum of 10 samples were tested in that week. The end of an epidemic was defined as the last of 2 consecutive weeks with at least 10% of the samples tested positive for RSV and a minimum of 10 samples tested in that week. The peak week was defined as the week with the highest percentage of positive tests.

Results: From the 57,026 RSV detection tests, 15,176 (26.6%) were positive. Most cases (61.7%) were younger than 9 months and more boys (54.3%) than girls tested positive for RSV. The majority (92.7%) of positive tests occurred within the identified RSV epidemics. Table 1 presents the total number of RSV tests, the number of positive tests, the start, end, and peak week, and the duration of the RSV epidemic by season. The onset of epidemics ranged from week 39 (end of September/start of October) to week 44 (start of November), except for the very early start of the 2008–2009 season in week 30 (end of July). The end of epidemics ranged from week 6 to week 10, except for season 2007–2008 (week 17), season 2008–2009 (week 3), and season 2010–2011 (week 13). The median (range) peak week was week 50 (47–52). Epidemic duration ranged from 17 to 21 weeks, except for the longer epidemics in 2007–2008 (28 weeks), 2008–2009 (26 weeks), and 2010–2011 (25 weeks).

Conclusion: Although the timing and duration of most RSV epidemics was relatively consistent, outbreaks were exceptionally early or long in some seasons, complicating the timing of Palivizumab treatment. The identification of environmental drivers of RSV activity would support efficient management of resources and timely delivery of Palivizumab. Further studies of our group will thus focus on how to link RSV epidemic characteristics to meteorological parameters.

Keywords: respiratory syncytial virus, RSV, seasonality, epidemics

Table 1 RSV detection tests and characteristics of the RSV epidemics, Belgium, 2004–2014

Season	No. of tests	No. of positive tests	Start week	End week	Peak week	Duration (weeks)
2004–2005	2,375	704	44	7	52	17
2005–2006	3,839	1,179	40	8	48	21
2006–2007	4,682	1,291	42	9	49	20
2007–2008	4,407	1,104	42	17	52	28
2008–2009	6,076	1,562	30	3	47	26
2009–2010	5,894	1,670	43	10	50	21
2010–2011	6,853	1,622	41	13	50	25
2011–2012	7,331	1,761	41	8	49	20
2012–2013	7,500	2,169	39	6	48	20
2013–2014	8,069	2,114	43	8	50	18