Spread and clinical severity of respiratory syncytial virus A genotype ON1 in Germany, 2011–2017

Human Respiratory Syncytial Virus (RSV) remains one of the leading causes of pediatric acute respiratory tract infections (RTI) affecting virtually all children by the age of two years [1‐3]. In 2015, 33 million episodes of lower RTI due to RSV causing approximately 3 million hospital admissions and about 60,000 in-hospital deaths were estimated globally in children under 5 years of age [4]. The main clinical characteristics are cough, fever, dyspnea and wheezing [5]. Premature birth and young age are predisposing factors for a severe course of RSV disease and hospitalization due to RSV infection [2, 4‐6].

Phylogenetically, RSV is divided into the major antigenic groups A and B, each with various genotypes, based mainly on genetic variability of the viral surface G glycoprotein [1, 7, 8]. In 2010, a novel RSV-A genotype ON1 characterized by a nucleotide (nt) duplication of 72-nt in the G protein gene, appeared in Ontario (Canada) [9]. A similar 60-nt duplication in the G protein gene of RSV-B genotype BA had previously been associated with rapid spread and replacement of other genotypes [10], indicating selection advantages of such nt-duplications [11, 12]. Correspondingly, it has been suggested that the new RSV-A ON1 may have the potential to replace other (non-ON1) RSV-A strains [2, 11, 13].

In Germany, RSV-A ON1 was first detected in 2011/12 [14], and was already predominant in the season 2012/13 [11]. Preliminary observations in hospitalized children in 2011/12 indicated a more severe clinical course of illness for RSV-A ON1 than for RSV-A non-ON1 genotypes [14], whereas a monocentric hospital study conducted during the season 2012/13 found no such difference [11].

In the multicenter investigation presented here, we analyzed the overall genotype distribution of RSV-A and RSV-B strains in out- and inpatient pediatric treatment settings in Germany during the period 2010/11 to 2016/17. The main objectives were to investigate the spread of the new genotype RSV-A ON1 and to assess its disease severity compared to RSV-A non-ON1 genotypes.

After its first detection in Germany in 2011/12, RSV-A ON1 co-circulated with RSV-A non ON1 genotypes and had largely replaced RSV-A non-ON1 genotypes in 2013/14, thus demonstrating a very rapid and effective spread. Following the first reports of RSV-A ON1 in Canada [9] and Germany [14], genotype ON1 was detected in many other countries and regions [3, 13], including Asia [8, 17‐28], Africa [12, 29‐31], Europe [32‐39] and North and South America [40‐43]. In line with our results, several multi-season studies have previously reported ON1 to have rapidly replaced RSV-A non-ON1 genotypes [18, 19, 25, 28, 30‐33, 35, 37‐39, 42‐45]. Furthermore, ON1 has recently been reported to be the currently most prevalent RSV-A genotype worldwide [43].

During the first and the second season of RSV-A ON1 appearance in Germany, the proportion of RSV-A in our study population was considerably higher than the proportion of RSV-B, similar to reports from some other countries [19, 38, 43, 44]. It has been speculated that the rapid spread of RSV-A ON1 might also reduce RSV-B circulation in subsequent seasons [43, 44]. In our study, however, RSV-B strongly dominated the seasons 2013/14 and 2015/16. This finding suggests that the emergence of genotype ON1 has not had a noticeable effect on the overall epidemiological pattern of frequent shifts between RSV-A and RSV-B group dominance [2, 3], confirming similar observations reported from Spain [38] and Argentina [43].

The genotypes currently most prevalent worldwide (RSV-A ON1 and RSV-B BA) are both characterized by extensive nt-duplications in the G-gene. The mechanisms for the selective advantage of such nt-duplications are still poorly understood and may be associated with effects on virus replication fitness [46], or on the modulation or evasion of host immune response and host adaptation [3, 19]. It will be interesting to observe for how long these two genotypes remain predominant. For RSV-B genotype BA, first detected in 1999 [10], a considerable number of sub-genotypes based on point mutations are already known [3]. For RSV-A ON1, several new lineages have recently emerged [13], including the extreme case of a complete loss of the nt-duplication which has been observed under specific conditions, e.g. a lack of host’s immune system driven selective pressure [47]. The future evolution of these two most prevalent genotypes and the underlying selection mechanisms may have a considerable impact on the development of potential RSV vaccines [19].

Reports on the clinical disease severity of RSV-A ON1 compared to RSV-A non-ON1 genotypes are inconsistent and have thus far mainly been based on observations from inpatients [2]. Our investigation is one of only few studies describing RSV-A ON1 separately for children requiring outpatient primary care [25], and indicates similar disease severity for outpatients infected with RSV-A ON1 and those with RSV-A non-ON1 infections. Regarding inpatients, we also found a similar clinical severity of RSV-A genotypes in children admitted to a general PW. Interestingly, however, in the group of children requiring PICU treatment, our study suggested a somewhat more severe course of disease in children with genotype ON1 infection, such as a longer duration of hospital stay and of oxygen treatment, compared to children infected with RSV-A non-ON1.

Some of previous inpatient studies comparing RSV-A ON1 with RSV-A non-ON1 genotypes found no difference in clinical severity [11, 38], while others showed divergent correlations between genotype ON1 and viral virulence [2]. Several authors have suggested a higher virulence of RSV-A ON1, based on the observation of higher rates of PICU admission and/or mechanical ventilation [42], earlier hospitalization and a higher rate of lower respiratory tract infections [19] or more frequent refusal to feed [44]. In contrast, three studies reported a milder course of illness associated with genotype ON1, such as a lower severity score [32], a lower rate of bronchopneumonia / lower respiratory tract disease [33, 35] and a lower hospitalization rate [33]. These controversial observations might be explained by the heterogeneity of study designs, inclusion criteria, region, and the seasons/years when the surveillance was undertaken, resulting in different levels of genotype-specific herd immunity [2, 19, 44].

In populations immunologically naïve to a new genotype, an initially more severe clinical course of disease might be expected. A study conducted during the early phase of RSV-A ON1 spread in Vietnam indicated a higher hospitalization rate for genotype ON1 [19]. Furthermore, a study from Italy covering the first two years of genotype ON1 emergence [35] and also our own observations from the season 2011/12, the first year of genotype ON1 appearance in Germany [14], had suggested a higher rate of PICU admissions for RSV-A ON1 patients. However, our previous assumption was based solely on laboratory samples; clinical data to adjust for other factors with a potential impact on PICU admission was not available at that time [14]. The inter-setting analysis presented here was based on both laboratory and clinical data for a larger and more homogenous subgroup of PW and PICU patients admitted during the first two seasons of RSV-A ON1 emergence. In this analysis, we found no association of genotype ON1 with PICU treatment after adjustment for season, demographic and anamnestic characteristics. Furthermore, only very few of our within-setting comparisons pointed to a more severe clinical course of RSV-A ON1 during the early phase of ON1 circulation compared to later seasons, e.g. an increased refusal to feed in PW patients and longer CPAP treatment in PICU patients. Our study therefore showed no indication of a notably higher virulence of RSV-A ON1 during the first season(s) after its initial emergence.

The main difference between RSV-A genotypes observed in our analyses was the higher clinical severity in the group of PICU children infected with genotype ON1, compared to PICU children with RSV-A non-ON1 infection. The majority of children in this setting were younger than 6 months of age (median age two months), and likely experienced their first-ever RSV infection. RSV-A NA1 was the most prevalent RSV-A genotype in the population before the emergence of genotype ON1 and, hence, it might be speculated that maternal RSV-neutralizing antibodies transferred to the child before birth [48] were mainly directed against the NA1 genotype. As the mean half-life of RSV-neutralizing antibodies has been estimated as 26 days, it seems plausible that they may have provided at least partial immunity against the NA1 genotype during the first few months of life [48‐50], but no or only very limited protection against the new ON1 genotype [51].

Our study has several limitations. The data for the present analyses was obtained within the framework of several ongoing surveillance studies, each targeting clinical severity of respiratory viruses in a specific pediatric setting. Hence, although all studies included children with PCR-confirmed, RSV-associated RTI and enabled extensive within-setting comparisons, the scope for inter-setting comparison was limited, mainly due to the differences in design, study seasons, and patient age range. Even in a subgroup corrected for study seasons and age range, the results of the inter-setting comparison PW vs. PICU should still be regarded with caution, as the children from PW and from PICU pooled for this analysis belonged to different study populations, with only one hospital contributing to the PW dataset. Hence, regional factors, such as possible differences in the distribution of RSV-A genotypes, cannot be ruled out, which may lead to confounding of the association. Furthermore, some subgroup analyses within settings were limited by the small sample size, e.g. the comparison of RSV-A genotypes in the subgroup of PICU children with premature birth. However, the higher clinical severity of genotype ON1 found in patients within the PICU setting was indicated by several markers of severity, both overall and in stratified analyses, and, thus, appears to be a robust finding. Apart from small sample size, the less pronounced difference among RSV-A genotypes observed in PICU children with premature birth might be due to the fact that the general higher risk for a more severe course of RSV disease of these children has a stronger impact than variations in the RSV genotype.

The main strengths of our study are the collection of a largely comprehensive set of clinical and laboratory data from three different pediatric settings reflecting various degrees of clinical severity. In particular, our study includes an evaluation of RSV-A ON1-associated clinical severity in primary care outpatients, who represent the majority of medically treated RSV cases, and a separate evaluation of genotype ON1-associated severity in pediatric PICU patients.

In children, RSV-A ON1 largely replaced RSV-A non-ON1 genotypes in Germany within two epidemiological seasons and remained the prevalent RSV-A genotype during the following four seasons. Clinical severity of ON1 infections was similar to that of RSV-A non-ON1 infections both in out- and inpatient pediatric settings, apart from a longer treatment duration for ON1 patients within the group of children requiring PICU treatment.