## 1 Introduction  
Respiratory syncytial virus (RSV) is the most common cause of serious lower respiratory tract infection in young children worldwide and a frequent cause for hospitalization in children less than 2 years old in the USA. Severe infections early in life may predispose to asthma later. RSV also impacts the elderly and can be especially debilitating in those with underlying heart or lung disease and accounts for nearly as many hospitalizations as influenza [1, 2]. Currently, there are no vaccines licensed to prevent RSV infection, despite more than 50 years of effort. An FDA approved humanized anti-RSV-F monoclonal antibody prevents severe RSV disease but is only approved for use in pre-term and high-risk infants [3]. The development of an effective RSV vaccine has proven elusive, in part due to the immature immune system of infants and immune senescence of aged adults. Maternal antibodies may further complicate the immune response to RSV infection in young infants by suppressing the initial response to viral infection allowing reinfection to occur [4]. While it is now commonly accepted that most neutralizing antibodies target pre-F protein, there is also evidence demonstrating that both RSV-pre-F and -G antibodies can modulate RSV disease severity in young infants [5; 6]. Additionally, RSV-G antibodies are under evaluation for their role in complement-mediated antiviral activity, blockade of airway epithelial cell infection, and anti-inflammatory activity [7]. Accordingly, assays that detect anti-RSV-G antibodies can be used to evaluate antibody responses elicited either by immunization with G-containing vaccines or by natural infection to assess incidence rates during the RSV season. RSV is categorized into two distinct antigenic subgroups, RSV-A and RSV-B, which may co-circulate or independently dominate during yearly epidemics. The RSV-F protein is most conserved (>90%) between RSV-A and -B strains, thus making this protein an ideal target for vaccine design and therapeutics. In contrast, RSV-G protein is more variable across subgroups (53% conservation at the amino acid level), although there is a central conserved domain (CCD) region that is 100% conserved amongst all circulating strains. This central conserved region is essential for infectivity in vivo, since it mediates attachment to airway epithelial cells, and plays a role in the host immune response due to the resemblance to the CX3C chemokine motif [8; 9]. The CCD region of the RSV-G protein has been targeted for monoclonal antibody development and RSV-G based vaccines; however, areas outside the CCD region may also play a part in protection from infection [7; 10; 11; 12; 13; 14; 15; 16]. During clinical trials of candidate vaccines, RT-PCR assays are often used for case confirmation and to identify RSV-infected subjects. While these assays can be very sensitive, the RSV-infected cohort may sometimes be underestimated due to logistics involved in sample collection. Serological assays have been used to complement and support RT-PCR testing to identify the RSV-exposed population. Serological assays enhanced detection of RSV cases by almost 12% over RT-PCR methods alone [17]. Unfortunately, many of the currently available serological tests that detect anti-RSV IgG responses use lysates of RSV-infected cells, purified virus, or vaccine antigens to bind antibodies. These tests do not provide information about the subtype of the infecting strain and may not discriminate between responses elicited by vaccine from those seen following natural infection. In contrast, serological assays using non-vaccine, off-target antigens have successfully identified individuals undergoing RSV infection during the post-vaccination period [18; 19]. The immunoprecipitation assay described in this report uses two luciferase-tagged RSV antigens (RSV-G\({}_{\text{A}}\) and RSV-G\({}_{\text{B}}\)) to help fill the gap in serology testing.