Respiratory syncytial virus has come under the spotlight in recent years for a number of reasons. CLI chatted to Dr Adam Meijer (Center for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, the Netherlands) to find out more about why interest in detection of this virus has increased.

News about respiratory syncytial virus (RSV) suddenly seems to be everywhere – can you give us some background information about it, please?

Respiratory syncytial virus (RSV) is so called because it causes fusion of cells into large, multinucleated cells in cell culture and in the respiratory tract (which is the meaning of the word syncytial). There are several publications on the role fusion of infected cells plays in the disease development, although it is not obvious that this is the main reason for the complications that arise from an RSV infection. RSV was discovered in 1956, when it was isolated from chimpanzees with respiratory tract infection and in 1957 from children with respiratory disease. Vaccines were rapidly developed because RSV was recognized as a cause of severe disease in young children, especially newborns, as well as the frail elderly. The focus has mainly been on newborns because of the devastating impact of infection, which can lead to death. Currently, it’s an important cause of death in low-income countries. The main reason for that is because treatment options are mainly supportive and are not always available quickly enough in low- and middle-income countries, especially in rural areas. In high-income countries, if a baby has severe shortness of breath, then it’s quickly brought to hospital and given oxygen and respiration support that helps to prevent death from RSV infection. However, the burden, of course, for the patient – the baby – and the parents is high.

Hence, quickly, in the 1960s already, vaccines were developed against RSV. The main problem, however, was that with the first trials in the USA subsequent infections with RSV were worse than without being vaccinated: 80% of vaccinated patients ending up in hospital compared to 5% controls who were unvaccinated. This has held up vaccine development for decades and only in the last few years, 2023 and 2024, vaccines for active immunization (vaccination) have become available for use. Those are for maternal vaccination to prevent the infection in the newborn, and also for vaccination of the elderly and immunocompromised adults. However, for passive immunization (immunization), a monoclonal antibody became available in 1998 for pre-term babies, which has to be given every month and is very expensive. Very recently, available in 2022, a new, longer lasting monoclonal antibody has been developed, so the newborn receives one injection and then is protected against RSV for the whole season (usually winter).

So, one reason for the increased interest in RSV comes from having those vaccines and monoclonal antibodies available in order to have a very good estimate of the burden of disease, so that the impact of the mass use of immunization could be assessed.

A second reason for interest in RSV burden has to do with the seasonality of RSV outbreaks [1–3]. Before the Covid-19 pandemic [caused by the severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2)], the RSV outbreaks fairly consistently coincided with winter in the northern hemisphere (October to March, peaking in December and January) and the southern hemisphere, as well as more or less year round in the (sub)tropics. During the Covid-19 pandemic, RSV pretty much disappeared worldwide. However, when RSV began to reappear following the release of Covid-19 lockdowns, the specific pattern of circulation was completely disturbed, but seems to have now returned to pre-Covid-19 pandemic patterns. It is important to understand the timing of RVS circulation in order to plan the timing of immunization for children and vaccination for the frail elderly, but especially for maternal vaccination to protect babies who are going to be born during the RSV season. If RSV is present throughout the year, then a different vaccination and immunization plan is needed.

A third reason for increased interest in RSV detection was caused by having greater availability of the molecular point-of-care triple test for SARS-CoV-2, influenza virus and RSV. These triple tests are being used when a patient enters the emergency department or becomes hospitalized with respiratory disease during the winter season and it is necessary to determine what the patient is suffering from to put them in the right place for treatment and also to prevent nosocomial transmission.

Lateral flow assays are easy to use for self-testing but not as sensitive as molecular point-of-care tests (Adobe Stock.com)

Are there any therapies available for treatment of vulnerable patients?

Treatment is to prevent and counter the bronchiolitis that is the main cause of the problems from serious RSV infection. If this is not done in a timely manner, damage to the lungs will result, and this is especially the case in low-income countries. Additionally, the longer the duration of the RSV infection, the higher the risk of pneumonia caused by the virus itself or bacterial superinfection.

Anti-viral drugs

There is one anti-viral that has been approved to treat RSV infection [by the European Medicines Agency (EMA) and also by the U.S. Food and Drug Administration (FDA)], ribavirin, which is a broad spectrum anti-viral. Ribavirin has not been used often to help treat RSV infection and when it is used it’s usually for immunocompromised patients, especially those with hematologic malignancies or bone marrow transplants.

Supportive treatment

The main therapy for serious RSV infection is supportive: primarily respiratory support with oxygen – particularly for newborns and very young children as they quickly become exhausted and unable to breathe anymore. Maintaining hydration is also necessary, as when little children stop drinking they quickly become dehydrated. Additionally, treatment to reduce fever can be given.

Vaccination and immunization

The interest in vaccination and immunization is driven because the respiratory support therapy at that level is quite invasive. Active immunization can be achieved with vaccination, for example, AREXVY® (GSK) and mRESVIA® (Moderna) for (older) adults and ABRYSVO® (Pfizer) for (older) adults and pregnant individuals. Also, as mentioned above, passive immunization for pre-term babies and newborns can be achieved with injection of monoclonal anti-bodies; SYNAGIS® (AstraZeneca), BEYFORTUS® (AstraZeneca, Sanofi) and ENFLONSIA® (Merck). The latter two are long lasting monoclonal antibodies. The following link to the PATH charity provides a frequently updated landscape of RSV and associated vaccination and immunization trials and market approved options: https://www.path.org/our-impact/resources/rsv-vaccine-and-mab-snapshot/.

Attention is also turning to human metapneumovirus (hMPV), the little sibling of RSV, which was discovered decades later and causes a similar pattern of infection in children a little older than those mostly affected by RSV. Development of combination vaccines, i.e. several vaccines that create immunity to RSV and hMPV, are already underway.

How is it detected?

Detection or identification of RSV is not really needed for clinical diagnostic/therapeutic purposes, as GPs and especially pediatricians say that, often, if a child enters the clinic or hospital during the RSV season, they see the symptoms and that’s already enough to know this is an RSV infection and they can begin supportive treatment. However, for a definitive laboratory-confirmed diagnosis, detection of RSV or the immune response against RSV is needed.

From the laboratory side, virus isolation was the first technique used for laboratory diagnosis, which moved to molecular techniques. Antigen detection can also be used to identify RSV, but antigen detection, often in the form of lateral flow assays, are less sensitive compared to molecular detection of the RSV genetic material.

Respiratory support is used to treat a baby with RSV infection
(Adobe Stock.com)

Targeted molecular assays

Molecular detection has also improved a lot, especially the molecular point-of-care tests where it is now possible within 15 minutes or half an hour to have a very sensitive result about whether a child or an adult has an acute RSV infection. These tests often use nasal secretions or nasal or throat swab material. A lot of studies have looked into detection of RSV in adults in particular, as they have a lower viral load compared to children. This is because adults have usually developed immunity that can to a certain extent protect against RSV, but it is possible to become reinfected with RSV several times during a lifetime. The reduced viral load means that false-negative results can arise if specimen collection for testing does not happen early after infection in the acute phase of disease. For studies using serology, a second specimen collected after 14 days is often needed to be sure about the diagnosis of a recent RSV infection. Therefore, serology is excellent for epidemiology studies in the elderly, to analyse the burden of disease and how many people are infected. For molecular detection, sputum or a lower respiratory tract specimen would be better but you don’t generally collect a bronchoalveolar lavage sample from a patient when they come to the GP – you need to use a convenient specimen and that’s usually a naso- or oropharyngeal swab, or a nasal aspirate. A molecular assay done using these samples can detect an RSV infection with really good sensitivity.

These techniques are often PCR based, but there are other methods. For example, several of the molecular point-of-care tests use isothermal amplification, which is far less expensive than the equipment (thermal cycler, etc.) needed for PCR. This means that for a large GP practice, the costs of isothermal-amplification based molecular testing could be comparable to lateral flow antigen assays which are less sensitive. Hence, the more sensitive molecular point-of-care tests could be the method of choice. Currently, this technology is not used for self-testing, but manufacturers are trying to develop lateral flow assays that incorporate isothermal amplification techniques – which would be the ideal – and we may see these in the future.

Untargeted sequencing approaches

Nowadays, whole genome next generation sequencing targeted at RSV is widely used. Untargeted sequencing approaches with metagenomics are still quite expensive but are very useful for surveillance, in addition to diagnostics, because then you can detect all the pathogens present, not just the ones that you are specifically looking for, and obtain their genomic information simultaneously. Studies of this nature have already been done – for example, we have also done such a study with the Leiden University Medical Center [4]. This additional analysis on the genomic material is useful because we want to assess the effect of the increasingly wide use of vaccination and immunization on the evolution of the virus and particularly on the development and emergence of escape variants.

Table top PCR-based molecular testing instrument (Cepheid GeneXpert® 4) This device uses the self-contained test cartridge (placed on the left top of the machine for illustration). The cartridge contains all the nucleic acid extraction reagents and the nucleic acid amplification reagents. The sample is pipetted in the cartridge with a simple sample transfer pipette. The cartridge is inserted in the machine, the start button pushed and the operator simply has to wait for the results to be displayed on the machine screen in less than 1 hour. (Photo credit: A. Meijer)

How accurate are the assays, are they affected by viral strain evolution?

In-house versus commercially-available tests
In our recent paper [5], where we focus on External Quality Assessment (EQA) data generated after the return of RSV following the Covid-19 pandemic, around a quarter to one third of all of the data sets were generated by laboratories using tests developed in-house and the rest using commercially available tests. This has changed over the last 20 to 15 years, where analysis was mostly done by in-house testing, and the move has been driven by the need to become compliant with the EU In Vitro Diagnostics Regulations (IVDR) by 2027. This means that new tests that are brought onto the market should already be IVDR compliant but there is still some time for older, existing methods to move to being completely IVDR compliant. Laboratories have to move away from in-house testing or they need to have very good reasons for keeping in-house testing. For example, we are having a lot of discussions with the EU Commission and bodies responsible for the IVDR about why it is important for surveillance activities, especially the emergence of new strains, to have in-house testing available which can quickly be adapted if a test fails due to the evolution of the virus and the target site on the gene modifies in a way that that hampers the performance of the test. An in-house test can be change very quickly, but for a manufacturer it can be a challenge to keep up to date when it takes around 6 months to adapt an assay and have it fully validated again to comply with the IVDR and CE marking.

However, several of those commercial assays have now also implemented more than one target gene into their assay so that if testing for one of the target genes fails there is a backup. Additionally, multiplexed packages of tests are becoming more available commercially (for example, a respiratory virus package), which allows several viruses to be tested for at the same time. There is already even a package that tests for 31 pathogens in one array, so there are big drivers for moving from in-house to commercially available tests.

Test performance

In our recent paper, we did a broad analysis of the EQA data for RSV using data from Quality Control for Molecular Diagnostics (QCMD; https://www.qcmd.org/en/) [5]. Usually, with EQA, you want to use the same virus and concentration in a sample in multiple rounds of assessment to be able to properly compare performance. However, this approach has one important drawback in that if you have a virus emerging with a mutation which has an impact on the performance of a certain test, that effect won’t be seen immediately in your EQA because that specific virus has specially to be added in to your EQA. That takes time. This has been recognized by the World Health Organization (WHO) initiative for RSV EQA, which has just completed the fourth round of testing. The WHO initiative is focusing more on reference laboratories which should be capable of detecting the most recent variants of the virus and characterize them genomically, rather than primary diagnostic labs where you just want to ensure that they can correctly detect RSV. This brings the challenge of needing to adapt the EQA but also wanting to restrict the changes to a minimum, otherwise it’s difficult to compare laboratory performance through the different rounds of EQA testing. In our study, we saw a small downward blip in performance in 2021, probably because of the emergence of new, triple tests on the market that came about because of the Covid-19 pandemic, but then we saw performance was restored to the usual high level in subsequent years. The value of the EQA and the objective of QCMD is to notice these observations and to feed the results back to the manufacturers so that they have the opportunity to react and give an explanation or to modify their tests. So, the EQA process is important in two ways – it allows ISO accreditation for the diagnostic labs but it also helps manufacturers to shape their tests and to fit with the current evolution of the viruses.

How can detection of RSV be improved?

I think that we have already touched on some of those elements, but, to summarize, I think future improvement lies in the development of the molecular point-of-care tests. These are becoming more available and are particularly useful when they combine detection of several pathogens, such as the triple tests mentioned above. I can imagine that fairly soon we will have quadruple tests, with the inclusion of hMPV. I think the EQA already shows a consistently high performance for these tests but some need a little tweaking to increase sensitivity. This is particularly important for the analysis of a suboptimal specimen, which has been collected late in the infection or when diagnosing the institutionalized elderly where the specimen is not as ‘clean’ as compared to a newborn child or an adult who maintains good nose and throat hygiene.

For the antigen detection tests, it would be good to improve the sensitivity to a level comparable to that for the SARS-CoV-2 tests. Then, these tests may also become a tool for surveillance activities and possible outbreak investigation. It’s not possible really to improve on the type of specimen. It’s not possible to have a lower respiratory tract specimen for every patient, but as long as the timing of collection is good the current practice of a nasal aspirate or the combination of nasopharyngeal and oropharyngeal swabs provides good sensitivity.

Lastly, for answering certain questions, such as estimating immune-escape variants, increased availability of being able to simultaneously detect the virus and obtain genomic information will be very valuable. These metagenomic techniques are very promising but still expensive and take time to generate results, as well as needing highly trained personnel for the complex bioinformatics that is required to process the obtained massive amount of genetic information for a final result. It will be very beneficial if this technique can become more widely available and less expensive.

 References
1. Obando-Pacheco P, Justicia-Grande AJ, Rivero-Calle I et al. Respiratory syncytial virus seasonality: a global overview. J Infect Dis 2018;217(9):1356–1364 (https://doi.org/10.1093/infdis/jiy056).
2. Shan S, Zhang W, Gao H et al. Global seasonal activities of respiratory syncytial virus before the Coronavirus disease 2019 pandemic: a systematic review. Open Forum Infect Dis. 2024 Apr 25;11(5):ofae238 (https://doi.org/10.1093/ofid/ofae238).
3. Thindwa D, Li K, Cooper-Wootton D et al. Global patterns of rebound to normal RSV dynamics following COVID-19 suppression. BMC Infect Di 2024;24(1):635 (https://doi.org/10.1186/s12879-024-09509-4).
4. Mourik K, Sidorov I, Meijers E, van den Brink S, Bos S, Aarts L, Kuttiyarthu Veetil N, Boers S, Eggink D, Meijer A, de Vries J. Viral metagenomic sentinel surveillance of acute respiratory infections in primary care as a public health strategy: a prospective national pilot study. Presented at 27th European Society for Clinical Virology (ESCV) Annual Meeting, September 2025, Thessaloniki, Greece.
5. Presser LD, Yousef A, McCulloch E et al. Evaluation of molecular detection for respiratory syncytial viruses in World Health Organization Europe region laboratories, 2020-2023. J Clin Virol 2025;179:105832 (https://doi.org/10.1016/j.jcv.2025.105832).

The interviewee

Adam Meijer PhD, Senior scientist respiratory viruses; Head RIVM location of the Dutch National Influenza Centre; Head of the National Reference Laboratory for Respiratory Viruses

Center for Infectious Disease Control, National Institute for Public Health and the Environment, 3720 BA Bilthoven, the Netherlands

Email: adam.meijer@rivm.nl