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Studying the Effect of Nirsevimab Immunization on Antibody Development After RSV Infection in Healthy Infants

Verified siteRegistered drugNo placebo
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What is this trial about?

A plain-language summary of the goals, design and what participants do

This study focuses on Respiratory Syncytial Virus (RSV) infections in infants. RSV is a common virus that can cause serious respiratory diseases in babies. The study will examine how nirsevimab immunization affects an infant's immune system response to RSV infection. Nirsevimab is a medication that provides protection against RSV by giving babies ready-made antibodies that can fight the virus.

The purpose of the research is to determine how nirsevimab affects the development of a baby's own natural immune response (antibody levels) after encountering RSV. The study will involve two groups: a nirsevimab group of infants aged 0-3 months who receive the immunization, and a control group of children around 12-15 months of age who do not receive nirsevimab.

Participants will have blood samples collected at various timepoints to measure RSV-specific antibodies in their blood. These measurements will be taken before immunization and then at 2, 4, 6, and 12 months afterward to track how the immune system responds over time. Parents will also complete questionnaires about their child's health and development throughout the study period.

The research process

The trial runs in 5 steps – from screening to follow-up. Each step says what happens and what the team monitors.

  1. Step 1

    Initial immunization

    You will receive the nirsevimab immunization (brand name Beyfortus) as a single injection. This medication helps protect against respiratory syncytial virus (RSV), which can cause serious lung infections.

    The dose will be either 50 mg or 100 mg depending on your infant's weight, given as a solution for injection in a pre-filled syringe.

    Before receiving the immunization, a blood sample will be collected to measure your infant's initial RSV antibody levels.

  2. Step 2

    2-month follow-up visit

    Two months after receiving the nirsevimab immunization, you will return for a follow-up visit.

    During this visit, a blood sample will be collected to measure your infant's RSV-specific antibody levels.

    You may be asked to complete a questionnaire about your infant's age and weight.

  3. Step 3

    4-month follow-up visit

    Four months after the immunization, you will attend another follow-up appointment.

    A blood sample will be collected to measure RSV-specific antibody levels.

    The healthcare team will continue monitoring your infant's response to the immunization.

  4. Step 4

    6-month follow-up visit

    Six months after receiving nirsevimab, you will return for another follow-up visit.

    A blood sample will be collected to measure RSV-specific antibody levels.

    The healthcare team will assess how the antibody levels have changed over time.

  5. Step 5

    12-month final assessment

    The final study visit will take place 12 months after the initial immunization.

    A final blood sample will be collected to measure RSV-specific antibody levels.

    The healthcare team will evaluate the long-term effectiveness of the nirsevimab immunization by measuring various types of antibodies, including RSV-specific antibody neutralization and functionality.

Who can join the trial?

7 criteria

  • Infants must be receiving nirsevimab immunization (a medication that helps prevent serious diseases caused by RSV infection) or be in the control group
  • For the nirsevimab group: Babies must be born before or at the start of RSV season and be 0-3 months old when receiving the immunization
  • For the control group: Children must be around 12-15 months of age at the start of RSV season (September/October)
  • For the nirsevimab group: Parents or legal representatives must have decided that their infant will receive the nirsevimab immunization
  • Children must be healthy according to the standard health criteria used in well baby clinics (minor issues like slight temperature increases or colds are still considered normal health)
  • Parents or legal representatives must agree to their child's participation in the study
  • Parents or legal representatives must provide signed informed consent after receiving both oral and written information about the study

Who cannot join the trial?

10 criteria

  • You cannot participate if you have a diagnosed or suspected congenital (present at birth) heart defect
  • You cannot participate if you have a chronic lung disease, such as conditions affecting your lungs over a long period
  • You cannot participate if you have any condition affecting the immune system (the body's defense against infections)
  • You cannot participate if you have any allergies to the study medication ingredients
  • You cannot participate if you have received blood products (like blood transfusions) or immune globulins (antibody treatments) in the past 90 days
  • You cannot participate if you have been previously infected with RSV (Respiratory Syncytial Virus)
  • You cannot participate if you received or plan to receive any RSV vaccine other than nirsevimab
  • You cannot participate if you have received or plan to receive any other vaccine within 14 days before or after the study medication
  • You cannot participate if you have previously participated in any other RSV vaccine study
  • You cannot participate if you have any serious medical condition that would interfere with the study or pose a risk to your health
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Investigated drugs

Nirsevimab is a medication that helps protect infants against respiratory syncytial virus (RSV). It contains antibodies that specifically target RSV, which is a common virus that can cause respiratory infections. Nirsevimab works by preventing the virus from entering cells in the respiratory system, which helps reduce the severity of RSV infections or prevent them altogether.

What is already known about the treatment

Nirsevimab - Nirsevimab is a long-acting monoclonal antibody administered as a single intramuscular injection that provides protection against respiratory syncytial virus (RSV) infection in infants. It works by specifically targeting the RSV F protein, preventing the virus from fusing with host cell membranes and blocking viral entry into respiratory cells. Nirsevimab has demonstrated effectiveness in clinical trials for preventing RSV-related lower respiratory tract infections and hospitalizations in infants, and it has received regulatory approval in several countries for use during RSV seasons. Unlike older preventive options, nirsevimab offers the advantage of season-long protection with a single dose and does not require monthly administrations.

Investigated diseases

Respiratory Syncytial Virus (RSV) Infection – A viral respiratory illness that affects the lungs and breathing passages. RSV typically causes mild, cold-like symptoms in healthy adults and older children, but can lead to more severe lower respiratory tract infections in infants, young children, and elderly individuals. The virus spreads through respiratory droplets when an infected person coughs or sneezes, or through direct contact with contaminated surfaces. Symptoms typically begin with a runny nose, decreased appetite, and cough, sometimes progressing to wheezing, difficulty breathing, and bronchiolitis or pneumonia in severe cases. RSV infections commonly occur during fall, winter, and spring, with most children experiencing at least one RSV infection by age two.
Trial detailsLast updated 2 Oct 2026
Age0-17PhasePhase IIITrial ID2025-521801-42-00Protocol codeIIV-724Estimated enrolment250 patientsSponsorRijksinstituut voor Volksgezondheid en Milieu (RIVM)

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