In young children, lower respiratory tract infections, such as pneumonia and bronchiolitis, are mainly caused by respiratory syncytial virus (RSV).¹ Nirsevimab is a recombinant human IgG1 kappa monoclonal antibody that interlocks with the RSV fusion (F) protein (binds to F1 and F2 subunits) in its prefusion conformation to prevent viral entry into host cells.² Infants who have received a single intramuscular injection of nirsevimab before the RSV season are protected from RSV-associated lower respiratory tract infection.²

Children with acute lower respiratory infection (ALRI) are most commonly affected by the human RSV.¹ In 2005, ALRI claimed the lives of 55,000 to 1,99,000 children who were under 5 years of age.¹ Studies showed that most RSV-ALRI episodes (93%) and RSV-ALRI mortalities (99%) occurred primarily in the developing countries.¹ Since 2012, there has been tremendous growth in RSV vaccine development.¹ More than 60 candidate RSV vaccines have been used in pregnant women, neonates and young children.¹ In view of this tremendous progress in RSV vaccine production, the Product Development for Vaccines Advisory Committee (PDVAC) of the World Health Organization (WHO) stated RSV as “the most likely big new vaccine area with a vaccine likely to be available in the next 5-10 years”.¹

A recent study showed the implications of using nirsevimab in the healthy late-preterm and term infants.² In this study, infants (N=1,490) who were born at a gestational age of 35 weeks and above were randomly assigned (2:1) to receive 1 dose of nirsevimab or placebo, prior to the RSV season.² The primary and secondary efficacy endpoints were medically attended RSV-associated lower respiratory tract infection (150 days after injection) and hospitalization due to the same circumstances.²

According to the results, 12 out of 994 infants (1.2%) who received nirsevimab, and 25 out of 496 infants (5%) who received placebo showed to have medically attended RSV-associated lower respiratory tract infection.2 These findings demonstrated that nirsevimab had an efficacy of 74.5% (95% CI: 49.6-87.1; p<0.001).² The secondary efficacy point of hospitalization occurred in 6 out of 994 infants (0.6%) and 8 out of 496 infants (1.6%) in the nirsevimab and placebo groups, respectively, with an efficacy of 62.1% (95% CI: -8.6 to 86.8; p=0.07).² Among infants who received nirsevimab and placebo, 67 out of 987 (6.8%) and 36 out of 491 (7.3%) showed serious adverse events, respectively.² Antidrug antibodies (data until day 361) were present in 58 out of 951 infants (6.1%) and 5 out of 473 (1.1%) infants in the nirsevimab and placebo groups, respectively.²

There is an urgent need to protect the late-preterm and term infants from the most common pathogen-RSV.² At present, there are 3 approaches to protect these infants at various stages of clinical development: vaccinating the infants; vaccinating mothers so that the infants acquire passive immunity, and injecting antibodies directly to the infants.² Recent findings from a phase 3 trial have shown that no significant results were attained when mothers were vaccinated with an RSV F protein nanoparticle, in terms of the primary endpoint. The efficacy against medically significant RSV-associated lower respiratory tract infection was 39.4% (97.52% CI: -1.0 to 63.7).²

In conclusion, nirsevimab with an extended half-life is found to be effective in reducing the rate of medically attended RSV-associated lower respiratory tract infection in infants.²