In September 2023, in the Autonomous Community of Madrid, Spain, less than 15 days before the introduction of immunization with nirsevimab,1 there was no official document to inform parents who had to decide whether or not to immunize their newborn children. Furthermore, in the developed world, there has been a worrying rise in anti-vaccination movements for more than two decades.2,3 It was in this context that we undertook the present study to explore the potential hesitancy of parents in regard to immunization with nirsevimab in a public secondary care hospital, analyzing potential associated factors that could influence the decision.

We conducted a prospective observational descriptive study by means of a voluntary survey of parents of newborns between October 1, 2023 and March 31, 2024. We estimated that obtaining data for more than 85% of infants born during the study period would achieve a sufficient sample size with an error of less than 1%, a level of heterogeneity of 50% and a 95% confidence level. The statistical analysis was performed with the software SPSS version 29.0.

During the study period, 312 infants were born, of who 41 were excluded: 30 because the parents refused to participate, two because they required transfer to another hospital, seven because they were twins and a single response was obtained per pair of twins and 2 due to a language barrier. We analyzed data from 271 parental responses (86.9% of newborns), 6 corresponding to single-parent families.

Of the total sample, 92.3% of parents chose to administer nirsevimab, and the main reason was, most frequently, the recommendation of the neonatal care team (85%). Only 38% reported having received sufficient information about it before the child was born.

When we analyzed the differences between those who decided to immunize and those who decided against it, we found that parents with a university education were more likely to opt to immunize their offspring (mothers: P<.05, OR [95% CI]=2.47 [1.00–6.17]; fathers: P<.01, OR [95% CI]=3.30 [1.32–9.00]) and, in relation to geographical origin, parents from Morocco were more likely to exhibit hesitancy (mothers, P<.05; fathers, P<.001). Thirty-two percent of participating families reported a history of bronchiolitis in a previous child, and these families were more likely to agree to immunization (P<.05; OR [95% CI], 3.95 [1.06–25.7]). With regard to the use of social media as a source of health information, 48.7% used social media frequently or continuously, and these parents exhibited greater hesitancy (P=.012; OR [95% CI], 0.2 [0.06–0.69]). Of those who reported hesitancy, 2.5% expressed resistance to the future administration of other vaccines (P<.001; OR [95% CI], 32.1 [5.38–217]). The most frequent reasons for hesitancy were fear of side effects (81%), lack of information (75%) and the belief that nirsevimab is an experimental drug (56%) (Tables 1 and 2).

Studies performed4–6 a year after the introduction of immunization with nirsevimab have focused on assessing the effectiveness, impact and decrease in medical visits and hospitalization due to bronchiolitis, with very encouraging results. In Spain, high immunization coverage rates were achieved during the 2023–2024 season,4 with a mean coverage rate of 92% in infants born during the season and 88% in infants born before season onset. The overall coverage rate for the Autonomous Community of Madrid was 86%.6

The main limitation of the study is a direct consequence of the chosen methodology, as we relied on parental responses without verifying the provided information. Furthermore, in 29 cases we were unable to explore the reasons why parents decided against immunizing their children. The main strength of the study is that it is the first study in Spain analyzing the reasons why parents are reluctant to have their children immunized with nirsevimab.

Identifying factors that may affect the administration of nirsevimab is important in order to maintain or even increase coverage. To our knowledge, no studies published to date have analyzed the potential reasons for hesitancy in the administration of nirsevimab. According to our findings, the factors associated with hesitancy were not having a university education, Moroccan origin, a history of bronchiolitis in a previous child (regardless of severity), “vaccine phobia” and the use of social media as one of the main sources of health information before childbirth.