Discussion
This report is the first analysis conducted at CDC to provide population-based estimates of the percentage of infants who were immunized against RSV in numerous U.S. states and to analyze the timing of immunization. IIS data from 33 states and DC found that during the first season after approval of nirsevimab for infants and an RSV vaccine for pregnant women, 28.9% of infants born during October 2023–March 2024 were immunized against RSV, either through receipt of nirsevimab or through maternal RSV vaccination. The percentages of infants covered varied widely by state, from 10.8% to 53.1%. Only 38.1% of infants who received nirsevimab received the antibodies within the first week of life (0–6 days after birth), the optimal timing for maximum protection.§§
An analysis of data from an internet panel survey conducted by CDC during March 26–April 11, 2024, estimated that approximately double the percentage (55.8%) of U.S. infants received protection against RSV through maternal RSV vaccination and nirsevimab during the 2023–24 season as the percentage estimated in this report (8). Differences between the findings of that study and those in this report might be due to an overestimation of coverage in the survey (from self-selection bias, recall bias, or limited sample size) or an underestimation in IIS-based estimates of infant protection in some states because of jurisdictional policies and variation in reporting by immunization providers that might limit the capture of all immunizations administered.¶¶ Some RSV immunization providers, including birthing hospitals (those with more than one birth within the previous year or at least one registered maternity bed) and outpatient obstetric health care providers who administer immunizations less frequently, might not participate in their jurisdictional IIS, which might limit the data that were used in this report. Several other recent studies (4–6,9) also reported higher immunization coverage estimates than those in this report; however, most of those analyses were from a single institution or health care network with targeted interventions to promote RSV immunization. A population-based study in Wisconsin using IIS data reported coverage estimates similar to those in this report (36.2%) (7).
Several factors might have contributed to low RSV immunization coverage during the 2023–24 respiratory virus season. First, although nirsevimab was recommended in August 2023, supply issues limited its availability in some areas, particularly at the beginning of the season (10). Second, lack of familiarity among patients and providers about nirsevimab for infants and maternal RSV vaccine, as well as the complexity of the related pediatric and maternal recommendations, might have contributed to limited acceptance or delayed administration. Third, cost concerns might have also played a role. Private health insurers are allowed a 1-year grace period before they are required to cover ACIP-recommended vaccines under the Affordable Care Act.*** In addition, because nirsevimab might have been too expensive for some hospitals to include as part of routine newborn care, they might have opted not to stock it. Finally, this report only included immunizations administered during the ACIP-recommended months for most geographic areas (2,3); some doses might have been administered outside those months. Preliminary data from the 2024–25 respiratory virus season suggest RSV immunization coverage among infants aged <8 months through maternal vaccination or nirsevimab increased to 57% nationally.††† Receipt of nirsevimab among infants during the first week of life was more common toward the end of the season, potentially indicating improvements in supply, administration in birthing hospitals, and increased familiarity with nirsevimab use, among other factors. Continued monitoring of when infants receive nirsevimab will be important during the upcoming respiratory virus season. Nirsevimab coverage by birth month decreased across the season, which might reflect shorter duration of eligibility to receive nirsevimab or increased opportunities for maternal vaccination among infants born later in the season.
The findings in this report indicate wide variation in both nirsevimab and maternal RSV vaccine use across U.S. states and DC. Differences in age at receipt of nirsevimab were also found according to infants’ VFC eligibility status, with VFC-eligible infants less likely to receive nirsevimab within 3 days of birth, the period in which most infants are likely to be in a hospital after their birth. This might be an indication of a limited number of birthing hospitals being accredited as VFC providers. Despite ACIP approval occurring shortly before the respiratory virus season and challenges around nirsevimab supply and insurance reimbursement, in six states and DC in this analysis, ≥40% of infants were covered by one of the RSV immunization products. Another monoclonal antibody (clesrovimab) was also recently approved for use by the Food and Drug Administration and recommended by ACIP for infants aged <8 months who are not immunized through maternal vaccination, allowing additional options to provide infant immunization against RSV in upcoming seasons.§§§ This baseline information from the first season after RSV immunization product approval can be used by pediatric and obstetric health care providers and public health professionals to guide focused immunization strategies for future respiratory virus seasons.
Limitations
The findings in this report are subject to at least seven limitations. First, although adult women who received RSV vaccine (Abrysvo) were assumed to be pregnant, IIS data do not include pregnancy status. Therefore, inclusion of vaccinated women who were not pregnant would have resulted in an overestimation of coverage. Second, because maternal and infant records could not be linked through the deidentified IIS data reported to CDC, determining whether some infants received immunization coverage both through nirsevimab and maternal vaccination was not possible. The proportion of infants receiving immunization coverage was estimated by summing the total number of adult women who received RSV vaccine and the number of infants who received nirsevimab then dividing the total by the total number of live births. This might have led to overestimation of the numerator for infant coverage if a mother and infant collectively were immunized through both methods; however, this is only recommended in rare instances and has not been reported frequently in other studies (5,7,9). Third, IIS data do not identify multiple pregnancies (which account for approximately 3% of live births¶¶¶) or stillbirths (approximately 0.6% of births****). Thus, IIS dose data might not fully align with the denominator of live births by state. Fourth, although infants born to mothers aged <18 and >49 years were not included in the denominator for calculations of infant coverage, nirsevimab doses received by these infants could not be identified and removed from the numerator. This might have resulted in an overestimate of the percentage of infants covered; however, this is likely to represent a small number because births to adolescents and women aged ≥50 years represent a small proportion of total live births. Fifth, each jurisdiction’s IIS data might not include all doses administered because of consent and provider reporting policies, particularly among adult populations. This might have led to an underestimation of numerators for estimates of infant coverage. Sixth, 22% of infant records were missing VFC eligibility status, limiting the ability to fully interpret those data. Finally, because these results are from 33 states and DC, they might not be generalizable to the entire U.S. population.
Implications for Public Health Practice
Additional efforts are needed to increase infant protection against severe RSV through maternal or infant immunization. Continued work is also needed to increase birthing hospital enrollment in VFC and improve obstetric provider reporting of immunizations to the IIS. IISs provide timely, population-based data that can be used to estimate state-level infant RSV immunization coverage and monitor trends. Preliminary data from the 2024–25 season suggest increases in RSV immunization coverage, and the recent ACIP recommendation for an additional monoclonal antibody, clesrovimab, could increase access to RSV protection for infants in the 2025–26 respiratory virus season.††††