At IDWeek 2025, (the joint annual meeting of the Infectious Diseases Society of America, the Society for Healthcare Epidemiology of America, the HIV Medicine Association, the Pediatric Infectious Diseases Society and the Society of Infectious Diseases Pharmacists) held from 19 to 21 October in Atlanta, Georgia, Dr Avika Dixit, a director in clinical development at Moderna Inc, presented findings from a Phase III open-label trial that evaluated the safety and immunogenicity of Moderna’s mRNA-1273.815 (Spikevax XBB.1.5) in Covid-19 vaccine-naïve children aged two to four.
Due to infection and/or immunisation, SARS-CoV-2 seroprevalence rates in children aged 2 to 4 have increased and are now comparable to those aged five and older, and these younger children may benefit from a single-dose vaccine regimen currently recommended for those five and above. The data presented at IDWeek 2025 could influence prescribing guidelines and protect a younger cohort.
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Data from October 2022 to March 2025 showed that within the paediatric age group that had access to vaccination, the cumulative rates of Covid-19-associated hospitalisations were highest in those aged from six months to four years. During this time, the recommended dosing for mRNA-1273 under emergency use authorisation was a two-dose series in previously unvaccinated children, based on proven safety and efficacy data in a predominantly seronegative population. However, as SARS-CoV-2 has become endemic, a significant proportion of children have acquired immunity through infection or vaccination. However, infants and toddlers are less likely to have pre-existing immunity than older children. These age-related differences emphasise the need to vary dosing strategies by age.
This study (NCT05436834) assessed a single dose of mRNA-1273.815 in children aged two to four versus a two-dose series in toddlers aged from six to 23 months. Healthy vaccine-naïve participants were enrolled into two aged-based cohorts. Cohort A consisted of 199 children aged two to four who received a single dose of the vaccine. Cohort B consisted of 399 six-to-23-month-olds who received two doses of the vaccine. Blood samples for immunogenicity were collected 28 days after dosing, and participants were followed for three months after their last vaccination. The primary objectives were to evaluate the safety and reactogenicity in both cohorts, as well as to assess the immunogenicity of a single dose of the vaccine in cohort A participants with prior infection versus cohort B participants without prior infection.
mRNA-1273.815 was shown to be safe and well tolerated. Most solicited adverse reactions in both cohorts were mild or grade 1 in severity. Nasopharyngitis was the most common unsolicited adverse event, although no events were considered to be related to the vaccine. There were no deaths or product discontinuations in this trial. A single dose of the vaccine was as immunogenic in predominantly seropositive children aged between two and four as the two-dose series in predominantly seronegative children aged six to 23 months.
This data therefore supports the use of a single dose of mRNA-1273 Covid-19 vaccine from the age of two onwards, regardless of prior immunisation status. Broadening the eligibility for Omicron-adapted vaccines to include younger children would likely reduce transmission rates and mitigate severe disease burden among vulnerable paediatric populations.
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