Serum from JN.1 mRNA vaccine recipients cross-neutralizes BA.3.2.2; sipavibart retains neutralizing activity
Background
Cross-neutralization of the SARS-CoV-2 BA.3.2.2 variant was observed in serum from recipients of a JN.1-based messenger RNA (mRNA) vaccine. Neutralizing antibody titers were lower than against JN.1, however, and this study did not establish protection against infection or treatment efficacy.
The researchers said BA.3.2.2 had emerged as the predominant sublineage within the BA.3.2 lineage worldwide in late 2025. The study examined the antigenic relationship between this variant and immunity induced by a JN.1 vaccine. Its description as the predominant sublineage within BA.3.2 should not be interpreted as meaning it was the predominant SARS-CoV-2 variant overall or had been officially designated a variant of concern.
Key findings
The researchers analyzed neutralizing antibody responses to eight variant viruses in serum from 25 recipients of a JN.1 mRNA vaccine. Participants were grouped according to whether they tested negative or positive for antibodies against the nucleocapsid (N) protein. The provided abstract does not describe how prior infection was established, so this classification alone cannot determine whether participants had previously been infected.
After vaccination, neutralizing antibody titers against all tested variants rose significantly in both groups. Cross-neutralization of BA.3.2.2 was detected in both groups, but titers were lower than those against JN.1. The abstract does not provide specific titers or fold reductions. It is therefore not possible to determine how much protection these antibody responses might provide in people.
Antigenic cartography placed BA.3.2.2 antigenically apart from all the tested JN.1-descendant variants. Thus, cross-neutralization was observed in serum from JN.1 vaccine recipients even as the antigenic map distinguished BA.3.2.2 from the JN.1 descendants.
Monoclonal antibody neutralizing activity varied by variant. Sipavibart (AZD3152) retained strong neutralizing activity against BA.3.2.2 but completely lost activity against the tested JN.1-descendant variants carrying F456L. By contrast, pemivibart (VYD222) and the SA55 antibody were reported to retain broad neutralizing activity. These comparisons assessed in vitro neutralizing activity; clinical superiority in patients was not established.
The researchers noted that BA.3.2.2 retains wild-type phenylalanine (F) at position 456 of the spike protein. They interpret this feature as linked to preservation of epitopes, the sites recognized by class 1 and 2 antibodies, providing a mechanistic basis for cross-neutralization. The provided abstract does not support extending this interpretation to claim that every relevant epitope remained unchanged or that antibody-binding affinity was directly confirmed.
Significance and outlook
The study shows both cross-neutralization in serum from vaccinated people and differences in activity among monoclonal antibodies. The researchers suggested that the potential use of sipavibart could be explored if BA.3.2.2 spreads more widely, while explicitly stating that clinical confirmation is needed.
Whether in vitro neutralization translates into protection in people or treatment benefit for patients remains to be established in follow-up clinical studies. The provided abstract also omits group sizes, blood-draw timing, specific assay conditions, and quantitative titers, limiting assessment of the magnitude and durability of the responses.
The study population and main findings are available in the Europe PMC bibliographic record and abstract.
The SARS-CoV-2 BA.3.2.2 sublineage has emerged globally as the dominant branch of BA.3.2 by late 2025, yet its antigenic relationship with JN.1 vaccine-induced immunity remains unclear. We evaluated neutralizing antibody responses in 25 JN.1 mRNA vaccinees against eight variants, stratified by anti-nucleocapsid antibody serostatus. Post-vaccination titers increased significantly against all variants in both N antibody-negative and -positive groups. Cross-neutralization against BA.3.2.2 was detected in both groups despite lower titers compared to JN.1. Antigenic cartography revealed that BA.3.2.2 was antigenically isolated from all JN.1-descendant variants. AZD3152/sipavibart retained potent neutralization against BA.3.2.2 but completely lost activity against all F456L-harboring JN.1-descendant variants, while VYD222/pemivibart and SA55 maintained broad activity. Retention of wild-type F456 in BA.3.2.2 preserves class 1/2 antibody epitopes, providing a mechanistic basis for cross-neutralization and suggesting a potential therapeutic window for sipavibart should BA.3.2.2 expand globally, pending clinical confirmation.
These results provide research evidence connecting variant surveillance with the evaluation of antibody candidates. For example, if BA.3.2.2 spreads further, the observed neutralizing activity of sipavibart could help identify candidates for follow-up clinical testing. The finding that the same antibody lost activity against the tested F456L-bearing JN.1 descendants also needs to be considered. However, no data were provided to support immediate prescribing based on an individual patient's viral sequence or efficacy in any specific high-risk group. Cross-neutralization in serum from vaccinated people also does not directly demonstrate prevention of infection or the duration of protection.