Immunogenicity and safety of LP.8.1 variant-containing mRNA COVID-19 vaccines

The LP.8.1 variant-containing mRNA vaccines (mRNA-1273.251 and mRNA-1283.251) were well tolerated and elicited robust neutralizing antibody responses against the matched LP.8.1 variant and circulating SARS-CoV-2 subvariants in individuals aged 12 and older.

The Gist

Imagine your immune system is a highly trained security team guarding a fortress (your body). For years, this team has been practicing against a specific villain: the SARS-CoV-2 virus. But viruses are tricky; they keep changing their masks and costumes to sneak past the guards.

This paper is a report card on a new training session for that security team. The "trainers" (Moderna scientists) introduced two new types of training manuals (vaccines) designed to teach the guards how to recognize a specific new costume the virus was wearing called LP.8.1.

Here is the breakdown of what happened, explained simply:

1. The New Threat: The "LP.8.1" Disguise

The virus has been evolving. By late 2024 and 2025, a new version called LP.8.1 became very common in the US. It was like the virus putting on a new, slightly different mask. Because the virus was changing, the old training manuals (previous vaccines) weren't as effective. So, scientists created two new "updated manuals" for the 2025–2026 season:

• mRNA-1273.251: A full-length manual that shows the guards the virus's entire body.
• mRNA-1283.251: A "highlight reel" manual that focuses only on the most critical parts of the virus (the face and the hands) to teach the guards faster and with less "ink" (lower dose).

2. The Experiment: The Training Camp

The researchers gathered two groups of people to test these new manuals:

• Group A: Seniors (65+ years old).
• Group B: Younger adults (12–64) who have health conditions that make them more vulnerable if they get sick.

They gave everyone one single dose of either the "Full Manual" or the "Highlight Reel." Then, they waited 29 days to see how well the security team performed.

3. The Results: The Guards Are Ready!

The results were excellent news:

• The Main Target: When the guards were tested against the specific new mask (LP.8.1), they reacted strongly. The "Full Manual" group saw their defense levels jump about 15 to 26 times higher than before. The "Highlight Reel" group saw an even bigger jump, about 36 to 53 times higher!
• The Bonus Skill (Cross-Training): The researchers also tested if the guards could recognize other new masks they hadn't seen before (like XFG, NB.1.8.1, and a tricky one called BA.3.2.2).
  • The guards were surprisingly good at recognizing these, too. It's like teaching a guard to spot a specific type of hat, and they immediately get really good at spotting similar hats they've never seen before.
  • However, one specific new mask (BA.3.2.2) was a bit harder to spot. The computer models predicted this would be the hardest one to catch, but the guards still managed to recognize it well enough to be safe.

4. Safety: No Accidents

The most important part of any training is that it doesn't hurt the trainees.

• The study checked for any bad side effects (like high fevers, allergic reactions, or serious illness).
• Result: Zero serious problems. The few minor complaints (like a toothache or a tingly feeling) were completely unrelated to the vaccine. The training was safe for everyone.

5. The Big Picture: Why This Matters

Think of the virus as a criminal who changes their disguise every few months. If the police (your immune system) only have an old "Wanted" poster, the criminal walks right past them.

This study proves that:

1. Updating the poster works: Giving people the new LP.8.1 vaccine refreshed their memory and made their immune system much stronger against the current version of the virus.
2. It's safe: You can get the update without worrying about getting sick from the vaccine itself.
3. It's a shield against the future: Even though the virus might change again (like the BA.3.2.2 variant), this new training gives the body a strong foundation to fight back, even if the virus tries a slightly different trick.

In short: The new vaccines are like a high-tech, up-to-date security system that successfully trained the body's defenders to spot the latest virus disguises, keeping people safe with zero major side effects.

Technical Summary

1. Problem Statement

The SARS-CoV-2 virus continues to evolve, with the JN.1 lineage driving successive waves of subvariants. By late 2024 and early 2025, the LP.8.1 subvariant (carrying nine additional spike mutations relative to JN.1) became dominant in the United States. Concurrently, new recombinant variants (e.g., XFG) and divergent strains (e.g., NB.1.8.1 and the long-branch BA.3.2.2) emerged, posing risks of immune escape and reduced vaccine efficacy.

The challenge was to rapidly evaluate the immunogenicity and safety of updated mRNA vaccine formulations containing the LP.8.1 antigen in high-risk populations (adults ≥65 years and individuals aged 12–64 with comorbidities) to ensure protection against both the matched strain and emerging circulating variants.

2. Methodology

The study utilized a multi-faceted approach combining clinical trials, fit-for-purpose laboratory assays, and predictive modeling:

• Clinical Studies: Two independent, ongoing, open-label, single-arm Phase 3b/4 trials were conducted in the US:
  • Vaccine A: mRNA-1273.251 (50 µg dose), encoding the full-length prefusion-stabilized LP.8.1 Spike (S) protein.
  • Vaccine B: mRNA-1283.251 (10 µg dose), encoding only the Receptor-Binding Domain (RBD) and N-Terminal Domain (NTD) of the LP.8.1 S protein.
  • Participants: 103 participants in the mRNA-1273.251 arm (median age 64) and 172 in the mRNA-1283.251 arm (median age 59).
  • Timeline: Single-dose administration on Day 1, with assessments through Day 29.
• Immunogenicity Assessments:
  • Primary: Pseudovirus neutralization assays measuring neutralizing antibodies (nAbs) against the vaccine-matched LP.8.1 variant at Day 1 (baseline) and Day 29.
  • Exploratory: A fit-for-purpose assay on a randomly selected subset (n=56) tested cross-neutralization against emerging variants: BA.3.2.2, XFG, and NB.1.8.1.
• Safety Assessments: Monitoring of unsolicited adverse events (AEs), serious AEs (SAEs), AEs leading to withdrawal, and AEs of special interest (AESIs) through Day 29.
• Predictive Modeling: An in silico risk calculator (based on deep mutational scanning and statistical modeling) was used to estimate the relative fold-reduction in neutralization titers (immune escape potential) for emerging variants compared to the vaccine strain.

3. Key Contributions

• Validation of Updated Formulations: Provided the first clinical data confirming that LP.8.1-containing mRNA vaccines (mRNA-1273.251 and mRNA-1283.251) are safe and immunogenic in the 2025–2026 recommended composition.
• Cross-Variant Neutralization Data: Generated empirical data on the ability of these vaccines to neutralize not only the matched LP.8.1 strain but also genetically distinct emerging variants (BA.3.2.2, XFG, NB.1.8.1).
• Predictive Modeling Integration: Correlated clinical neutralization data with in silico immune escape predictions, validating the model's ability to forecast reduced efficacy against specific divergent strains (specifically BA.3.2.2).
• Dose-Response Comparison: Offered comparative immunogenicity data between the full-length S protein vaccine (50 µg) and the RBD/NTD-only vaccine (10 µg), demonstrating robust responses even at the lower dose.

4. Key Results

Immunogenicity (Vaccine-Matched LP.8.1):
Both vaccines elicited robust neutralizing antibody responses by Day 29.

• Geometric Mean Fold Rise (GMFR):
  • mRNA-1273.251: GMFR of 26.3 (ages 12–64) and 15.4 (ages ≥65).
  • mRNA-1283.251: GMFR of 53.0 (ages 12–64) and 36.7 (ages ≥65).
• The lower-dose mRNA-1283.251 demonstrated a higher fold-rise compared to the full-length mRNA-1273.251.

Cross-Neutralization (Emerging Variants):
Both vaccines effectively neutralized circulating variants in the exploratory subset:

• GMTs at Day 29 (mRNA-1273.251): LP.8.1 (1973.3), XFG (952.4), BA.3.2.2 (823.6), NB.1.8.1 (2064.7).
• GMTs at Day 29 (mRNA-1283.251): LP.8.1 (3827.2), XFG (2050.6), BA.3.2.2 (1549.3), NB.1.8.1 (3818.4).
• Trends: The highest titers were observed against NB.1.8.1 and the matched LP.8.1. The lowest titers were observed against BA.3.2.2, consistent with its genetic divergence.

Safety:

• No vaccine-related Serious Adverse Events (SAEs), AESIs, or deaths were reported.
• No participants withdrew due to AEs.
• Reported AEs (paraesthesia, hypersensitivity, tooth abscess) were considered unrelated to the vaccine.

Predictive Modeling:

• The model predicted the highest immune escape risk for BA.3.2.2 (4.58-fold reduction in ID50 titers).
• Moderate risk was predicted for XFG (2.62-fold reduction).
• Minimal risk (1.0 score) was predicted for the matched LP.8.1.
• These predictions aligned with the clinical data showing lower GMTs against BA.3.2.2.

5. Significance

This study confirms that the updated LP.8.1-containing mRNA vaccines are well-tolerated and induce potent, broad-spectrum neutralizing antibody responses in high-risk populations.

• Public Health Impact: The data supports the inclusion of LP.8.1 in the 2025–2026 vaccine composition, ensuring protection against the dominant strain while maintaining cross-protection against other JN.1-lineage variants (XFG, NB.1.8.1).
• Surveillance Importance: The identification of BA.3.2.2 as a variant with the highest predicted immune escape highlights the need for continued genomic surveillance and the potential necessity for future strain updates, particularly for this genetically divergent lineage.
• Platform Validation: The results reinforce the flexibility of Moderna's mRNA platform, demonstrating that both full-length S protein and RBD/NTD-only constructs can be rapidly adapted to new variants while maintaining high immunogenicity and safety profiles.