Experimental Whole-Virus Flu Vaccine Shows Safety and Broad Immune Signals in First Human Trial

Influenza remains one of humanity’s most persistent infectious threats, and yet the vaccines deployed against it every year share a fundamental limitation: they are built to match specific strains predicted to circulate in a given season. When the prediction misses, protection collapses. A team of researchers at the National Institutes of Health has now reported the first human test of a radically different approach, a whole-virus vaccine designed to confer broad protection across influenza A subtypes, including strains that have never circulated widely in people. The results, published in Nature Communications, indicate that the experimental vaccine BPL-1357 is well tolerated and capable of eliciting robust antibody responses, particularly when delivered by intramuscular injection.

BPL-1357 is unlike any licensed influenza vaccine in composition or intent. It consists of four wild-type, low-pathogenicity avian influenza viruses that have been chemically inactivated with beta-propiolactone, a reagent that destroys infectivity by alkylating viral nucleic acids while leaving the structural proteins, including the surface glycoproteins hemagglutinin and neuraminidase, largely intact. Because the vaccine presents the complete repertoire of viral proteins rather than only a strain-matched hemagglutinin head, the immune system is exposed to more conserved targets that vary far less between influenza lineages. In animal models, the vaccine had already demonstrated remarkable protection against a wide range of influenza viruses, raising the prospect of a true universal influenza vaccine platform that could be manufactured in advance of any pandemic and deployed regardless of which subtype emerges.

The newly published study was a phase 1 clinical trial designed to answer the two questions that must precede any discussion of efficacy: is the vaccine safe, and does it provoke measurable immunity in humans? Forty-five healthy adult participants were enrolled and randomized to receive BPL-1357 intramuscularly, intranasally, or placebo. The study employed a double-blinded, double-dummy, placebo-controlled design, meaning that neither participants nor investigators knew who received the active product, and each participant received both an injection and a nasal preparation so that the route of administration could be compared without unblinding. Vaccination followed a prime and boost strategy, with the second dose administered 28 days after the first, mirroring the way many inactivated vaccines achieve their full effect.

The primary safety endpoints tracked adverse events and serious adverse events from enrollment through 28 days after the boost, with additional monitoring extending to 182 days after the second dose. The verdict was reassuring. Adverse effects were mostly mild, and apart from mild injection-site pain reported more frequently in the intramuscular group, reactions occurred in only a minority of participants. No serious safety signals emerged that would halt development. For a vaccine built from wild-type avian viruses, the mildness of the reactogenicity profile is a notable finding, and it validates the beta-propiolactone inactivation process as a way to strip infectivity while preserving the immunogenic architecture of the intact virion.

The immunogenicity data told a more differentiated story depending on the route of delivery. Participants who received BPL-1357 intramuscularly mounted strong serum IgG responses against hemagglutinin antigens from three distinct influenza subtypes: H3, which circulates seasonally in humans; H5 and H7, both avian subtypes with pandemic potential. Specifically, 71.4 percent of intramuscular recipients demonstrated a serologic response against H3, 71.4 percent against H5, and 85.7 percent against H7, all clearly exceeding rates observed in the placebo group. This breadth is the central promise of the whole-virus design. A conventional seasonal vaccine would not be expected to generate antibodies against H5 or H7 at all, yet the avian antigens in BPL-1357 appear to have primed immune recognition of those very subtypes.

Immunity to influenza involves more than hemagglutinin, however, and the trial’s secondary endpoints were designed to capture a fuller immunological picture. Intramuscular vaccination also induced serum neutralizing antibody activity and responses directed against neuraminidase, the second surface glycoprotein of the virus and an increasingly appreciated target for broad protection. Neuraminidase-directed immunity does not block viral entry but limits viral release and spread, and a vaccine that stimulates both hemagglutinin and neuraminidase responses across subtypes offers multiple layers of defense. What the intramuscular route failed to produce, though, was mucosal immunity: participants showed no nasal IgA responses against either hemagglutinin or neuraminidase. Since influenza infection begins at the respiratory mucosa, secretory IgA at that interface is considered an important first line of defense that systemic antibodies cannot fully replicate.

This is where the intranasal arm of the trial provided its own lessons. Participants vaccinated by the nasal route developed only subtle mucosal and serum responses, an outcome consistent with what the investigators had observed in preclinical experiments. The finding underscores a persistent challenge in mucosal vaccinology: delivering an inactivated, non-replicating product to the nasal cavity rarely generates the kind of vigorous mucosal immune response that live attenuated vaccines, which briefly replicate in the airway, can achieve. At the same time, the fact that the intranasal preparation produced any measurable response at all, without safety concerns, suggests that route-specific optimization, including dose escalation or formulation changes, remains a viable avenue for future studies aimed at inducing mucosal IgA.

The trial’s design reflects careful attention to the scientific questions at stake. By comparing two delivery routes in a randomized, blinded framework with a double-dummy placebo control, the investigators could isolate the immunological consequences of route from the effects of the antigen itself. The measurement of both serum IgG and nasal IgA against both major viral glycoproteins, and the inclusion of multiple hemagglutinin subtypes in the assay panel, allowed the team to quantify breadth as well as magnitude. Such breadth is precisely what a universal vaccine must demonstrate, because the value of the approach lies not in replacing the seasonal flu shot with an identical match, but in creating a baseline of immunity that protects against strains outside the vaccine, including subtypes that could spark the next pandemic.

The investigators themselves are explicit that these results mark a beginning rather than a conclusion. The trial tested a single dose level, and the authors note that more work is needed to assess other dose levels, further characterize the immunity induced, and evaluate efficacy against actual influenza infection and disease. A phase 1 trial in 45 participants cannot establish protection; it can only establish that the vaccine is safe enough to test further and that the immune responses it generates have the desired breadth. Larger studies, ideally including controlled exposure to influenza virus or follow-up through natural influenza seasons, will be required to translate the encouraging serologic signals into evidence of clinical benefit.

Even so, the trial represents a meaningful step in a field where progress has been slow and incremental. Most universal influenza vaccine candidates in clinical development target conserved regions of hemagglutinin, such as the stalk, or internal proteins through novel platforms such as mRNA or viral vectors. BPL-1357 takes a different philosophical approach: rather than engineering a single conserved epitope, it delivers four entire avian viruses inactivated in a way that preserves their native antigenic presentation, betting that the breadth of the antigenic payload will pull the immune response toward conserved targets. The demonstration that this approach is well tolerated in humans, and that a prime-boost intramuscular regimen can elicit cross-subtype serum IgG, neutralizing activity, and neuraminidase-specific antibodies, provides the first clinical validation of that strategy. As avian H5N1 continues to spread in wild birds, dairy cattle, and occasionally into people, the need for vaccines that protect against a wide range of influenza viruses has never been clearer. The BPL-1357 results suggest that a whole-virus, wild-type avian antigen approach can safely clear its first human hurdle, and that the road toward a genuinely universal influenza vaccine now has at least one more promising candidate on it.

Subject of Research: A phase 1 trial of the BPL-1357 inactivated whole-virus universal influenza vaccine evaluating safety and immunogenicity in humans

Article Title: Safety and immunogenicity of a BPL-inactivated, whole-virus, universal influenza vaccine: a randomized, double-blinded, double-dummy, placebo-controlled, phase 1 clinical trial

Article References: Safety and immunogenicity of a BPL-inactivated, whole-virus, universal influenza vaccine: a randomized, double-blinded, double-dummy, placebo-controlled, phase 1 clinical trial. (n.d.). https://doi.org/10.1038/s41467-026-78583-w

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78583-w

Keywords: influenza, universal vaccine, BPL-1357, phase 1 trial, whole-virus vaccine, beta-propiolactone inactivation, hemagglutinin, neuraminidase, mucosal IgA, avian influenza, NIAID, vaccine immunogenicity