• A short piece of synthetic DNA called CpG oligodeoxynucleotide, which switches on an immune sensor called toll-like receptor 9 (TLR9).
• A β-glucan (a sugar-based molecule from fungi) called schizophyllan.

The team tested whether one dose sprayed into the nose could protect mice. The results, in short:

• Treated mice lost less weight and survived influenza A infection at higher rates than control mice.
• Protection was still detectable 100 days after the single dose, though it weakened over time.
• The treatment also protected susceptible mice against SARS-CoV-2.
• It did not substantially cut viral levels in the lungs.

That last point matters. The compound appears to help the host tolerate infection and limit tissue damage rather than directly blocking the virus from copying itself.

Does the delivery route matter?

Yes. Nasal delivery produced stronger protection than injecting K3-SPG under the skin or into the bloodstream. That fits the logic of the approach: the drug lands directly in the airways, where respiratory viruses strike first.

How does the protection work?

Using several analytical techniques, including single-cell RNA sequencing — a method that measures gene activity in individual cells — the team mapped a two-stage immune response:

• Early phase: Macrophages, the immune system's debris-clearing cells, carried most of the load.
• Later phase: Innate lymphoid cells, a family of immune cells in tissues, took over.

Some of these innate lymphoid cells showed changes in their chromatin, the DNA–protein scaffolding that controls which genes are switched on or off. That kind of reprogramming suggests the cells had been durably retrained — a possible explanation for why protection persisted for 100 days.

The response also depended on two molecular players: TLR9, the receptor K3-SPG activates, and TNF-α, an inflammatory signaling molecule.

What did the researchers say?

"Our research has identified unique mechanisms by which vaccine adjuvants can induce protective innate immunity against respiratory viral infections, suggesting that adjuvants may have potential applications beyond their traditional role in enhancing vaccine responses," Ishii said.

He also pointed to the pandemic-preparedness angle: "Because an adjuvant-based preventive approach may not require prior knowledge of the specific pathogen, it could potentially provide an additional layer of protection while pathogen-specific vaccines are being developed and manufactured."

Such an approach could complement initiatives like the 100 Days Mission, an international effort to develop vaccines within 100 days of a new outbreak, by buying time before targeted shots arrive.

Do the results apply to humans?

Not yet — and the researchers are explicit about this. The findings come primarily from mice, and mice are not people. Human TLR9 biology differs from that of rodents, and delivering a compound safely to the human respiratory tract raises its own challenges.

Further studies must assess K3-SPG's safety, effectiveness, and suitability for human use before any claim about protecting people can be made. The 100-day protection figure is a mouse result, not a clinical one.

Still, the study demonstrates a principle worth watching: a single nasal dose of a well-characterized adjuvant can tune the airways' innate defenses against more than one virus family — influenza A and SARS-CoV-2 — for months in animals. Whether that principle survives contact with human biology is the next, and harder, question.