On Friday, January 23, the ASIJ Health Center issued a warning in the Daily Bulletin, cautioning students and parents that Norovirus and Influenza were circulating on campus. Both illnesses commonly spread during the winter months; however, there’s one key difference between the two: Influenza has vaccines developed and distributed every year, yet no vaccine has been approved for the Norovirus.

So why does the Norovirus have no vaccine, and how close are we to developing one?

Let’s start with some basic information about the Norovirus. Norovirus is highly contagious and can cause diarrhea, vomiting, nausea, and stomach pain. It can be contracted through contact with people infected with the virus, by consuming contaminated food or beverages, or by putting unwashed fingers in the mouth after touching contaminated surfaces. Prevention strategies include thorough, frequent handwashing and disinfecting contaminated surfaces.

The first challenge in creating a Norovirus vaccine is its diversity. There are about 50 distinct strains of Norovirus, and a different immune response is required to protect people from each strain. Thus, a Norovirus vaccine would, similarly to the flu vaccine, be based on an educated guess about which strains will spread in the future.

The second challenge is that the Norovirus mutates rapidly. Why this poses a challenge when creating vaccines is best illustrated by the case of the flu vaccine: the immune system recognizes proteins on the virus’s surface and triggers an immune response, but as the virus accumulates mutations over time, the body may struggle to recognize or effectively fight it, as it has become sufficiently different from the original virus. This is the same basic problem that forces influenza vaccines to be reformulated regularly, and it helps explain why a future norovirus vaccine may need to be updated over time as well.

So, where are we with vaccine development now?

Despite these two challenges, vaccine development is well underway. In January 2025, it was reported that Moderna was preparing a phase 3 trial of a potential Norovirus vaccine. The plan at the time was to sign up volunteers in the spring and follow them for 6 months or more, with the report on vaccine efficacy expected to “take longer.” 

In short, Norovirus lacks a vaccine because its high mutation rate and diversity make it extremely difficult to combat effectively. Still, with Moderna’s candidate now in late-stage trials, we could plausibly see the first licensed norovirus vaccine sooner than many people expect.