Further, detailed examination of the participants’ immune responses showed not only systemic response, but responses in distant mucus membranes. In the blood, two types of antibodies (IgA and IgG) increased by several fold after vaccination compared with the placebo group. The group with the largest responses was the one that received the high dose.
A test that acts as a surrogate for neutralizing antibody responses to norovirus indicated that the antibodies spurred by the vaccine could block the virus. Additional tests found that cellular immune responses were also activated and that the systemic responses result in protection in places far from the intestines—namely the mouth and nose. Saliva tests and nasal swabs found significant jumps in secreted IgA against norovirus.
Immune responses were strongest in the first two months after vaccination and diminished over time, but some persisted for nearly seven months. When the scientists looked at differences between the two age groups (55–65 and 65–80), they didn’t see significant differences, suggesting the vaccine was equally effective in the older group.
Overall, the scientists at Vaxart concluded that the vaccine “has the potential to inhibit infection, viral shedding, and transmission.”
“Overall, VXA-G1.1-NN administration in older adults led to robust and durable immunogenicity detected both in circulation and multiple mucosal sites, an exciting outcome considering that diminished cellular and mucosal immunity are typical in older populations,” they wrote.
Not so good news
The outlook isn’t entirely rosy, though—there is some bad news. While immune responses rose in statistically significant measures during this small early-stage trial, it’s unclear if that equates to real-life protection. And there’s some good reason to be wary. In 2023, Vaxart released results of a challenge study, in which 141 brave souls (76 vaccinated and 65 given a placebo) were deliberately exposed to norovirus to see if the vaccine was protective. The results were weak: 53 placebo-group members (81.5 percent) became infected with norovirus, as determined by a PCR test looking for genetic evidence of the virus in their stool—and so did 76 vaccinated people (60 percent). That worked out to the vaccine offering only a 29 percent lower relative risk of getting infected. Looking at whether infected people developed symptoms of acute gastroenteritis, the vaccine had a protective efficacy of about 21 percent: 34 vaccinated people (48 percent) versus 37 placebo-group members (57 percent) developed symptoms.
