RESULTS
Analysis of the demographic characteristics of the subjects in the two SMV trials indicated no significant differences in sex, age, and secretor status between subjects challenged with SMV Inoculum 1 and Inoculum 2 (Table 2).
We compared infection and illness rates between the subjects who received SMV Inoculum 1 (n=15) to those who received SMV Inoculum 2 (n=33). Overall, SMV infection occurred in 9 of 15 (60%) subjects challenged with Inoculum 1 and in 25 of 33 (75.7%) subjects challenged with Inoculum 2. Illness occurred in 7 of 15 (46.7%) subjects challenged with Inoculum 1 and in 9 of 33 (27.3%) subjects challenged with Inoculum 2. There were no statistically significant differences between the infection (60.0% vs. 75.7%) and illness (46.7% vs. 27.3%) rates in the two trials (Table 3).
In addition to examining the infection and illness rates following SMV challenge, we explored possible differences in severity of illness in the 16 infected subjects who met our definition of illness (Table 4). Subjects infected with SMV Inoculum 1 had a mean severity score of 6.00 (95% CI: 4.97, 7.03], whereas the mean severity score for subjects infected with SMV inoculum 2 was significantly lower 2.94 (95% CI: 1.74, 4.14, P = 0.003) (Table 4, Figure 1). Furthermore, our analysis indicates that Inoculum 1 was associated with more severe acute gastroenteritis even at doses more than 100-fold lower than the doses used for the Inoculum 2 challenge. In addition, we found secretor-positive subjects had significantly higher mean modified Vesikari severity scores compared to secretor-negative subjects (P < 0.001) at the dose of 1.2×107 genome copies (Figure 1). When we combined all data from both inocula and different challenge doses, we did not find a significant association between log10 inoculum dose and modified Vesikari scores for either inoculum (Inoculum 1 and 2, P = 0.951 and P = 0.905, respectively).
The RT-PCR results (Table 5) indicated that the NV positive rate was not significantly different in subjects infected with Inoculum 1 vs. inoculum 2 in stool samples collected during the first three days post challenge, but subjects infected with Inoculum 1 had significantly more (P < 0.001) PCR-positive stools during days 4-6 post challenge (46.6%) compared to none of the subjects infected with Inoculum 2 having PCR-positive stool samples during that period of time. When shedding duration was compared between these two groups, SMV RNA was detected in three stool samples (9.1%) from days 15-45 post-challenge from subjects challenged with Inoculum 2, but no subject challenged with Inoculum 1 shed virus at 15 and 45 days post-challenge.
Finally, we compared anti-SMV serum IgG conversion (>4-fold vs. pre-challenge) in subjects challenged with Inoculum 1 and Inoculum 2 (Table 5). During days 1 - 3 post challenge, none of subjects in both groups showed anti-SMV serum IgG conversion. During days 4 - 6 post-challenge, eight subjects (57.1%) infected with Inoculum 1 showed anti-SMV serum IgG conversion compared to none of the subjects infected with Inoculum 2 (P < 0.001) during that period. Most serum IgG conversion occurred between 15 and 30 days post challenge, but there was no significant difference between the two trials in the overall proportion of subjects with seroconversion.