Of the 34 vaccinees in the community-based trial, four reported fever of a few hours’ duration during the first two study days. Similarly, two of 21 vaccinees experienced fever during this time frame in clinic-based trials reported here and previously (3
) (Table ). The cumulative 12% rate of transient, febrile reactions is comparable to the rate of febrile reactions attributed to licensed parenteral vaccines such as those for diphtheria, Haemophilus influenzae
type b, Lyme disease, measles-mumps-rubella, and varicella zoster (www.cdc.gov/nip
). An additional outpatient vaccinee suffered a more protracted febrile illness, with no change in bowel habits 2 weeks after vaccination. The timing and nature of this illness raise some doubt as to whether it was vaccine related. Nonetheless, we have included this occurrence in our cumulative reactogenicity data in Table .
Cumulative reactogenicity for all SC602 trials at a 104-CFU dosea
Because of the circumstances of data collection, diarrhea was defined by a specific number of liquid stools (three to five) for outpatients and by a specific weight of liquid stools for inpatients. Although different definitions were applied, the overall rates of diarrhea were similar in all of the inpatient (9%) trials and in the outpatient (12%) trial. The cumulative rate of diarrhea for all SC602 trials was 10% (Table ). With the caveat of different parameters of diarrhea and different methods for detecting fever in inpatient and outpatient trials, a total of 11 of 58 vaccinees receiving the 104-CFU dose of SC602 experienced either fever or diarrhea, and one vaccinee experienced both symptoms, for a cumulative rate of 19% for any clinical reaction. In placebo controls from an earlier dose-ranging trial of SC602, 2 of 16 volunteers had diarrhea or fever, for a cumulative rate of 12%.
We also collected data on subjective symptoms that might have an impact on consumer acceptance of SC602. Headache and abdominal cramps were cited by 5 of 58 vaccinees (9%) as “severe” complaints inhibiting normal activities (Table ). None of the placebo controls in the initial dose-ranging trial reported any severe symptoms, but the rate of mild to moderate symptoms was similar to that of vaccinees (Table ) (3
). We tentatively conclude that the rate of short-term fever, diarrhea, severe headache, or severe intestinal cramps elicited by a 104
- CFU dose of SC602 in North American adults is approximately 30%, compared to 13% in placebo controls (Table ). However, an expanded double-blind, placebo-controlled, community-based phase 2 trial of SC602 would be required to formally assess the rate of reactogenicity of this vaccine.
The ELISPOT antibody-secreting cell assay for peripheral blood lymphocytes expressing anti-2a lipopolysaccharide is a sensitive assay for quantifying immune responses evoked in the intestinal mucosa. When IgA antibody-secreting cell data from the 12 inpatients receiving a 3-log or 4-log dose of SC602 were compared to those for a previous group of 12 patients receiving a single 4-log dose (3
), the geometric mean titers were almost identical (46 versus 47 antibody-secreting cells/106
peripheral blood lymphocytes). Eight vaccinees had a positive antibody-secreting cell response (≥10 antibody-secreting cells) for at least one antibody isotype in the first inpatient trial, and 10 vaccinees were positive in the present trial (overall positive response rates of 75% for any isotype and 66% for IgA).
As a point of reference, volunteers experiencing severe shigellosis after challenge with virulent S. flexneri
2a had an IgA geometric mean titer of 174 antibody-secreting cells/106
peripheral blood lymphocytes, with an 86% positive response (3
). Threefold serum IgA ELISA responses were detected in half the outpatient and inpatient vaccinees, and this level of antibody response correlated with protection against all symptoms of shigellosis (including diarrhea) in the previous efficacy trial of SC602 vaccinees. All vaccinees were protected against fever, dysentery, and severe shigellosis, but not against diarrhea.
The self-plating technique devised for the outpatient trial (18
) successfully detected excretion of vaccine organisms by 33 of 34 vaccinees, with a mean duration of colonization of approximately 12 days. An unexpected finding in the outpatient trial was the intermittent isolation of shigellae. As many as six negative cultures over the course of 12 days were bracketed by periods of positive cultures, and two subjects were intermittently culture positive for over 4 weeks. Infrequent long-term excretion of shigellae (15
) and Shigella flexneri
) is consistent with past reports, showing that SC602 is not attenuated regarding excretion. Because the original criterion of two consecutive culture-negative stools did not prove to be a reliable standard for determining the persistence of SC602, all volunteers were asked to return as many inoculated HEA plates as possible for 1 month.
In order to clearly terminate the outpatient trial, all volunteers were treated with ciprofloxacin on days 35 to 40. Since we were unable to always detect SC602 in stool samples from vaccinees who were colonized with SC602, the duration of colonization in a small proportion of vaccinees could be longer than 35 days. In the inpatient trial, SC602 was not detected by microbiological culture in the stools of three of nine volunteers ingesting a 4-log dose of SC602, while all three inpatients ingesting the 3-log dose excreted vaccine up to the end of the 7-day study. In previous inpatient trials, all vaccinees receiving a 4-log dose of SC602 were culture positive (3
). Cumulative analysis of inpatient and outpatient trials indicates that 90% of vaccinees receiving this dose of SC602 were culture positive on day 7. SC602 was not transmitted to control volunteers during the initial inpatient dose-ranging trials (3
Dose selection trials of a second icsA
) candidate (S. sonnei
WRSS1) have yielded promising safety and immunogenicity results in phase 1 trials (10
). More recent community-based trials of WRSS1 have included active microbiological monitoring of stools of all household contacts for adventitious spread of the excreted vaccine. Like SC602, WRSS1 colonizes a majority of volunteers who ingest as few as 103
CFU after bicarbonate buffer, and the vaccine strain was excreted for an average of 5 days. However, when 32 household contacts were exposed to vaccinees excreting WRSS1 for a cumulative total of 164 days, there was no evidence of accidental transmission of the vaccine (unpublished data). These data suggest that adventitious spread of attenuated Shigella
vaccines to individuals with normal standards of hygiene will be an uncommon event. A similar community-based study of SC602 household contacts is planned as a key step in further development of this vaccine. Regardless of the outcome of such phase 1 trials, however, the possibility remains that excreted Shigella
vaccines will occasionally colonize bystanders.
Fortunately, the icsA
gene deletion is a powerful attenuation that prevents development of hemorrhagic lesions in the intestinal mucosa even when 108
CFU of SC602 is ingested with bicarbonate (3
). In primates inoculated with an icsA
mutant, small foci of inflammation occur in lymphoid follicles, but the infection does not progress to colonic lesions, presumably because the shigellae cannot invade contiguous epithelial cells (17
). The failure of shigellae to propagate by intercellular spread is an attenuating feature that should be effective even in immunocompromised individuals. The Food and Drug Administration has recently addressed the issue of live, attenuated vaccines with a potential for adventitious transmission by approving the live, attenuated FluMist intranasal vaccine for general use only in healthy people aged 18 to 50 years. Prescriptions are required for youths aged 5 to 17 years, and the killed influenza vaccine is preferred to FluMist for anyone who comes into close contact with people with weakened immune systems. Likewise, live vaccines such as SC602 and WRSS1 would presumably be contraindicated for consumers with immunocompromised household or workplace contacts.
The cumulative results of SC602 clinical trials have demonstrated that this icsA
) mutant is a substantially attenuated candidate vaccine strain that can evoke protection against the most severe symptoms of shigellosis in a stringent human challenge model of disease. The short-term symptoms associated with SC602 (headache, cramps, diarrhea, and fever) have thus far accompanied all invasive Shigella
vaccines that induce substantial immune responses (9
). These symptoms could presumably be lessened by palliative treatments without sacrificing vaccine efficacy, and for certain high-risk customers (e.g., soldiers and adventure travelers), the reactions would probably represent an acceptable inconvenience. As noted previously, further studies of SC602 would be required to confirm the safety profile of the vaccine for vaccinees and for bystanders.