Effect of Extended Dose Intervals on the Immune Response to Oral Cholera Vaccine

Cholera is a life-threatening disease if prompt actions are not taken. The most recent estimates of the global burden of cholera estimate that there are more than 1.3 billion people at risk. Of which, 2.86 million (range: 1.3-4.0 million) will contract cholera and 95,000 (21,000-143,000) will die each year. A safe, effective, and affordable killed whole-cell oral cholera vaccine (OCV) is now being used widely to prevent cholera in areas at risk. This regimen demonstrated 65% efficacy retained for at least 3 years and even up to 5 years in an endemic setting.

As described in the package insert, the standard dosing schedule of the OCV is two doses with the second dose given 2 weeks after the first. In several campaigns, it was felt to appropriate to give a single dose to twice as many people and to give the second dose at a later time when this was logistically possible. In fact, modelling of the impact of OCV during an outbreak finds that when the vaccine supply is limited (as it is currently), more cases are prevented if a single dose strategy is used since a single dose can be provided to twice as many people. Even if the efficacy is a bit lower, the number of cases prevented will be higher. Even the individual person in this situation will be better off if more of his neighbors also receive vaccine since he benefits from the herd protection when more people are vaccinated.

A single dose strategy was used during an outbreak in Zambia in 2015-16, but this first dose was then followed up with a second dose after 6 to 8 months. I was also used in Haiti in 2016-17 following the hurricane. While the delayed second dose strategy has been used and will likely continue to be used during outbreaks or during humanitarian crises, there is no feasible way to assess the relative effectiveness against clinical cholera comparing a two-week interval with a delayed second dose strategy.

While clinical effectiveness trials are not feasible, serological responses comparing different dose intervals are possible. One such study found that vibriocidal titers were similar if the second dose was given either 2 week or 4 weeks after the first , but studies have not been done with longer dose intervals, as was used in Zambia and Haiti.

The proposed study will determine if giving the second dose of the OCV using a longer interval will result in a response to the second dose that is not inferior, or perhaps even results in a more prolonged elevated vibriocidal titre. From an immunological standpoint, there could be advantages to a longer dose interval if this resulted in a true booster response which sustained high titre of antibody. While acknowledging that the vibriocidal titer is not an established correlate of protection, it is the best correlate of the immune response following vaccination. It seems logical that a sustained high titer is likely to be more effective. Unfortunately, the clinical trials comparing the 2 week and 4 week interval with Shanchol only examined the serum titers shortly after the dosing (about 2 weeks). Follow-up serum samples were not obtained to discern how these dose intervals compared in terms of duration of elevated serum titres.

Recent studies have found that children <5 years of age are less well protected than older individuals even though their serum antibody response rates (take rates) were similar to the older subjects when the serum was collected about two weeks after the vaccine doses were administered. These immunogenicity studies have not, however, examined antibody titers when blood samples were obtained after a longer time interval. Therefore, it is not known if the antibody titers decline at the same rate as older subjects. It could be that GMT titers are lower or that titers fall more rapidly in the young children, and a different dosing interval could maintain higher titres. To compare antibody responses in different age groups, we plan to stratify groups into age 1-4, 5-14, and 15 years and older.

Inclusion criteria

• Age ≥1 year, stratified into different age groups
• Living in the Waya Clinic Catchment Area
• Good health condition, without clinically significant medical history (by participant or guardian, in case of minor)
• Not pregnant for female subjects.
• Available to participate for the study duration, including all planned follow-up visits for up to 9 months from screening.
• Signed informed consent

Exclusion criteria

• Presence of a significant medical or psychiatric condition (Examples include: Diagnosis and treatment of tuberculosis (TB) or HIV; renal insufficiency; hepatic disease; oral or parenteral medication known to affect the immune function, such as corticosteroids, other immunosuppressant drugs; or behavioural or memory issues)
• Ever having received oral cholera vaccine.
• Receipt of an investigational product (within 30 days before vaccination).
• History of diarrhoea in 7 days prior to first dose of vaccine (defined as ≥3 unformed loose stools in 24 hours).
• History of chronic diarrhea (lasting for more than 2 weeks in the past 6 months)
• Current use of laxatives, antacids, or other agents to lower stomach acidity?
• Planning to become pregnant in the next 2 years.