Profiling long-term, antigen-specific memory B cells to oral cholera vaccine in an endemic population

Zambia is classified as one of the cholera endemic countries by WHO and has set up a national elimination program with a target to eliminate cholera by 2025. Part of the pillars of the elimination program is the use of OCV for pre-emptive purposes. However, to date, very few studies have been conducted to understand what immune criteria to use before individuals are deemed in need of revaccination with OCV in hot spot area. There is need to understand the persistence of memory responses alongside antibody responses.

In 2016, Shanchol™ was deployed to Zambia for the first-time targeting individuals within hotspot areas to mitigate the epidemic outbreak. Over a million individuals were vaccinated and the majority with one dose only. Recently, another stock of OCV (Shanchol) was received and is being offered to individuals in listed hotspot areas regardless of prior vaccination status during the 2016 campaign.

While this is a positive step by the Zambian government, it is not based on any empirical evidence as to the ideal time for re-vaccination, nor is there specific support that specific memory responses will be stimulated in the previously immunized individuals four years back.

We propose to conduct a prospective cohort study to describe memory B cell responses to OCV using a population we have been following up for 48 months in a prior study which was aimed at characterising longevity of vibriocidal titres to OCV. We propose to amend the currently approved protocol to include this work to be done in 3 categories of OCV recipients: those previously administered 2 doses, those who got 1 dose only, and those who never received any.

The specific objectives include;

1. Describe proportions of memory B cells and corresponding serum antibodies specific to OCV across 3 groups by OCV history among individuals in Cholera hotspots.
2. Determine the prevalence of exposure to wild type Vibrio cholerae in the past 4 years by measuring IgG, IgA and IgM cholera specific immunoglobulins.

• Compare the trend of immune response (MBS) across the three groups over 3 months follow up period

1. Determine the association between baseline MBS and seroconversion

The team at CIDRZ have the infrastructure, equipment, trained staff in PBMC collection/processing, vibriocidal and access to the ELISPOT reader. Adam Cunningham will provide technical support for the assays and mentorship on detailed cellular immunology. Findings from this study will provide detailed understanding on long-term immunity to cholera and propose when to administer cholera booster doses.

Zambia is among the countries in sub-Saharan Africa that contribute to the 2.9 million cases and 95,000 deaths resulting from cholera globally [1]. The periodicity of outbreaks cannot be predicted. For example, in the last two decades, Zambia experienced multiple, intermittent outbreaks biennially until 2016. There is inadequate knowledge on long term immunity to cholera after natural infection or vaccination, which if improved could help identify ways to improve vaccination strategies.

During the 2016 outbreak, the oral cholera vaccine Shanchol™ (OCV) was introduced into Zambia, with >420,000 doses given to >70% of the target population. In 2021, re-vaccinations are being offered in known hotspot areas to bolster immunity against cholera. Here, we propose to collect peripheral mononuclear cells (PBMCs) and serum from previously-vaccinated individuals currently under follow-up and those unvaccinated but exposed to cholera. Memory B Cells (MBCs) from fully or partially-vaccinated (received dose one only) and exposed vaccine naïve individuals will be profiled to determine whether the development of long-term memory to cholera is inferior in naïve individuals’ compared to vaccinated individuals.

Dr. Caroline C Chisenga at Centre for Infectious Disease Research in Zambia (CIDRZ) is investigating Shanchol™-induced long term immunity. Professor Adam Cunningham has experience on how adaptive immunity to pathogens and their component antigens are induced, maintained and function. He will provide support on T and B cell work using PBMCs. Findings will inform global thinking on memory to cholera; when it is lost and potentially when cholera re-vaccination should be implemented in hotspot areas.

Since the first recorded case of cholera in 1977, Zambia has relied on efforts of promoting proper use of water, sanitation, and hygiene (WASH) as effective means for fighting cholera. Although there were annual cholera epidemics in Zambia between 2003 and 2011, no confirmed cases of the disease were reported in the country in 2012–2015. This silence including the cause of the outbreak after 4 years could not be clearly explained.

In 2016, the oral cholera vaccine (Shanchol) was first introduced to Zambia in an effort to curb the fast spreading outbreak occurring after a period of 4 years. Even though no outbreak has been reported since then, a re-vaccination pre-emptive campaign was recently embarked on with the view of further protecting the population assuming that the all individuals either previously fully vaccinated, partially or are vaccine naïve are at equal risk and must be vaccinated.

Although this initiative is good, it is also costly in that we do not know for example if re-vaccinating individuals that were initially vaccinated is much more beneficial compared to individuals that were either partially vaccinated or have not been previously vaccinated.

With the limited GAVI stockpile there is urgent need to generate scientific evidence on when memory to cholera is lost using the MBCs and develop a guided and impactful vaccine implementation plan.

This plan will useful for;

1. Determining individuals that must be prioritised for re-vaccination
2. Reducing cost of re-vaccination and improve case management if vaccination is targeted
3. Averting the number of cases and deaths resulting from cholera due to boosted immunity if re-vaccination is timely
4. Improved time of productivity if cholera is controlled

Subsequently, all these will contribute to the national plan for cholera elimination in Zambia by 2025.

Furthermore, if we are to achieve and contribute to the national strategic plan for cholera elimination, there is also need for improved lab capacity in country. With our proposed collaboration and work on detailed cellular immunology, we will build laboratory capacity and expertise for understanding immunity to cholera which currently is lacking.

Additionally, the proposed work is also foundational for the imminent cholera human infection challenge work which is critical for cholera prone areas. Also knowing that challenge studies are the direction science is taking now and with our already existing stored cholera isolates, it is needful that this level of immunology work in undertaken.