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microscopic image of Vibrio cholerae
Cholera can reach epidemic proportions, as is currently taking place in Haiti. Other recent outbreaks of cholera in Asia and Africa have occurred in Sudan, Guinea-Bissau, Liberia, Nigeria, Cameroon, Angola, South Africa, Zimbabwe, Malawi, Tanzania, Kenya, Ethiopia, Somalia, Iraq, India, Afghanistan and Vietnam. It is estimated that four out of every 100 persons who acquire the illness die. A person whose stomach contains normal gastric acid is not at much risk for acquiring cholera, but during an epidemic exposure, persons clearly become ill in large numbers.
There is an excellent discussion of the possibilities for cholera immunization in an article entitled, “A national cholera vaccine stockpile – a new humanitarian and diplomatic resource,” which appeared recently in the New England Journal of Medicine (N Engl J Med 363;24:2279-2282, 2010) authored by Matthew Waldor, MD and colleagues. Although the World Health Organization (WHO) estimates that there are three to five million annual cases worldwide with 100,000 to 130,000 deaths, no country currently requires immunization (vaccination) against cholera, and the Centers for Disease Control and Prevention (CDC) does not recommend cholera vaccination for travel. However, some localities require proof of cholera immunization for travellers entering from a territory that still reports the disease. For this purpose, documentation of a single dose of oral vaccine (not FDA-approved or licensed for use [and therefore, not available] in the U.S.) generally is sufficient.
Injectable vaccines have not yet been proven effective. Oral vaccines, however, have been shown effective. There are three oral cholera vaccines most commonly used. One, called Dukoral (not yet approved in the U.S., but available in Canada), is comprised of killed Vibrio cholerae cells (serotype O1) formulated with recombinant cholera toxin B. This vaccine has been licensed in more than 60 countries. The other two vaccines, called Shanchol and mORC-VAX, are comprised of killed bacterial cells (serotypes O1 and O139) without the toxin. The authors point out that there are fewer than 400,000 total doses of oral cholera vaccines currently available for shipment from the manufacturers. It is likely that as cholera vaccines become more available, they will be used more often in areas where the infection is endemic or epidemic. However, it is to be noted that the utility of cholera vaccines during an epidemic would likely be limited by the requirement of two doses to develop significant immunity.
The following information is from the WHO position paper on cholera vaccines (published March 26, 2010):
Manufacturers’ recommended schedules
Dukoral: 2 oral doses for 7 or more days (but <6 weeks) apart for all aged 6 years or older. For age group 2–5 years: 3 doses 7 or more days, but <6 weeks, apart. If 2nd (or 3rd) dose not given within 6 weeks of the previous, restart primary immunization.
A booster after 2 years for individuals aged 6 years or older. If >2 years since previous dose, restart primary immunization. For age group 2–5 years, 1 booster every 6 months; if >6 months since previous dose, restart primary immunization.
Shanchol and mORCVAX: 2 oral doses 14 days apart for all aged 1 year or older. A booster dose is recommended after 2 years.
WHO position on oral cholera vaccines
Cholera control should be a priority in endemic areas.
Given the availability of 2 oral cholera vaccines and data on their efficacy, field effectiveness, feasibility and acceptance in cholera-affected populations, these vaccines should be used in conjunction with other prevention and control strategies in areas where the disease is endemic, and immunization should be considered also in areas at risk for outbreaks.
Cholera vaccination should be used in conjunction with other interventions. Vaccination provides an immediate short-term response while the longer term Interventions, such as improving water and sanitation, are put into place.
Although all age groups are vulnerable to cholera, where resources are limited, immunization should be targeted at high-risk children aged ?1 year (Shanchol or mORCVAX) or ?2 years (Dukoral).
Documented duration of significant protection induced by current cholera vaccines is 2 years. Hence, initial vaccination with 2 doses should be followed by a booster every second year.
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