In the light of the recently increased threat of cholera and the progress in vaccine biotechnology, it is anticipated that vaccines will again be considered as tools for cholera prevention. Recent field trials in Bangladesh and Vietnam showed that available oral vaccines are safe, effective, inexpensive, and simple to administer. The new vaccines have not yet been recommended by the World Health Organization.

Starting in 1817, cholera (see box) has raged in seven pandemics, with a possible eighth pandemic emerging superimposed on the seventh. The first six were caused by the Vibrio's classical biotype and originated in the Ganges River delta in the Indian subcontinent, the place of origin of cholera. The seventh and current one is caused by the El Tor strain, which began in 1961 in Indonesia, and has gradually affected most of Asia and Africa, and incidentally parts of Europe.
Three recent developments mark the resilience of cholera as a re­emerging infection:

• In 1991, the pandemic reached South America, where cholera had not been seen for more than a hundred years. Since then, it has spread over the whole of Latin America, causing more than a million cases and hundreds of importations into the USA and Canada;
• In 1992, a novel Vibrio cholerae of a different serogroup, called O139 Bengal, emerged in southern Asia. Marine ecosystems in the Bay of Bengal were experiencing a pandemic of coastal algal blooms, apparently harbouring and amplifying the cholera bacteria as a reflection of an "environmental distress syndrome." This new variant has developed potential for pandemic spread;
• In 1994, in the Rwandan refugee camps in eastern Zaire, an estimated 70,000 cases and 12,000 deaths occurred during the first weeks of an explosive cholera epidemic. The relatively high number of deaths was due to the absence of adequate health facilities and to oral rehydration.

Disappointing performance of cholera vaccines
Ever since 1883, when Robert Koch discovered Vibrio cholerae as the causing agent of cholera, numerous attempts have been made to produce an effective vaccine. With the exception of an oral vaccine on the basis of bovine bile, used in trials in Asia during the early part of the century, all vaccines were administered in injectable form. The only currently­licensed vaccine is also injected, and has an estimated protective efficacy of 30 per cent for 3 to 6 months. Because of this low performance, and to avoid leaving populations­at­risk with a false sense of security, the World Health Organisation (WHO) stopped recommending this vaccine in 1973.
In response to the low performance of injected vaccines, the insight emerged during the 1970s that oral administration might be preferable to induce optimal (mucosal) immunity against cholera. The intestinal mucosal lining not only constitutes a barrier against pathogens, it also represents a vast opportunity to induce immunity through oral vaccines. Oral formulations have been pursued that would raise specific antibodies in the small intestine itself. Since then, several trials of both killed and live vaccines have been conducted, which show that the oral vaccine administration does indeed result in higher and longer­lasting immunity.

Bangladesh
From 1985 to 1990, in a large randomized, placebo­controlled field trial, imported oral vaccines manufactured by the French company Merieux were tested by the International Centre for Diarrhoeal Disease Research, Bangladesh. The trial made use of the Centre's Demographic Surveillance System which monitors life and health events among 200,000 persons in rural Bangladesh. The trial showed that 3 doses of a combined vaccine of the toxin's B subunit (that part of the toxin that attaches the toxin to a specific receptor in the lining of the mucosal cells in the intestine, and which is highly immunogenic) with killed whole­vibrio cell vaccines conferred better protection during the first 8 months than did the whole cell vaccine alone (85 per cent versus 58 per cent vaccine efficacy). Over 3 years of follow­up this relative advantage of the combined vaccine disappeared, and protection of both vaccines was just above 50 per cent. Protection was lowest among children of 2 to 5 years of age and disappeared entirely during the third year after vaccination. Protection against the classical biotype of the cholera bacteria was higher than against the El Tor strain. The combined vaccine also conferred a 3 month cross­protection against diarrhoea caused by Escherichia coli strains that produce heat­labile enterotoxines. No rebound of increased susceptibility of vaccinated persons for cholera was found after 5 years of follow­up, as was the case in cholera and typhoid trials in the 1960s.
Based on these results WHO decided not to recommend these vaccines for public health use. It determined that a new candidate vaccine should be at least 50 per cent protective to children under 5 years of age, effective against El Tor cholera, inexpensive, and preferably single­dosed.

Cholera Cholera is an acute watery diarrhoea in humans caused by a bacterial infection of the small intestine due to Vibrio cholerae. After invading the small­intestinal mucosal lining, the Vibrio's enterotoxin triggers a cascade of intracellular, metabolic changes that may lead to diarrhoea and fatal dehydration within a few hours. If the infection remains untreated, severe cholera leads to death in 50 per cent of the cases. Cholera treatment consists of two components: rehydration of the patient, which is critical, and antimicrobial treatment, which is optional and intended to shorten the duration of illness and save supplies. Severely dehydrated patients, who make up for five per cent of all clinical cases, require intravenous rehydration fluids. If only moderately dehydrated, patients can be treated using oral rehydration solution (ORS), a mixture of sugars, salts, and water.  Measures to prevent cholera at the community and household levels focus on interrupting the faecal­oral infection route. This includes the prevention of contamination of drinking water by sewage; disinfection of drinking water; safe food preparation particularly of seafood (shellfish) and of vegetables and fruits; basic sanitation; hygiene measures such as hand washing with soap.  Cholera bacteria favour brackish water. Survival in seawater for more than 50 days has been demonstrated. The association of Vibrio cholerae with zooplankton has contributed to its global distribution. Under unfavourable conditions, the bacteria can survive in a dormant state, i.e. viable but not culturable. Dormant bacteria contribute to the occurrence of seasonal epidemics.

Promising results in Vietnam
In 1992, a killed oral cholera vaccine, manufactured locally in Vietnam and administered in two doses, has performed better. In an open field trial in Hué, it showed a reduction of the incidence of cholera in vaccinated versus unvaccinated households by 66 per cent. Protection for children aged from 1 to 5 years was similar to that of older persons. This outcome has been ascribed to the presence of certain colonisation antigens, toxins and haemagglutinins in the Vietnam vaccine that were not included in the vaccines used in Bangladesh. The findings apply to cholera caused by the pandemic El Tor biotype; the classical strain does not circulate in Vietnam. Capitalizing on the findings of the Bangladesh trial, further research has been conducted on the merits of using the cholera toxin's B subunit in oral vaccine formulations.
Concurrent trials in areas where cholera occurs throughout the year, and in principally cholera­free areas where individuals lack specific immunological memory for cholera, have shown the safety and immunogenicity of a single dose of attenuated V. cholerae 01 vaccine strain CVD 103­HgR, prototype and the most advanced of all live oral vaccines. The vaccine has been developed by the Center for Vaccine Development (CVD) of the University of Maryland, USA. It confers complete and early protection against cholera caused by either biotype. It has also been shown to be highly effective against carriers of blood type O, that is predominant (73%) among the population of Latin America and predisposes for El Tor cholera. When combined with a Salmonella typhi Ty21a vaccine against typhoid fever, the vaccine's potential for public health use is further enhanced.
Finally, in addition to vaccines effective against classical and El Tor cholera, initial studies have been conducted of a live oral vaccine against the new V. cholerae O139 strain. As mentioned before, this Bengal strain, which emerged in 1992 in southern Asia, has assumed potential for pandemic spread. The vaccine has also been developed by CVD, and might appear effective against the classical, the El Tor and the new Bengal strains.

However, before recommending the use of these more effective, oral cholera vaccines for public health purposes, several considerations have to be taken into account:

• The cost­effectiveness of the vaccines needs to be weighed against that of other preventive measures (see box);
• The safety of, in particular, live vaccines in populations with HIV infection has to be ascertained. Because of their weakened immunity, HIV infected individuals are anticipated to be at greater risk of incurring cholera following exposure to live vaccines;
• The transient state of enhanced susceptibility post­vaccination has to be addressed. Vaccinated people are at increase risk of developing cholera during a period lasting from a few hours to a few days following vaccine administration;
• The delayed onset of protection may question the vaccines' usefulness in emergency settings where vaccines would be most welcome, such as refugee camps established overnight.

Academic Hospital Utrecht, Interne Kliniek DIGD, Heidelberglaan 100, 3584 CX Utrecht, the Netherlands. Phone (+31) 30 250 6307; Fax (+31) 30 251 8328; E­mail f.vanloon@digd.azu.nl
Sources
M.M. Levine (1997), "Oral Vaccines Against Cholera: Lessons from Vietnam and elsewhere." Lancet, vol. 349, pp.220­1.

J.D. Clemens et al. (1990), "Field Trial of Oral Cholera Vaccines in Bangladesh: Results from three year follow­up." Lancet, vol. 335, pp.270­3.

F.P.L. Van Loon et al. (1996), "Field Trial of Inactivated Oral Cholera Vaccines in Bangladesh: Results from five years of follow­up." Vaccine, 14(2), pp.162­7.

D.D. Tach et al. (1997), "Field Trial of a Locally Produced, Killed, Oral Cholera Vaccine in Vietnam." Lancet, vol. 349, pp.231­5.

C.O. Tacket et al. (1995), "Initial Studies of CVD 112 Vibrio cholerae O139 Live Oral Vaccine: Safety and efficacy against experimental challenge." Journal of Infectious Diseases, 172(3). pp.883­6.