Vaccines for Cholera Control: Does Herd Immunity Play a Role

Cholera is a diarrhoeal disease caused by Vibrio cholerae O1 and O139, transmitted through the faeco-oral route. The disease occurs in outbreaks but can establish itself permanently. The full impact of the disease is difficult to assess. The currently preferred measure of disease burden, disability-adjusted life years, fails to capture the enormous impact of a cholera outbreak, which spares no age group and paralyses the economy in severely affected areas. The seventh cholera pandemic began in Indonesia in 1961 and spread quickly to other Asian countries, which became the epicentre of cholera outbreaks. With the economic emergence of Asia the number of cholera cases reported from that region has decreased. There are several possible reasons to explain this decline. First, massive investment has been made in providing a safe water supply and in sanitation. Second, reporting of cholera has become less reliable, because global trade— especially trade in seafood—and tourism are negatively affected by cholera outbreak reports. In 1970 Vibrio cholerae O1 El Tor invaded sub-Saharan Africa, which had not experienced cholera for more than 100 years. In 2006, Africa reported 234,349 cases of cholera to the World Health Organization (WHO), accounting for 99% of the officially notified global cholera [1]. Between 1995 and 2005, 66% of cholera outbreak reports to ProMedmail (a global electronic reporting system for outbreaks of emerging infectious diseases and toxins, run by the International Society for Infectious Diseases, at http://www.promedmail. org/) came from sub-Saharan Africa [2]. There is growing evidence of the large and increasing burden of cholera in Africa. Most recently the US-led invasion of Iraq has been accompanied by a re-emergence of cholera in that country. As of September 2007, nearly 7,000 cholera cases from the Sulaymaniyah and Kirkuk Governates have been reported to WHO [3]. Cholera was eliminated from the industrialized world through safer water supplies, better sanitation, and improved food hygiene. These have been the accepted control mechanisms for the disease, but as the emergence of cholera in Iraq illustrates, the provision of safe water and sanitation breaks down during wars and complex humanitarian emergencies. In addition to these crisis situations, cholera also thrives in the ever-increasing slums of some megacities such as Kolkata (formerly Calcutta), India, which are not quickly accessible to improvements in infrastructure. In 2002, WHO mentioned for the first time the potential use of oral cholera vaccines in endemic and epidemic situations [4]. Up to that point …