A new modeling study appearing this week in PLOS Medicine supports consideration of vaccination campaigns using a single dose of cholera vaccine versus campaigns using the recommended two doses given two weeks apart. Justin Lessler and colleagues, of Johns Hopkins Bloomberg School of Health, Baltimore, Epicentre, Paris and Médecins Sans Frontières, Geneva focus their modelling analyses on comparing the number of lives that could be saved by adopting a single vaccine dose, which could be more rapidly administered to more people than the internationally licensed two dose protocol.
The authors calculate the minimum relative single-dose efficacy at which a single-dose campaign is expected to be as or more effective than a two-dose one. They base this analysis on mathematical models of cholera epidemics and use data from cholera outbreaks in Haiti, Zimbabwe and Guinea to illustrate their results. They estimate that a single dose of vaccine would need to be 35-56% as effective as two doses to prevent the same number or more deaths. Interestingly this threshold decreases when vaccination is delayed until later in an epidemic. The 2010 outbreak of cholera in Port-au-Prince, Haiti, for example, resulted in over 119,000 cases of cholera. The authors estimate that a one-dose vaccination campaign reaching half the population would have averted over 78, 000 cases and prevented 738 deaths.
If stockpiles of the cholera vaccine were limitless (the current global stockpile has less than 2 million doses) one would not have to make such difficult decisions as to whether to modify a vaccination policy consistent with clinical guidelines; however, in outbreaks or following natural disasters when normal sanitation is derailed, difficult decisions need to be made to save as many lives as possible. This study highlights the fact that while using a single dose may provide less protection to those who are vaccinated if they are actually exposed to cholera, a single-dose approach quickly maximizes the number of people immune to cholera, ultimately saving more people. The implications may trigger a re-evaluation of vaccination policies in response to cholera outbreaks and in crisis situations where cholera risk is high but limited doses of vaccine are available.