I would like to know what is the best approach between prevented number of cases for every 1000 vaccinated individuals and the decreasing lethality of cholera in the vaccinated region when it comes to assess the Oral Cholera Vaccine Efficacy during a mass vaccination campaign in Sub Saharan Africa.
Hey Diego - most relevant question. First, let us both read these 2 papers:
We will continue with the discussion. Good day !!
It might be a ''before-after" study. Incidence rate of cholera using person-time determined before (risk in exposed) and after (risk in unexposed) vaccination campaign can be compared. The number of preventable cases is calculated as follows : number of person-time in exposed group * risk difference. The etiologic fraction in exposed to assess the vaccine efficacy is : (RR-1)/RR = (number of preventable cases)/(number of cases in exposed group = (risk difference)/(risk in exposed).
NB : RR = risk ratio.
IN your study you must be sure to allow enough time for the disease to develop after people are vaccinated. Otherwise you run the risk of believing your vaccine is more effective than it really is.
You might also consider two outcome measures: severity (or duration) of disease, and death. But you will have to be sure you have good ascertainment of cases, i.e., every outcome is counted in both the vaccinated and unvaccinated groups.
It is possible that the vaccine will make the the cholera infection less severe rather than prevent it altogether. Just look at the rates in the vaccinated versus the unvaccinated groups, considering person-time.
Ciao Diego,
you have to set up a disease surveillance for cholera in the subgroup of subjects you intend to evaluate for vaccine efficacy. You need to have a case defintion-or more than one according severity-as Jerry suggests- but mainly according the level of laboratory/clinical ascertainment you'll have in the field.
You'll need to have cholera incidence over a certain period of time in the same population before vaccination. You'll then be able to estimate the sample size of the population you intend to follow for cholera after vaccination to be able to estimate VE with some precision.
Good luck and keep me posted how it goes.
Best wishes, Simonetta
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