There is no fixed cut-off figure of disease burden that dictates a
national vaccination policy for an infectious disease. This decision has
to be based on calculations taking into account burden of disease (number,
complications, morbidity/mortality), epidemiology with respect to host and
organism, transmission pattern, efficacy and effectiveness of the
intervention (vaccine), safety profile, absolute cost of vaccine and
vaccination program, cost-effectiveness, expected short and long term
outcome, and the likely impact of the absence of a policy on the same.
Although the investigators of the paper(1) claimed that the burden of
typhoid is large enough to warrant vaccination in India, their data do not
support this assertion.
The importance of a specific definition of typhoid
(based on blood culture) is that (i) this is what has been used to
calculate disease burden in various studies; (ii) calculation of
vaccine efficacy from various trials is based on this definition; (iii)
the ratio of blood-culture negative to blood-culture positive ‘typhoid
cases’ is not known; and (iv) if a more sensitive but less specific
definition/test of typhoid is used, many non-typhoid cases would be
included(2) in whom the vaccine(s) would be expected to be efficacious,
but will not be. Thereby overall effectiveness would decrease, and not
increase.
Neither the detection of culture proven typhoid cases
nor the ‘large’ number of suspected typhoid cases in young children can be
taken as evidence that "the incidence is going up even in children
around two years of age."
Cochrane reviews are meant to aid decision-making
processes, and not dictate the decision to be taken. However, it should be
noted that the review on typhoid vaccines(3) did not identify trials
comparing different typhoid vaccines against each other; in fact most
trials compared one of the typhoid vaccines with a placebo/control
vaccine. Therefore interpreting this information to suggest that a
particular typhoid vaccine is superior, indeed amounts to assumption by
extrapolation.
References
1. Ochiai RL, Acosta CJ, Danovaro-Holliday MC, Baiqing
D, Bhattacharya SK, Agtini MD, et al. A study of typhoid
fever in five Asian countries: disease burden and implications for
controls. Bull WHO 2008; 86: 260-268.
2. Mweu E, English M. Typhoid fever in children in
Africa. Trop Med Int Health 2008; 13: 532-540.
3. Fraser A, Goldberg E, Acosta CJ, Paul M, Leibovici
L. Vaccines for preventing typhoid fever. Cochrane Database Syst Rev 2007:
3: CD001261.