BMJ  2005;330:844-845 (9 April), doi:10.1136/bmj.330.7495.844-b

Letter

Non-specific effects of vaccination

Survival bias may explain findings

The first 150 words of the full text of this article appear below.

EDITOR—Vaugelade et al found the BCG and diphtheria-tetanus and pertussis vaccines (DTP) to be associated with reductions in mortality greater than expected from disease prevention.¹ However, as they admit, dead vaccinated children may have been misclassified because their vaccination cards were destroyed.

In a survival analysis using vaccination dates as time dependent covariates, surviving children change status at date of vaccination even though vaccinations were only known later, whereas dead vaccinated children and their follow up time remain in the unvaccinated group.^{2 3} As a result, risk free survival time will be allocated to the vaccinated group, creating survival bias.² Consequently, mortality is too high in unvaccinated children; 45% of children were vaccinated before age 6 months, but only 12 vaccinated children died compared with 435 unvaccinated children. This corresponds to an unlikely mortality that is 10-15 times as high among unvaccinated children younger than 6 months. In contrast, . . . [Full text of this article]

psb@mail.gtelecom.gw, Bandim Health Project, Danish Epidemiology Science Centre, Statens Serum Institute, Apartado 861, Bissau, Guinea-Bissau

Bandim Health Project, Danish Epidemiology Science Centre, Statens Serum Institute, Apartado 861, Bissau, Guinea-Bissau

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