Interpreting changes in measles genotype: the contribution of chance, migration and vaccine coverage.
BACKGROUND
In some populations, complete shifts in the genotype of the strain of measles circulating in the population have been observed, with given genotypes being replaced by new genotypes. Studies have postulated that such shifts may be attributable to differences between the fitness of the new and the old genotypes.


METHODS
We developed a stochastic model of the transmission dynamics of measles, simulating the effects of different levels of migration, vaccination coverage and importation of new genotypes on patterns in the persistence and replacement of indigenous genotypes.


RESULTS
The analyses illustrate that complete replacement in the genotype of the strain circulating in populations may occur because of chance. This occurred in >50% of model simulations, for levels of vaccination coverage and numbers of imported cases per year which are compatible with those observed in several Western European populations (>80% and >3 per million per year respectively) and for the given assumptions in the model.


CONCLUSIONS
The interpretation of genotypic data, which are increasingly being collected in surveillance programmes, needs to take account of the underlying vaccination coverage and the level of the importation rate of measles cases into the population.
Nojiri
Shuko
S
Department of Pharmacoepidemiology, Faculty of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. snojiri-circ@umin.ac.jp
Vynnycky
Emilia
E
Gay
Nigel
N
D003627:Data Interpretation, Statistical; D040961:Genetic Drift; D005838:Genotype; D006801:Humans; D007564:Japan; D008457:Measles / Q000453:epidemiology / Q000517:prevention & control / Q000635:transmission* / Q000821:virology; D008458:Measles Vaccine / Q000008:administration & dosage; D008459:Measles virus / Q000235:genetics*; D013269:Stochastic Processes; D014611:Vaccination
D008458:Measles Vaccine