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Week 26/2010: Vaccine-based polio is surprisingly dangerous

Polio can be prevented with vaccines, but over 30,000 polio infections are found globally every year. Almost one per cent of cases originate from a polio virus which has been refined to a vaccine and which has assumed a form that spreads the disease.

The majority of such cases – approximately 300 infections per year – are found in Nigeria, for which polio vaccinations pose quite a challenge. Almost half of Nigeria’s population of 150 million people is under 15 years of age. An annual total of about 3,600 cases of polio are reported in Nigeria, which means that over 8% of the infections reported in Nigeria are vaccine-based.

Usually, infections emerging from a mutant virus have been considered weakly infectious exceptional cases, so vaccinations have focused on the prevention of other virus types. However, an extensive analysis on the Nigerian situation revealed that vaccine-based polio may result in an epidemic just as easily as the usual disease.

“Only one polio vaccine in four in Nigeria is effective against vaccine-based polio. It should be the first choice in areas where the mutated virus constitutes a threat,” says Jukka Corander, Professor of Statistics of the University of Helsinki, who contributed to the study.

Corander was responsible for modelling the level of infections and the immunity of the population. The work required a complex probability model, since there were many unknown factors. “In Nigeria, you cannot accurately tell who has been administered which vaccine. There is only some information available on how many different vaccinations each age group has received.”

However, the results of the research, published last week in the New England Journal of Medicine, a leading publication in the sector, are clear. The models show how the results of a vaccination plan can clearly be improved by wisely combining different types of vaccines.

Text: Juha Merimaa
Photo: Veikko Somerpuro
Translation: AAC Global

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