Measles on Merseyside – failure of herd immunity
From 1 January to 30 June 2012, 359 confirmed and 157 probable cases of measles were reported in Merseyside, England. The most affected age groups were children under five years and young adults from 15 years of age. Most cases have been sporadic. There have been few outbreaks in nurseries; however, no outbreaks have been reported in schools. Of the cases eligible for vaccination, only 3% of the confirmed cases were fully immunised.
“This measles outbreak is by far the largest in the North West of England at least since 1996. Around a quarter of cases have, so far occurred in children too young to be vaccinated. This age group is normally dependant on passive immunity (maternal antibodies) or herd immunity for protection. Given that the uptake of the first dose of MMR vaccine locally in recent years has been relatively high, this large number of cases can partially be explained by the lower levels of immunity in older children and teenagers, and by lower levels of maternal antibodies passed on to babies from vaccinated mothers.”