Jesús Castillaa (b), Manuel García Cenoza (b), Maite Arriazua (b), Mirian Fernández-Alonso (c), Víctor Martínez-Artola (d), Jaione Etxeberria (a,b), Fátima Irisarri (a,b) and Aurelio Barricarte (a,b). Effectiveness of Jeryl Lynn-containing vaccine in Spanish children. Volume 27, Issue 15, 26 March 2009, Pages 2089-2093

(a) Instituto de Salud Pública de Navarra, Pamplona, Spain
(b) CIBER de Epidemiología y Salud Pública (CIBERESP), Spain
(c) Clínica Universitaria de Navarra, Pamplona, Spain
(d) Hospital Virgen del Camino, Pamplona, Spain

We evaluated the effectiveness of the Jeryl Lynn strain vaccine in a large outbreak of mumps in Navarre, Spain, 2006-2008. Each of the 241 cases of mumps occurring in children over 15 months of age born between 1998 and 2005 was compared with 5 controls individually matched by sex, birth date, district of residence and paediatrician. vaccination history was obtained blindly from clinical records. Conditional logistic regression was used to obtain the matched odds ratios (ORs), and effectiveness was calculated as 1 - OR. Some 70% of cases had received one dose of measles-mumps-rubella vaccine, and 24% had received two doses. Overall vaccine effectiveness was 72% (95% CI, 39-87%). Two doses were more effective (83%; 54-94%) than a single dose (66%; 25-85%). Among vaccinated children, risk was higher in those who had received the first dose after 36 months of age (OR = 3.1; 1.2-8.4) and those who had received the second dose 3 or more years before study enrolment (OR = 10.2; 1.5-70.7). Early waning of immunity in children after the second dose may contribute to reduced vaccine effectiveness for mumps prevention.

The vast majority of cases were in the vaccinated who received one or two doses of MMR. This is said to occur because a vaccine's "effectiveness" is never 100% and thus a percentage of vaccinated will still get the disease. It appears to be a good way to explain the failure of the vaccine and we will explore this issue in more detail in other articles, looking at "vaccine efficacy", the underlying assumptions involved, and the formulas used to calculate such efficacy.