|In 1998, 2859 cases of mumps were reported
to the communicable disease surveillance system (rate 7.3 cases per 100
000 population). In 1999, 4056 cases were reported (10.3/100 000); of
these, 55% occurred in the autonomous regions of the Balearic Isles,
Valencia, and the Canary Islands. These cases were mainly vaccinated
children aged under 6 years, in populations to which the Rubini strain
vaccine had been given in previous years.
Figure 2 shows that a high percentage of cases in children under 9
years of age occurred among the vaccinated segment, a pattern possibly
explained by the high vaccine coverage in such cohorts. From this age
upwards an increase is seen in the number of cases among unvaccinated
people. This situation illustrates the two types of outbreak currently
occurring in Spain - one in late childhood and adolescence, mostly among
an unvaccinated population, associated with large numbers of cases, the
other in preschool or young schoolchildren affecting a percentage of the
vaccinated population, but resulting in fewer cases.
The descriptive epidemiology of the two outbreaks, in Almoradí and
Manacor, was studied and a vaccine efficacy study was then carried out.
Information used for the descriptive study was obtained from a systematic
survey of patients, and completed with information from clinical
histories, vaccination registers, school records, vaccination cards, and
The case definition contained in the protocols of the national
epidemiological surveillance system (8) was adapted to the realities of
both outbreaks, with ‘case’ defined as any individual resident in
either epidemic territory who presented with acute uni- or bilateral
inflammation of the parotid or other gland that lasted two or more days,
between September 1998 and March 1999.
The term ‘vaccinated’ was applied to any person for whom
information was documented in any of the sources used, the term ‘unvaccinated’
to any person for whom no information was documented in any of the above
sources, and the term ‘unknown’ to anyone for whom no information was
To estimate the efficacy of the vaccine, a retrospective cohort study
was carried out. The following were taken as reference populations: in
Almoradí, children born in 1990-7 (n=1018); and in Manacor,
schoolchildren born from 1992 and enrolled at the primary school that
registered the highest attack rate (n=162). Cohorts were constructed
according to the respective time periods during which the different
strains of vaccine had been used in each region and were assigned a
reference population that, on the basis of its date of birth, had been
scheduled for vaccination in those periods. All subjects in the respective
cohorts were individually classified as case or non-case, and vaccination
status and date were ascertained from vaccination cards, clinical
histories, vaccination registers, and school records.
On the basis of this information, three cohorts were defined in
Almoradí: a Urabe/Jeryl-Lynn cohort comprising 328 children, a Rubini
cohort comprising 422 children, and a Jeryl-Lynn cohort comprising 268
children. In the case of Manacor, a Rubini cohort, based on a school
population of 124 children, was considered.
The measure of vaccine efficacy (VE) was calculated on the basis of
relative risk (RR), by applying the formula, VE=1-RR (9).
Both epidemics began in late September 1998 and lasted over seven
months (cases were enrolled in the study until April 1999, although
neither outbreak had ended by that time). The attack rates were 3% in
Almoradí and 1% in Manacor.
The two epidemics were similar in the presentation of cases in time,
age of people affected, and vaccination status. Spread took place through
day care centres and schools, which acted as the true nodal centres,
without any indication of clustering by home or neighbourhood.
The most notable feature of the two outbreaks was their distribution by
age – children aged 2 to 5 years were affected, although 93% of cases
were vaccinated. In Almoradí, the highest rates were in children aged 4
to 5 years, whereas in Manacor the population most affected was children
aged 2 to 4 years (table 1).
Epidemic outbreaks of mumps in
Almoradí and Manacor. Attack rate (%) by age