Vaccination Coverage
Between 1995 and 2002, quarterly and annual data on coverage of MMR in two and five year old children, collected as part of the COVER (Cover of Vaccination Evaluated Rapidly) programme, was available by former district health authorities (DHA) [8]. Since the reorganisation of the NHS in 2002, data for England is collected from primary care trusts (PCTs). To provide consistent data for all childhood cohorts, coverage from each PCT was aggregated to the district health authority (DHA) configuration prior to 2002 and to the Strategic Health Authority (SHA) configuration as over the period 2002–2006.
At age two years the denominator and the number of these children who have received one dose of MMR are reported; at age five years the denominator, the number of these children who have received at least one dose of MMR, and the number who have received two doses of MMR are reported. This enables the proportion of the cohort who have received no doses, one dose only and two or more doses to be calculated.
Three procedures were adopted to deal with missing and anomalous coverage data.
1) Interpolation was performed to eliminate inconsistent and missing data for any one quarter. When data for a single quarter was missing, the data was assumed to be the same as the previous quarter. Where data for more than one successive quarter was not available, linear interpolation from the last available quarter to the next available quarter was used.
2) The coverage of MMR in children born in 1990 and 1991 was not captured by routine data at five years of age, so coverage of the first dose at this age was assumed to be the same as for children born after March 1992 (available from COVER). Children born between January 1990 and March 1992 were neither in the target age group for the national MR catch-up campaign in 1994 nor eligible for routine MMR2, but were scheduled to receive a second dose in a catch-up in October 1996. Coverage in this group varied from 47.6% in South Thames region to 66.8% in Northern and Yorkshire and so it was assumed that 50% of these cohorts received the second dose.
3) Data at age two years is believed to be reasonably accurate, but coverage at age five years is thought to under-estimate the true coverage in many trusts [9]. This may be caused by children who have left the area not being removed from the denominator or by incomplete recording of vaccination history for children who move into the area. For example, eight DHAs reported a lower vaccination coverage for one dose at five years than for the same cohort at two years of age. To correct this problem, coverage at five years of age was corrected to be at least 3% higher than the two year old figure in each district. This increase in coverage was the mean increase observed between two and three years of age in sentinel trusts.
In addition, a sensitivity analysis was conducted to estimate the potential impact of under-estimating coverage. An audit of data quality for children born between July and September 1995 in 12 London PCTs in 2001 suggested that around 24% (201/836) of children recorded as unvaccinated for MMR at five years of age had received at least one dose of vaccine. Therefore, our analysis was repeated assuming that 10%, 20%, 30%, 40% and 50% of children recorded as unvaccinated had received one dose of vaccine and that 10%, 20%, 30%, 40% and 50% of those who were recorded as receiving a single dose had also received the second dose.
Susceptibility
The relevant quarterly birth cohorts were aggregated to approximate each school year (born September–August) and vaccine coverage for MMR1 and MMR2 calculated. For example, children born between October 1999 and September 2000 comprised the four year old cohort in the school year 2004/2005. Susceptibility was then estimated for the pre-school group (aged 0–4 years), infant and junior school (aged 5–10 years), secondary school (aged 11–17 years), college (18–24 years) and older (25 years or more) [10].
Cohorts born since 1990 have almost no exposure to natural measles infection, so the proportion susceptible in each age cohort is calculated as follows:
Children are assumed to be immune for the first six months of life through maternal antibody. For cohorts born before 1990, susceptibility was assumed to be 5% for secondary school and college (14–24 years) and 2% for older (25 years+) based upon sero-prevalence studies conducted after the national vaccination campaign [11, 12].
To calculate the susceptibility to measles in years after 2004/2005 it was assumed that vaccination coverage remained stable at 2004/2005 levels. This assumption was necessary for two reasons. First, coverage data from April–June 2005 to date has been incomplete due to problems with the implementation of new child health computer systems in several London PCTs [13]. Second, from October 2006 reconfiguration of the number of PCTs in England from 303 to 152, means that mapping these data to old DHA areas would be more complex [14].
Effective Reproduction Number
The effective reproduction number (R) for each previous DHA and SHA was calculated from the Next Generation Matrix (NGM) accounting for the proportion susceptible in each age group [11, 15]. This matrix accounts for age-specific heterogeneity in measles transmission. The values for a totally susceptible population are shown below, which give an R0 of 10.7.
For areas where R exceeds one, the potential outbreak size was estimated as twice the number of infections required to reduce R to 1 in that area [3, 16].