Identifying high-risk areas for sporadic measles outbreaks: lessons from South Africa
Benn Sartorius, C Cohen, T Chirwa, G Ntshoe, A Puren & K Hofman
Volume 91, Number 3, March 2013, 174-183
Table 1. Risk of measles in infants during an outbreak, South Africa, 2009–2011
|Parameter||Multivariate analysis (coverage)a
||Multivariate analysis (immunization)b
|RRc (95% BCId)||RRe (95% CI)||RRc (95% BCId)||RRe (95% CI)|
|Low- or medium-population-density nonmetropolitan area||1 (ref.)||NA||1 (ref.)||NA|
|High-population-density nonmetropolitan area||1.23 (0.64–2.43)||NA||1.24 (0.65–2.54)||NA|
|High-population-density metropolitan area||4.37 (2.27–8.56)||NA||4.42 (1.60–9.54)||NA|
|Proportion of infants not covered by vaccinationf in preceding yearg||7.84 (5.19–13.2)||NA||NA||NA|
|Proportion of infants not immunizedh in preceding yearg||NA||NA||12.88 (6.52–32.43)||NA|
|Interaction between infant population susceptible to measles and population densityi|
|Proportion of infants not covered by vaccinationf in the preceding year × high population density||NA||66.20 (40.49–108.25)||NA||NA|
|Proportion of infants not covered by vaccinationf in the preceding year × low or medium population density||NA||7.45 (4.72–11.77)||NA||NA|
|Proportion of infants not immunizedh in the preceding year × high population density||NA||NA||NA||89.35 (50.65–157.60)|
|Proportion of infants not immunizedh in the preceding year × low or medium population density||NA||NA||NA||8.89 (5.14–15.38)|
BCI, Bayesian credibility interval; CI, confidence interval; NA, not applicable; RR, risk ratio.
a The analysis considered infants aged under 1 year who were not covered by vaccination in the preceding year (i.e. they did not receive their first measles vaccine dose).
b The analysis considered infants aged under 1 year who were not immunized in the preceding year (i.e. they were not covered by vaccination or failed to seroconvert following the first vaccine dose).
c The multivariate association between parameters and the incidence of measles among infants was evaluated using a Bayesian Poisson conditional autoregressive convolution model.
d The 95% Bayesian credibility interval is analogous to the conventional 95% confidence interval.
e The multivariate association between parameters and the incidence of measles among infants was evaluated using a conventional Poisson approach.
f The proportion of infants that did not receive the first measles vaccine dose.
g The risk ratio is for a 1% increase, on a continuous scale, in the proportion of infants who were covered by vaccination or immunized.
h The formula used to calculate this proportion is described in the methods.
i Since there was a strong, significant correlation between population density and metropolitan area (i.e. Spearman ρ = 0.64, P < 0.001), the interaction between metropolitan area and high population density was not included.