Background
The largest recorded outbreak of a circulating vaccine-derived poliovirus (cVDPV),
detected in Nigeria, provides a unique opportunity to analyze the pathogenicity of
the virus, the clinical severity of the disease, and the effectiveness of control measures
for cVDPVs as compared with wild-type poliovirus (WPV).
Methods
We identified cases of acute flaccid paralysis associated with fecal excretion of type 2
cVDPV, type 1 WPV, or type 3 WPV reported in Nigeria through routine surveillance
from January 1, 2005, through June 30, 2009. The clinical characteristics of these
cases, the clinical attack rates for each virus, and the effectiveness of oral polio vaccines
in preventing paralysis from each virus were compared.
Results
No significant differences were found in the clinical severity of paralysis among the
278 cases of type 2 cVDPV, the 2323 cases of type 1 WPV, and the 1059 cases of type
3 WPV. The estimated average annual clinical attack rates of type 1 WPV, type 2
cVDPV, and type 3 WPV per 100,000 susceptible children under 5 years of age were 6.8
(95% confidence interval [CI], 5.9 to 7.7), 2.7 (95% CI, 1.9 to 3.6), and 4.0 (95% CI,
3.4 to 4.7), respectively. The estimated effectiveness of trivalent oral polio vaccine
against paralysis from type 2 cVDPV was 38% (95% CI, 15 to 54%) per dose, which
was substantially higher than that against paralysis from type 1 WPV (13%; 95% CI,
8 to 18%), or type 3 WPV (20%; 95% CI, 12 to 26%). The more frequent use of serotype
1 and serotype 3 monovalent oral polio vaccines has resulted in improvements
in vaccine-induced population immunity against these serotypes and in declines in
immunity to type 2 cVDPV.
Conclusions
The attack rate and severity of disease associated with the recent cVDPV identified
in Nigeria are similar to those associated with WPV. International planning for the
management of the risk of WPV, both before and after eradication, must include
scenarios in which equally virulent and pathogenic cVDPVs could emerge.