A spatial model of Wild Poliovirus Type 1 in Kano State, Nigeria: calibration and assessment of elimination probability.
BACKGROUND
Since the launch of the Global Polio Eradication Initiative, all but three countries (Nigeria, Pakistan, and Afghanistan) have apparently interrupted all wild poliovirus (WPV) transmission, and only one of three wild serotypes has been reported globally since 2012. Countrywide supplemental immunization campaigns in Nigeria produced dramatic reduction in WPV Type 1 paralysis cases since 2010 compared to the 2000's, and WPV1 has not been observed in Nigeria since July 24, 2014. This article presents the development and calibration of a spatial metapopulation model of wild poliovirus Type 1 transmission in Kano State, Nigeria, which was the location of the most recent WPV1 case and 5 out of 6 of the reported WPV1 paralytic cases in Nigeria in 2014.


METHODS
The model is calibrated to data on the case counts and age at onset of paralysis from 2003-2009. The features of the data drive model development from a simple susceptible-exposed-infective-recovered (SEIR) model to a spatial metapopulation model featuring seasonal forcing and age-dependent transmission. The calibrated parameter space is then resampled, projected forward, and compared to more recent case counts to estimate the probability that Type 1 poliovirus has been eliminated in Kano state.


RESULTS
The model indicates a 91 % probability that Type 1 poliovirus has been eliminated from Kano state as of October 2015. This probability rises to >99 % if no WPV1 paralysis cases are detected for another year. The other states in Nigeria have experienced even longer case-free periods (the only other state with a WPV1 case was Yobe, on April 19, 2014), and Nigeria is the last remaining country in Africa to experience endemic WPV1 transmission, so these results can be interpreted as an upper bound on the probability that WPV1 transmission is currently interrupted continent-wide.


CONCLUSIONS
While the results indicate optimism that WPV1 transmission has been interrupted in Kano state, the model also assumes that frequent SIAs with high coverage continue to take place in Kano state through the end of the certification period. We conclude that though WPV1 appears to be on the brink of continent-wide elimination (WHO officially removed Nigeria from the list of polio-endemic countries on September 25, 2015), it is important for the polio program to maintain vigilance in surveillance and vaccination activities to prevent WPV1 resurgence through the WHO's 3-year eradication certification period.
Intellectual Ventures Laboratory, 3150 139th Ave SE, Bellevue, WA, 98005, USA. kmccarthy@intven.com.
Kevin A
KA
McCarthy
Intellectual Ventures Laboratory, 3150 139th Ave SE, Bellevue, WA, 98005, USA.
Guillaume
G
Chabot-Couture
Federal Ministry of Health, Abuja, Nigeria.
Faisal
F
Shuaib
MMWR Morb Mortal Wkly Rep. 1999 Apr 23;48(15):312-6
10227799
Pediatrics. 1959 Jun;23(6):1041-62
13657588
Proc Natl Acad Sci U S A. 2004 Nov 30;101(48):16915-6
15557003
PLoS Med. 2005 Jan;2(1):e6
15696216
Rev Infect Dis. 1991 Sep-Oct;13(5):926-39
1660184
MMWR Morb Mortal Wkly Rep. 1997 Apr 18;46(15):321-5
9132585
PLoS Pathog. 2012;8(4):e1002599
22532797
MMWR Morb Mortal Wkly Rep. 1999 Jun 25;48(24):513-8
10401908
MMWR Morb Mortal Wkly Rep. 2001 Mar 30;50(12):222-4
11300627
BMC Infect Dis. 2015 Feb 18;15:66
25886823
J Infect Dis. 1997 Feb;175 Suppl 1:S176-82
9203713
N Engl J Med. 2008 Oct 16;359(16):1655-65
18923170
PLoS One. 2015 Aug 28;10(8):e0135765
26317401
Am J Epidemiol. 1996 Apr 15;143(8):816-22
8610692
Nature. 2007 Mar 29;446(7135):533-6
17392785
Br Med J. 1961 Jul 29;2(5247):266-9
13722671
Lancet. 2010 Nov 13;376(9753):1682-8
20980048
Lancet Glob Health. 2014 Feb;2(2):e90-7
25104665
Am J Epidemiol. 2012 May 1;175(9):936-49
22459121
Proc Biol Sci. 2006 Oct 7;273(1600):2541-50
16959647
Bull World Health Organ. 1987;65(4):495-501
3500802
Am J Epidemiol. 2010 Dec 1;172(11):1213-29
20978089
Am J Epidemiol. 1972 Oct;96(4):263-9
4342327
MMWR Morb Mortal Wkly Rep. 2014 Nov 14;63(45):1031-3
25393222
N Engl J Med. 2016 Feb 11;374(6):501-3
26863350
N Engl J Med. 2010 Jun 24;362(25):2360-9
20573924
Science. 2006 Nov 17;314(5802):1150-3
17110580
Proc Biol Sci. 2010 Sep 22;277(1695):2775-82
20427338
Ecol Lett. 2006 Apr;9(4):467-84
16623732
N Engl J Med. 2008 Oct 16;359(16):1666-74
18923171
Emerg Infect Dis. 2003 May;9(5):573-9
12737741
Biometrics. 2010 Dec;66(4):1162-73
20222935
Science. 2011 Dec 9;334(6061):1424-7
22158822
Bull World Health Organ. 1986;64(6):853-60
3493854
Bull World Health Organ. 1985;63(6):1151-69
3914926
Math Biosci. 1998 Jun;150(1):83-103
9654894
BMC Med. 2014 Jun 04;12:92
24894345
MMWR Morb Mortal Wkly Rep. 1999 May 28;48(20):416-21
10365631
Int J Infect Dis. 2014 Jan;18:4-13
24246740
PLoS One. 2012;7(2):e31743
22363717
J Immunol. 1960 Jul;85:46-55
13827179
J Infect Dis. 1997 Feb;175 Suppl 1:S113-6
9203702
Br Med J. 1976 Apr 3;1(6013):812
1260345