In 1988, when an estimated 350,000 cases of poliomyelitis occurred in 125 countries,
the World Health Assembly resolved to eradicate polio globally. Transmission of wild
poliovirus (WPV) continues uninterrupted in only three countries (Afghanistan, Nigeria,
and Pakistan) (
1
), and among the three serotypes, WPV type 1 (WPV1) remains the only confirmed circulating
type. This report describes global progress toward polio eradication during January
2016–March 2018, and updates previous reports (
2
). In 2017, 22 WPV1 cases were reported, a 41% decrease from the 37 WPV1 cases reported
in 2016. As of April 24, 2018, eight WPV1 cases have been reported (seven in Afghanistan
and one in Pakistan), compared with five cases during the same period in 2017. In
Pakistan, continuing WPV1 transmission has been confirmed in multiple areas in 2018
by isolation from wastewater samples. In Nigeria, ongoing endemic WPV1 transmission
was confirmed in 2016 (
3
); although WPV was not detected in 2017 or in 2018 to date, limitations in access
for vaccination and surveillance in insurgent-held areas in northeastern Nigeria might
permit continued undetected poliovirus transmission. Substantial progress toward polio
eradication has continued in recent years; however, interruption of WPV transmission
will require overcoming remaining challenges to reaching and vaccinating every missed
child. Until poliovirus eradication is achieved, all countries must remain vigilant
by maintaining high population immunity and sensitive poliovirus surveillance.
Routine Poliovirus Vaccination Coverage
Among infants aged 1 year, the estimated global coverage with 3 doses of poliovirus
vaccines (Pol3, mostly oral poliovirus vaccine [OPV]) through routine immunization
services was 85% in 2016 (the most recent year for which data are available). World
Health Organization (WHO)/United Nations Children’s Fund estimates for Pol3 coverage
in 2016 were 73% in the African Region, 92% in the Region of the Americas, 80% in
the Eastern Mediterranean Region, 94% in the European Region, 87% in the South-East
Asia Region, and 95% in the Western Pacific Region, with heterogeneity in coverage
among countries in all regions.* National Pol3 coverage with the third dose of OPV
(OPV3) in the three countries with endemic WPV transmission in 2016 was 60% in Afghanistan,
72% in Pakistan, and 49% in Nigeria. OPV3 coverage is substantially lower in areas
of WPV transmission, where children in high-risk mobile populations or areas of conflict
are repeatedly missed (
4
,
5
). Rarely, in areas with low vaccination coverage, Sabin-like viruses can spread and
revert to neurovirulence, resulting in outbreaks of disease caused by circulating
vaccine-derived polioviruses (cVDPV). Approximately 90% of cVPDV cases reported since
2006 have been caused by type 2 (cVDPV2). In countries with recent cVDPV detections,
Pol3 coverage was 74% in the Democratic Republic of the Congo (DRC), 48% in Syria,
47% in Somalia, and 83% in Laos (
6
). In these countries, OPV3 coverage was substantially lower in subnational areas
with cVDPV emergence and transmission.
Following certification of the eradication of WPV type 2 (WPV2) in 2015, a global,
synchronized withdrawal of trivalent OPV (tOPV, containing types 1, 2, and 3 live,
attenuated polioviruses), and switch to bivalent OPV (bOPV, containing types 1 and
3 only), was completed by the end of April 2016 (
7
). Starting in 2015, injectable trivalent inactivated poliovirus vaccine (IPV) was
introduced into routine immunization schedules in OPV-using countries, generally at
14 weeks of age. Some countries had to delay introduction of IPV until 2018 because
of global shortages of the vaccine.
Supplementary Immunization Activities
In 2016, 186 supplementary immunization activities (SIAs) were conducted in five WHO
regions, during which approximately two billion total OPV and IPV doses were administered
(Table 1), including 1,264,552,301 (63%) doses administered during national immunization
days, 710,995,110 (36%) during subnational immunization days, and 17,603,036 (1%)
doses during focused SIAs in areas of known or suspected poliovirus circulation (“mop-up”
activities). In the event of cVDPV2 outbreaks, on advice of the monovalent OPV type
2 (mOPV2) Global Advisory Group, the WHO Director-General releases mOPV2 for outbreak
response immunization. Of the administered doses, more than half (51%) were tOPV and
approximately half (47%) were bOPV; an additional 1.4% were mOPV2, 0.05% were IPV
plus bOPV, 0.2% were IPV alone, and 0.15% were fractional IPV (0.1 mL administered
intradermally).
TABLE 1
Number of supplementary immunization activities (SIAs) conducted, and number of oral
poliovirus vaccine (OPV) and inactivated poliovirus (IPV) doses administered, by World
Health Organization (WHO) region — worldwide, 2016–2017
Year/SIAs/Vaccine doses administered
Region
Global
AFR
AMR
EMR
EUR
SEAR
WPR
2016
SIAs (no.)
186
97
0
67
2
14
6
Vaccine (no. of doses administered)
mOPV2
28,357,599
28,357,599
0
0
0
0
0
bOPV
940,622,006
274,197,570
397,909,506
54,880,271
206,507,773
7,126,886
tOPV
1,017,074,205
407,366,635
0
103,470,392
1,097,605
496,401,815
8,737,758
IPV
3,293,021
1,943,763
134,9258
0
0
0
IPV + bOPV
904,050
0
0
904,050
0
0
0
fIPV
2,899,566
0
0
252,354
0
2,647,212
0
Total doses
1,993,150,447
711,865,567
0
503,885,560
55,977,876
705,556,800
15,864,644
2017
SIAs (no.)
172
82
0
79
2
8
1
Vaccine (no. of doses administered)
mOPV2
70,356,186
65,067,196
0
5,288,990
0
0
0
bOPV
1,705,913,274
519,920,180
0
488,368,342
389,314
696,180,796
1,054,642
tOPV
0
0
0
0
0
0
0
IPV
3,522,237
558,897
0
2,963,340
0
0
0
IPV + bOPV
8,920,134
0
8,920,134
0
0
0
fIPV
0
0
0
0
0
0
0
Total doses
1,788,711,831
585,546,273
0
505,540,806
389,314
696,180,796
1,054,642
Abbreviations: AFR = African Region, AMR = Region of the Americas; bOPV2 = bivalent
oral poliovirus, types 1 and 3; EMR = Eastern Mediterranean Region; EUR = European
Region; fIPV = fractional dose inactivated poliovirus vaccine (one fifth of a 0.5
mL intramuscular dose, given intradermally); IPV = inactivated poliovirus vaccine;
mOPV2 = monovalent oral poliovirus, type 2; SEAR = South-East Asia Region; tOPV2 = trivalent
oral poliovirus, types 1, 2, 3; WPR = Western Pacific Region.
In 2017, 172 SIAs were conducted in five WHO regions, during which approximately 1.79
billion total OPV and IPV doses were administered, including 1,110,923,756 (62%) doses
administered during national immunization days, 672,091,158 (38%) during subnational
immunization days, and 5,696,917 (0.3%) during mop-up activities. Of the administered
doses, 95% were bOPV, 3.9% were mOPV2, 0.5% were IPV plus bOPV, and 0.2% were IPV
alone.
Poliovirus Surveillance
Surveillance for acute flaccid paralysis (AFP) is the means of detecting polio cases
caused by WPV or cVDPV, confirmed by stool specimen testing through the Global Polio
Laboratory Network. The performance of AFP surveillance is assessed through two main
indicators: sensitivity and completeness of case investigation. An annual nonpolio
AFP rate of ≥1 case per 100,000 population aged <15 years for countries in the WHO
regions certified as poliofree, or ≥2 for all other countries is considered sufficiently
sensitive to detect a case of polio, should it occur. Case investigation is considered
to be sufficiently complete if at least 80% of reported AFP cases have adequate stool
specimens collected (i.e., two stool specimens collected ≥24 hours apart, within 14
days of paralysis onset, with arrival at a WHO-accredited laboratory in good condition).
In 2016, among the four countries reporting polio cases, three (Afghanistan, Nigeria,
Pakistan) met both performance indicators and one (Laos) did not. Among the five countries
reporting polio cases in 2017, four (Afghanistan, DRC, Nigeria, Pakistan) met both
performance indicators and one (Syria) did not. Although Nigeria and DRC meet AFP
surveillance indicators nationally and subnationally in most provinces, both countries
are affected by substantial issues in population accessibility and other impediments
to AFP surveillance (
1
). AFP surveillance has been supplemented by environmental surveillance through testing
of sewage in many countries, including poliofree countries as well as those with endemic
transmission (
1
).
Reported Poliovirus Cases
Countries reporting WPV cases. In 2016, 37 WPV cases were detected (Figure): 13 (35%)
in Afghanistan, 20 (54%) in Pakistan, and four (11%) in Nigeria. In 2017, 22 WPV cases
were identified: 14 (64%) in Afghanistan and eight (36%) in Pakistan. No WPV cases
have been identified in countries outside of Afghanistan, Nigeria, and Pakistan since
2014. During January 1–March 30, 2018, as of April 24, the low poliovirus transmission
season, eight WPV1 cases were reported (seven in Afghanistan; one in Pakistan) (Figure)
(Table 2).
FIGURE
Number of cases of wild poliovirus, by month of onset — worldwide, January 2015–March
2018*
* Data as of April 24, 2018.
The figure above is a histogram showing the number of worldwide cases of wild poliovirus,
by month of onset, during January 2015–March 2018.
TABLE 2
Number of reported polio cases, by country — Worldwide, January 1, 2016–March 30,
2018*
Classification/Country
2016 (Jan 1–Dec 31)
2017 (Jan 1–Dec 31)
2017 (Jan 1–Mar 30)
2018 (Jan 1–Mar 30)
WPV
cVDPV
WPV
cVDPV
WPV
cVDPV
WPV
cVDPV
Countries with endemic polio
Afghanistan
13
0
14
0
3
0
7
0
Pakistan
20
1
8
0
2
0
1
0
Nigeria
4
1
0
0
0
0
0
0
Total cases in endemic countries
37
2
22
0
5
0
8
0
Other countries with reported cVDPV cases
Laos
0
3
0
0
0
0
0
0
Democratic Republic of the Congo
0
0
0
22
0
0
0
3
Syria
0
0
0
74
0
0
0
0
Total cases in other countries
0
3
0
96
0
0
0
3
Total paralytic polio cases
37
5
22
96
5
0
8
3
Abbreviations: cVDPV = circulating vaccine-derived poliovirus; WPV = wild poliovirus.
* Data as of April 24, 2018.
Afghanistan reported 13 WPV1 cases in four districts in 2016, compared with 14 WPV1
cases in nine districts in 2017 (7.7% increase). In 2016, 54% of WPV1 cases in Afghanistan
were reported from Paktika province in the southeastern region. In 2017, 50% of WPV1
cases were reported from Kandahar province in the southern region. During January
1–March 30, 2018, seven WPV1 cases were detected (four in Kandahar province, one in
Nangahar province, and two in Kunar province; the latter two provinces are in the
eastern region), compared with three WPV1 cases detected during the same period in
2017.
Pakistan reported a 60% decrease in the number of WPV1 cases, from 20 cases in four
districts in 2016 to eight cases in seven districts in 2017. During January 1–March
30, 2018, one WPV1 case was reported (in Balochistan province), compared with two
reported during the same period in 2017. WPV1 continues to be isolated from environmental
surveillance sites in five provinces of the country (Balochistan, Islamabad, Khyber
Pakhtunkhwa, Punjab, and Sindh).
Nigeria reported four WPV1 cases in 2016. No WPV1 cases were reported in 2017 and
none to date in 2018.
Countries reporting cVDPV cases and isolations. In 2016, five cVDPV cases were reported
from three countries (
8
). In Laos, an outbreak that began with eight cVDPV type 1 cases in 2015 continued
into 2016 with three additional cases reported. One cVDVPV2 case was reported in 2016
in Nigeria and another in Pakistan. In 2017, a total of 96 cVDPV2 cases were reported,
including 74 cases from Syria (most recent case in September 2017) and 22 from DRC.
The outbreak in DRC has continued into 2018, with four cases to date, as of April
24, 2018 (the most recent case occurring in February) (
9
). Isolation of cVDVP2 from environmental samples in Mogadishu, Somalia, in late 2017
and early 2018, and related cVDPV2 from environmental samples in Nairobi, Kenya, in
early 2018, has confirmed long-term cVDPV2 transmission, in a broad area, although
no associated polio cases have been detected to date. cVDPV type 3 has been isolated
in Mogadishu from sewage samples collected in March 2018, again, with no associated
polio cases having been detected to date. In Nigeria, cVDPV2 has been recently detected
by environmental surveillance in two states in early 2018; no associated polio cases
having been detected to date. Response immunization is underway or planned for all
these cVDPV cases and isolations.
Discussion
Although substantial progress was made toward polio eradication during 2016–2017,
challenges remain in the countries with endemic transmission. Continued circulation
of WPV1 has been confirmed in Afghanistan and Pakistan in the 2018 low WPV season,
and it remains uncertain if WPV circulation has been interrupted in Nigeria (
3
).
The number of WPV cases in Afghanistan declined from 2015 to 2016, but the decrease
did not continue in 2017. Although negotiations to obtain local access are constantly
being undertaken, the number of children who were inaccessible to vaccination in the
south and east because of insecurity increased during 2017 (
5
). In Pakistan, a decline in WPV1 cases since 2014 continued during 2016 and 2017.
The detection of WPV in environmental surveillance samples in the absence of WPV-positive
AFP cases in several provinces might indicate either surveillance gaps or waning in
the intensity of transmission. Intensified SIA schedules and efforts to reach previously
unvaccinated children, along with expansion of community-based initiatives employing
local permanent vaccinators and ensuring worker safety have helped reduce the number
of WPV cases. Large-scale movement of high-risk populations across Pakistan’s border
with Afghanistan in both directions continues to pose a challenge to interrupting
WPV transmission, and crossborder collaborative vaccination efforts made in 2017 are
being enhanced in 2018 (
4
).
In Nigeria, WPV1 circulation went undetected from mid-2014 to mid-2016, and the discovery
of both endemic WPV1 and long-standing cVDVP2 transmission in 2016 in Borno State
illuminated gaps in surveillance. Continued inaccessibility of insurgent-held areas
hinders both immunization and surveillance efforts (
3
). Enhancement of initiatives for collaborating with the military to reach currently
unvaccinated children will be helpful in ensuring interruption of WPV transmission.
In the other countries of the Lake Chad basin bordering Borno State (Cameroon, Chad,
and Niger), problems with inaccessibility related to insecurity and a large number
of difficult-to-access islands have been addressed through progressive improvements
in microplanning and implementation of SIAs, but uncertainties remain regarding SIA
quality and success in interrupting undetected WPV transmission.
Global WPV2 eradication was certified in 2015 after no detection since 1999 (
2
). WPV type 3 has not been detected since 2012 (
2
). A minimum of 3 years of sensitive AFP surveillance without detection of WPV is
required to certify a WHO region as being poliofree (
10
). Four of six WHO regions (the Region of the Americas, European, South-East Asia,
and Western Pacific regions) have been certified free of indigenous WPV. Improvements
in AFP surveillance performance in critical subnational areas are required to achieve
poliofree certification of the African and Eastern Mediterranean regions.
Because efforts to increase immunity to poliovirus type 2 before the global tOPV to
bOPV switch did not reach all persistently unvaccinated children in hard-to-reach
areas, some cVDPV2 emergences have been detected following the switch. Reaching all
children for vaccination in areas with cVDPV2 transmission is also an ongoing challenge.
Although progress toward global polio eradication has continued, challenges in identifying
and vaccinating every missed child remain. Much of the recent progress reaching previously
missed children has been associated with recruitment of trusted community volunteers
who are invested in their locality for vaccination and surveillance efforts. Intensification
of efforts to improve the quality of immunization and surveillance activities and
to develop additional innovations in addressing persisting challenges is necessary.
Until poliovirus eradication is achieved, all countries must remain vigilant by maintaining
high population immunity and sensitive poliovirus surveillance.
Summary
What is already known about this topic?
Transmission of wild poliovirus type 1 (WPV1) has not been interrupted in Afghanistan,
Nigeria, and Pakistan. A global, synchronized switch to bivalent oral poliovirus vaccine
(bOPV, types 1 and 3 only) was completed in April 2016.
What is added by this report?
Compared with 2016, the number of WPV1 cases overall decreased in 2017. Some transmission
of circulating vaccine-derived poliovirus type 2 (cVDPV2) has been identified more
than 1 year following the switch to bOPV in 2016.
What are the implications for public health practice?
Interruption of transmission of WPV1 and of cVDPV2 will require addressing persistent
challenges to vaccinating every missed child. Until poliovirus eradication is achieved,
all countries must maintain high population immunity and sensitive poliovirus surveillance.