Faster Detection of Poliomyelitis Outbreaks to Support Polio Eradication

As the global eradication of poliomyelitis approaches the final stages, prompt detection of new outbreaks is critical to enable a fast and effective outbreak response. Surveillance relies on reporting of acute flaccid paralysis (AFP) cases and laboratory confirmation through isolation of poliovirus from stool. However, delayed sample collection and testing can delay outbreak detection. We investigated whether weekly testing for clusters of AFP by location and time, using the Kulldorff scan statistic, could provide an early warning for outbreaks in 20 countries. A mixed-effects regression model was used to predict background rates of nonpolio AFP at the district level. In Tajikistan and Congo, testing for AFP clusters would have resulted in an outbreak warning 39 and 11 days, respectively, before official confirmation of large outbreaks. This method has relatively high specificity and could be integrated into the current polio information system to support rapid outbreak response activities.

Spatial Dynamics and High Risk Transmission Pathways of Poliovirus in Nigeria 2001-2013

The polio eradication programme in Nigeria has been successful in reducing incidence to just six confirmed cases in 2014 and zero to date in 2015, but prediction and management of future outbreaks remains a concern. A Poisson mixed effects model was used to describe poliovirus spread between January 2001 and November 2013, incorporating the strength of connectivity between districts (local government areas, LGAs) as estimated by three models of human mobility: simple distance, gravity and radiation models. Potential explanatory variables associated with the case numbers in each LGA were investigated and the model fit was tested by simulation. Spatial connectivity, the number of non-immune children under five years old, and season were associated with the incidence of poliomyelitis in an LGA (all P < 0.001). The best-fitting spatial model was the radiation model, outperforming the simple distance and gravity models (likelihood ratio test P < 0.05), under which the number of people estimated to move from an infected LGA to an uninfected LGA was strongly associated with the incidence of poliomyelitis in that LGA. We inferred transmission networks between LGAs based on this model and found these to be highly local, largely restricted to neighbouring LGAs (e.g. 67.7% of secondary spread from Kano was expected to occur within 10 km). The remaining secondary spread occurred along routes of high population movement. Poliovirus transmission in Nigeria is predominantly localised, occurring between spatially contiguous areas. Outbreak response should be guided by knowledge of high-probability pathways to ensure vulnerable children are protected.

Immunogenicity of a new routine vaccination schedule for global poliomyelitis prevention: an open-label, randomised controlled trial

BACKGROUND

Polio eradication needs a new routine immunisation schedule--three or four doses of bivalent type 1 and type 3 oral poliovirus vaccine (bOPV) and one dose of inactivated poliovirus vaccine (IPV), but no immunogenicity data are available for this schedule. We aimed to assess immunogenicity of this vaccine schedule.

METHODS

We did an open-label, randomised controlled trial in four centres in India. After informed consent was obtained from a parent or legally acceptable representative, healthy newborn babies were randomly allocated to one of five groups: trivalent OPV (tOPV); tOPV plus IPV; bOPV; bOPV plus IPV; or bOPV plus two doses of IPV (2IPV). The key eligibility criteria were: full-term birth (≥37 weeks of gestation); birthweight ≥2·5 kg; and Apgar score of 9 or more. OPV was administered at birth, 6 weeks, 10 weeks, and 14 weeks; IPV was administered intramuscularly at 14 weeks. The primary study objective was to investigate immunogenicity of the new vaccine schedule, assessed by seroconversion against poliovirus types 1, 2, and 3 between birth and 18 weeks in the per-protocol population (all participants with valid serology results on cord blood and at 18 weeks). Neutralisation assays tested cord blood and sera collected at 14 weeks, 18 weeks, 19 weeks, and 22 weeks by investigators masked to group allocation. This trial was registered with the India Clinical Trials Registry, number CTRI/2013/06/003722.

FINDINGS

Of 900 newborn babies enrolled between June 13 and Aug 29, 2013, 782 (87%) completed the per-protocol requirements. Between birth and age 18 weeks, seroconversion against poliovirus type 1 in the tOPV group occurred in 162 of 163 (99·4%, 95% CI 96·6-100), in 150 (98·0%, 94·4-99·6) of 153 in the tOPV plus IPV group, in 153 (98·7%, 95·4-99·8) of 155 in the bOPV group, in 155 (99·4%, 96·5-100) of 156 in the bOPV plus IPV group, and in 154 (99·4%, 96·5-100) of 155 in the bOPV plus 2IPV group. Seroconversion against poliovirus type 2 occurred in 157 (96·3%, 92·2-98·6) of 163 in the tOPV group, 153 (100%, 97·6-100·0) of 153 in the tOPV plus IPV group, 29 (18·7%, 12·9-25·7) of 155 in the bOPV group, 107 (68·6%, 60·7-75·8) of 156 in the bOPV plus IPV group, and in 121 (78·1%, 70·7-84·3) of 155 in the bOPV plus 2IPV group. Seroconversion against poliovirus type 3 was achieved in 147 (90·2%, 84·5-94·3) of 163 in the tOPV group, 152 (99·3%, 96·4-100) of 153 in the tOPV plus IPV group, 151 (97·4%, 93·5-99·3) of 155 in the bOPV group, 155 (99·4%, 96·5-100) of 156 in the bOPV plus IPV group, and 153 (98·7%, 95·4-99·8) of 155 in the bOPV plus 2IPV group. Superiority was achieved for vaccine regimens including IPV against poliovirus type 3 compared with those not including IPV (tOPV plus IPV vs tOPV alone, p=0·0008; and bOPV plus IPV vs bOPV alone, p=0·0153). 12 serious adverse events occurred (six in the tOPV group, one in the tOPV plus IPV group, three in the bOPV group, zero in the bOPV plus IPV group, and two in the bOPV plus 2IPV group), none of which was attributed to the trial intervention.

INTERPRETATION

The new vaccination schedule improves immunogenicity against polioviruses, especially against poliovirus type 3.

FUNDING

WHO, through a grant from Rotary International (grant number 59735).

Affordable inactivated poliovirus vaccine: strategies and progress

After polio eradication is achieved, the use of live-attenuated oral poliovirus vaccine (OPV) must be discontinued because of the inherent risk of the Sabin strains to revert to neurovirulence and reacquire greater transmissibility that could potentially result in the reestablishment of polio transmission. In 2008, the World Health Assembly mandated that the World Health Organization establish a strategy for developing more-affordable inactivated poliovirus vaccine (IPV) options for low-income countries. In 2012, the Strategic Advisory Group of Experts (SAGE) on Immunization recommended universal IPV introduction as a risk-mitigation strategy before the phased cessation of OPV (starting with Sabin type 2) and emphasized the need for affordable IPV options. In 2013, SAGE reiterated the importance of attaining the long-term target price of IPV at approximately $0.5 per immunizing dose and encouraged accelerated efforts to develop lower-cost IPV options. This article outlines the 4-pronged approach that is being pursued to develop affordable options and provides an update on the current status and plans to make IPV affordable for developing-country use.

The new polio eradication end game: rationale and supporting evidence

Polio eradication requires the removal of all polioviruses from human populations, whether wild poliovirus or those emanating from the oral poliovirus vaccine (OPV). The Polio Eradication & Endgame Strategic Plan 2013-2018 provides a framework for interruption of wild poliovirus transmission in remaining endemic foci and lays out a plan for the new polio end game, which includes the withdrawal of Sabin strains, starting with type 2, and the introduction of inactivated poliovirus vaccine, for risk mitigation purposes. This report summarizes the rationale and evidence that supports the policy decision to switch from trivalent OPV to bivalent OPV and to introduce 1 dose of inactivated poliovirus vaccine into routine immunization schedules, and it describes the proposed implementation of this policy in countries using trivalent OPV.

Polio eradication. Efficacy of inactivated poliovirus vaccine in India

Inactivated poliovirus vaccine (IPV) is efficacious against paralytic disease, but its effect on mucosal immunity is debated. We assessed the efficacy of IPV in boosting mucosal immunity. Participants received IPV, bivalent 1 and 3 oral poliovirus vaccine (bOPV), or no vaccine. A bOPV challenge was administered 4 weeks later, and excretion was assessed 3, 7, and 14 days later. Nine hundred and fifty-four participants completed the study. Any fecal shedding of poliovirus type 1 was 8.8, 9.1, and 13.5% in the IPV group and 14.4, 24.1, and 52.4% in the control group by 6- to 11-month, 5-year, and 10-year groups, respectively (IPV versus control: Fisher's exact test P < 0.001). IPV reduced excretion for poliovirus types 1 and 3 between 38.9 and 74.2% and 52.8 and 75.7%, respectively. Thus, IPV in OPV-vaccinated individuals boosts intestinal mucosal immunity.

Key issues in the persistence of poliomyelitis in Nigeria: a case-control study

BACKGROUND

The completion of poliomyelitis eradication is a global emergency for public health. In 2012, more than 50% of the world's cases occurred in Nigeria following an unanticipated surge in incidence. We aimed to quantitatively analyse the key factors sustaining transmission of poliomyelitis in Nigeria and to calculate clinical efficacy estimates for the oral poliovirus vaccines (OPV) currently in use.

METHODS

We used acute flaccid paralysis (AFP) surveillance data from Nigeria collected between January, 2001, and December, 2012, to estimate the clinical efficacies of all four OPVs in use and combined this with vaccination coverage to estimate the effect of the introduction of monovalent and bivalent OPV on vaccine-induced serotype-specific population immunity. Vaccine efficacy was determined using a case-control study with CIs based on bootstrap resampling. Vaccine efficacy was also estimated separately for north and south Nigeria, by age of the children, and by year. Detailed 60-day follow-up data were collected from children with confirmed poliomyelitis and were used to assess correlates of vaccine status. We also quantitatively assessed the epidemiology of poliomyelitis and programme performance and considered the reasons for the high vaccine refusal rate along with risk factors for a given local government area reporting a case.

FINDINGS

Against serotype 1, both monovalent OPV (median 32.1%, 95% CI 26.1-38.1) and bivalent OPV (29.5%, 20.1-38.4) had higher clinical efficacy than trivalent OPV (19.4%, 16.1-22.8). Corresponding data for serotype 3 were 43.2% (23.1-61.1) and 23.8% (5.3-44.9) compared with 18.0% (14.1-22.1). Combined with increases in coverage, this factor has boosted population immunity in children younger than age 36 months to a record high (64-69% against serotypes 1 and 3). Vaccine efficacy in northern states was estimated to be significantly lower than in southern states (p≤0.05). The proportion of cases refusing vaccination decreased from 37-72% in 2008 to 21-51% in 2012 for routine and supplementary immunisation, and most caregivers cited ignorance of either vaccine importance or availability as the main reason for missing routine vaccinations (32.1% and 29.6% of cases, respectively). Multiple regression analyses highlighted associations between the age of the mother, availability of OPV at health facilities, and the primary source of health information and the probability of receiving OPV (all p<0.05).

INTERPRETATION

Although high refusal rates, low OPV campaign awareness, and heterogeneous population immunity continued to support poliomyelitis transmission in Nigeria at the end of 2012, overall population immunity had improved due to new OPV formulations and improvements in programme delivery.

FUNDING

Bill & Melinda Gates Foundation Vaccine Modeling Initiative, Royal Society.

The potential impact of routine immunization with inactivated poliovirus vaccine on wild-type or vaccine-derived poliovirus outbreaks in a posteradication setting

The "endgame" for worldwide poliomyelitis eradication will entail eventual cessation of the use of oral poliovirus vaccine (OPV) in all countries to prevent the reintroduction of vaccine-derived polioviruses--exposing some populations to an unprecedented, albeit low, risk of poliovirus outbreaks. Inactivated poliovirus vaccine (IPV) is likely to play a large part in post--OPV management of poliovirus risks by reducing the consequences of any reintroduction of poliovirus. In this article, we examine the impact IPV would have on an outbreak in a partially susceptible population after OPV cessation, using a mathematical model of poliovirus transmission with a realistic natural history and case reporting. We explore a range of assumptions about the impact of IPV on an individual's infectiousness, given the lack of knowledge about this parameter. We show that routine use of IPV is beneficial under most conditions, increasing the chance of fadeout and reducing the expected prevalence of infection at the time of detection. The duration of "silent" poliovirus circulation prior to detection lengthens with increasing coverage of IPV, although this only increases the expected prevalence of infection at the time of the OPV response if IPV has a very limited impact on infectiousness. Overall, the model predicts that routine use of IPV will be advantageous for the posteradication management of poliovirus.

Priming after a fractional dose of inactivated poliovirus vaccine

BACKGROUND

To reduce the costs of maintaining a poliovirus immunization base in low-income areas, we assessed the extent of priming immune responses after the administration of inactivated poliovirus vaccine (IPV).

METHODS

We compared the immunogenicity and reactogenicity of a fractional dose of IPV (one fifth of a full dose) administered intradermally with a full dose administered intramuscularly in Cuban infants at the ages of 4 and 8 months. Blood was collected from infants at the ages of 4 months, 8 months, 8 months 7 days, and 8 months 30 days to assess single-dose seroconversion, single-dose priming of immune responses, and two-dose seroconversion. Specimens were tested with a neutralization assay.

RESULTS

A total of 320 infants underwent randomization, and 310 infants (96.9%) fulfilled the study requirements. In the group receiving the first fractional dose of IPV, seroconversion to poliovirus types 1, 2, and 3 occurred in 16.6%, 47.1%, and 14.7% of participants, respectively, as compared with 46.6%, 62.8%, and 32.0% in the group receiving the first full dose of IPV (P<0.008 for all comparisons). A priming immune response to poliovirus types 1, 2, and 3 occurred in 90.8%, 94.0%, and 89.6% of participants, respectively, in the group receiving the fractional dose as compared with 97.6%, 98.3%, and 98.1% in the group receiving the full dose (P=0.01 for the comparison with type 3). After the administration of the second dose of IPV in the group receiving fractional doses, cumulative two-dose seroconversion to poliovirus types 1, 2, and 3 occurred in 93.6%, 98.1%, and 93.0% of participants, respectively, as compared with 100.0%, 100.0%, and 99.4% in the group receiving the full dose (P<0.006 for the comparisons of types 1 and 3). The group receiving intradermal injections had the greatest number of adverse events, most of which were minor in intensity and none of which had serious consequences.

CONCLUSIONS

This evaluation shows that vaccinating infants with a single fractional dose of IPV can induce priming and seroconversion in more than 90% of immunized infants. (Funded by the World Health Organization and the Pan American Health Organization; Australian New Zealand Clinical Trials Registry number, ACTRN12610001046099.).

The effect of mass immunisation campaigns and new oral poliovirus vaccines on the incidence of poliomyelitis in Pakistan and Afghanistan, 2001-11: a retrospective analysis

BACKGROUND

Pakistan and Afghanistan are two of the three remaining countries yet to interrupt wild-type poliovirus transmission. The increasing incidence of poliomyelitis in these countries during 2010-11 led the Executive Board of WHO in January, 2012, to declare polio eradication a "programmatic emergency for global public health". We aimed to establish why incidence is rising in these countries despite programme innovations including the introduction of new vaccines.

METHODS

We did a matched case-control analysis based on a database of 46,977 children aged 0-14 years with onset of acute flaccid paralysis between Jan 1, 2001, and Dec 31, 2011. The vaccination history of children with poliomyelitis was compared with that of children with acute flaccid paralysis due to other causes to estimate the clinical effectiveness of oral poliovirus vaccines (OPVs) in Afghanistan and Pakistan by conditional logistic regression. We estimated vaccine coverage and serotype-specific vaccine-induced population immunity in children aged 0-2 years and assessed their association with the incidence of poliomyelitis over time in seven regions of Afghanistan and Pakistan.

FINDINGS

Between Jan 1, 2001, and Dec 31, 2011, there were 883 cases of serotype 1 poliomyelitis (710 in Pakistan and 173 in Afghanistan) and 272 cases of poliomyelitis serotype 3 (216 in Pakistan and 56 in Afghanistan). The estimated clinical effectiveness of a dose of trivalent OPV against serotype 1 poliomyelitis was 12·5% (95% CI 5·6-18·8) compared with 34·5% (16·1-48·9) for monovalent OPV (p=0·007) and 23·4% (10·4-34·6) for bivalent OPV (p=0·067). Bivalent OPV was non-inferior compared with monovalent OPV (p=0·21). Vaccination coverage decreased during 2006-11 in the Federally Administered Tribal Areas (FATA), Balochistan, and Khyber Pakhtunkhwa in Pakistan and in southern Afghanistan. Although partially mitigated by the use of more effective vaccines, these decreases in coverage resulted in lower vaccine-induced population immunity to poliovirus serotype 1 in FATA and Balochistan and associated increases in the incidence of poliomyelitis.

INTERPRETATION

The effectiveness of bivalent OPV is comparable with monovalent OPV and can therefore be used in eradicating serotype 1 poliomyelitis whilst minimising the risks of serotype 3 outbreaks. However, decreases in vaccination coverage in parts of Pakistan and southern Afghanistan have severely limited the effect of this vaccine.

FUNDING

Poliovirus Research subcommittee of WHO, Royal Society, and Medical Research Council.

Waning intestinal immunity after vaccination with oral poliovirus vaccines in India

BACKGROUND

The eradication of wild-type polioviruses in areas with efficient fecal-oral transmission relies on intestinal mucosal immunity induced by oral poliovirus vaccine (OPV). Mucosal immunity is thought to wane over time but the rate of loss of protection has not been examined.

METHODS

We examined the degree and duration of intestinal mucosal immunity in India by measuring the prevalence of vaccine poliovirus in stool samples collected 4-28 days after a "challenge" dose of OPV among 47 574 children with acute flaccid paralysis reported during 2005-2009.

RESULTS

Previous vaccination with OPV was protective against excretion of vaccine poliovirus after challenge, but the odds of excretion increased significantly with the time since the child was last exposed to an immunization activity (odds ratio, 1.39 [95% confidence interval .99-1.97], 2.04 [1.28-3.25], and 1.31 [1.00-1.70] comparing ≥6 months with 1 month ago for serotypes 1, 2, and 3, respectively). Vaccine administered during the high season for enterovirus infections (April-September) was significantly less likely to result in excretion, especially in northern states (odds ratio, 0.57 [95% confidence interval, .50-.65], 0.58 [.41-.81], and 0.48 [.40-.57] for serotypes 1, 2, and 3).

CONCLUSIONS

Infection with OPV (vaccine "take") is highly seasonal in India and results in intestinal mucosal immunity that appears to wane significantly within a year of vaccination.

A statistical model of the international spread of wild poliovirus in Africa used to predict and prevent outbreaks

BACKGROUND

Outbreaks of poliomyelitis in African countries that were previously free of wild-type poliovirus cost the Global Polio Eradication Initiative US$850 million during 2003-2009, and have limited the ability of the program to focus on endemic countries. A quantitative understanding of the factors that predict the distribution and timing of outbreaks will enable their prevention and facilitate the completion of global eradication.

METHODS AND FINDINGS

Children with poliomyelitis in Africa from 1 January 2003 to 31 December 2010 were identified through routine surveillance of cases of acute flaccid paralysis, and separate outbreaks associated with importation of wild-type poliovirus were defined using the genetic relatedness of these viruses in the VP1/2A region. Potential explanatory variables were examined for their association with the number, size, and duration of poliomyelitis outbreaks in 6-mo periods using multivariable regression analysis. The predictive ability of 6-mo-ahead forecasts of poliomyelitis outbreaks in each country based on the regression model was assessed. A total of 142 genetically distinct outbreaks of poliomyelitis were recorded in 25 African countries, resulting in 1-228 cases (median of two cases). The estimated number of people arriving from infected countries and <5-y childhood mortality were independently associated with the number of outbreaks. Immunisation coverage based on the reported vaccination history of children with non-polio acute flaccid paralysis was associated with the duration and size of each outbreak, as well as the number of outbreaks. Six-month-ahead forecasts of the number of outbreaks in a country or region changed over time and had a predictive ability of 82%.

CONCLUSIONS

Outbreaks of poliomyelitis resulted primarily from continued transmission in Nigeria and the poor immunisation status of populations in neighbouring countries. From 1 January 2010 to 30 June 2011, reduced transmission in Nigeria and increased incidence in reinfected countries in west and central Africa have changed the geographical risk of polio outbreaks, and will require careful immunisation planning to limit onward spread. Please see later in the article for the Editors' Summary.

Immunogenicity of bivalent types 1 and 3 oral poliovirus vaccine: a randomised, double-blind, controlled trial

BACKGROUND

Poliovirus types 1 and 3 co-circulate in poliomyelitis-endemic countries. We aimed to assess the immunogenicity of a novel bivalent types 1 and 3 oral poliovirus vaccine (bOPV).

METHODS

We did a randomised, double-blind, controlled trial to assess the superiority of monovalent type 2 OPV (mOPV2), mOPV3, or bOPV over trivalent OPV (tOPV), and the non-inferiority of bivalent vaccine compared with mOPV1 and mOPV3. The study was done at three centres in India between Aug 6, 2008, and Dec 26, 2008. Random allocation was done by permuted blocks of ten. The primary outcome was seroconversion after one monovalent or bivalent vaccine dose compared with a dose of trivalent vaccine at birth. The secondary endpoints were seroconversion after two vaccine doses compared with after two trivalent vaccine doses and cumulative two-dose seroconversion. Parents or guardians and study investigators were masked to treatment allocation. Because of multiple comparisons, we defined p≤0·01 as statistically significant. This trial is registered with Current Controlled Trials, ISRCTN 64725429.

RESULTS

900 newborn babies were randomly assigned to one of five vaccine groups (about 180 patients per group); of these 70 (8%) discontinued, leaving 830 (92%) for analysis. After the first dose, seroconversion to poliovirus type 1 was 20% for both mOPV1 (33 of 168) and bOPV (32 of 159) compared with 15% for tOPV (25 of 168; p>0·01), to poliovirus type 2 was 21% (35 of 170) for mOPV2 compared with 25% (42 of 168) for tOPV (p>0·01), and to poliovirus type 3 was 12% (20 of 165) for mOPV3 and 7% (11 of 159) for bOPV compared with 4% (7 of 168) for tOPV (mOPV3 vs tOPV p=0·01; bOPV vs tOPV; p>0·01). Cumulative two-dose seroconversion to poliovirus type 1 was 90% (151 of 168) for mOPV1 and 86% (136 of 159) for bOPV compared with 63% (106 of 168) for tOPV (p<0·0001), to poliovirus type 2 was 90% (153 of 170) for mOPV2 compared with 91% (153 of 168) for tOPV (p>0·01), and to poliovirus type 3 was 84% (138 of 165) for mOPV3 and 74% (117 of 159) for bOPV compared with 52% (87 of 168) for tOPV (p<0·0001). The vaccines were well tolerated. 19 serious adverse events occurred, including one death; however, these events were not attributed to the trial interventions.

INTERPRETATION

The findings show the superiority of bOPV compared with tOPV, and the non-inferiority of bOPV compared with mOPV1 and mOPV3.

FUNDING

GAVI Alliance, World Health Organization, and Panacea Biotec.

Implications of a circulating vaccine-derived poliovirus in Nigeria

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.

Asymptomatic wild-type poliovirus infection in India among children with previous oral poliovirus vaccination

BACKGROUND

Mucosal immunity induced by oral poliovirus vaccine (OPV) is imperfect and potentially allows immunized individuals to participate in asymptomatic wild-type poliovirus transmission in settings with efficient fecal-oral transmission of infection.

METHODS

We examined the extent of asymptomatic wild-type poliovirus transmission in India by measuring the prevalence of virus in stool samples obtained from 14,005 healthy children who were in contact with 2761 individuals with suspected poliomyelitis reported during the period 2003-2008.

RESULTS

Wild-type poliovirus serotypes 1 and 3 were isolated from the stool samples of 103 (0.74%) and 104 (0.74%) healthy contacts, respectively. Among contacts of individuals with laboratory-confirmed poliomyelitis, 27 (12.7%) of 213 and 29 (13.9%) of 209 had serotypes 1 and 3, respectively, isolated from their stool samples. The odds ratio of excreting serotype 1 wild-type poliovirus was 0.13 (95% confidence interval, 0.02-0.87) among healthy children reporting 6 doses of OPV, compared with children reporting 0-2 doses. However, two-thirds of healthy children who excreted this virus reported >or=6 doses, and the prevalence of this virus did not decrease with age over the sampled range.

CONCLUSIONS

Although OPV is protective against infection with poliovirus, the majority of healthy contacts who excreted wild-type poliovirus were well vaccinated. This is consistent with a potential role for OPV-vaccinated children in continued wild-type poliovirus transmission and requires further study.

Mucosal immunity after vaccination with monovalent and trivalent oral poliovirus vaccine in India

BACKGROUND

Persistent wild-poliovirus transmission, particularly in India, has raised questions about the degree of mucosal immunity induced by oral poliovirus vaccine (OPV) in tropical countries.

METHODS

Excretion of vaccine poliovirus after challenge with OPV was measured in stool samples collected from children identified by the acute flaccid paralysis surveillance program in India during 2005-2007. The effectiveness of trivalent and monovalent OPV against excretion of each poliovirus type was estimated.

RESULTS

Vaccine poliovirus was isolated from 4994 (5.2%) of 96,641 children with 2 stool samples. The relative odds of excreting challenge poliovirus among children with 5 reported previous doses of trivalent OPV compared with 0 previous doses was 0.24 (95% confidence interval [CI], 0.12-0.45), 0.08 (95% CI, 0.04-0.14), and 0.40 (95% CI, 0.19-0.85) for serotypes 1, 2, and 3, respectively, but the relative odds increased to 0.62 (95% CI, 0.44-0.88), 0.44 (95% CI, 0.20-0.99), and 0.66 (95% CI, 0.41-1.06), respectively, in the northern states of Uttar Pradesh and Bihar. In these 2 states, the relative odds of excretion of serotype 1 was 0.32 (95% CI, 0.26-0.41) after 5 doses of type 1 monovalent OPV.

CONCLUSIONS

The mucosal immunity induced by OPV in India varies by location, serotype, and vaccine formulation. These findings have implications for global eradication and the potential role played by inactivated vaccine in this setting.

Effectiveness of immunization against paralytic poliomyelitis in Nigeria

BACKGROUND

The number of cases of paralytic poliomyelitis has declined in Nigeria since the introduction of newly licensed monovalent oral poliovirus vaccines and new techniques of vaccine delivery. Understanding the relative contribution of these vaccines and the improved coverage to the decline in incident cases is essential for future planning.

METHODS

We estimated the field efficacies of monovalent type 1 oral poliovirus vaccine and trivalent oral poliovirus vaccine, using the reported number of doses received by people with poliomyelitis and by matched controls as identified in Nigeria's national surveillance database, in which 27,379 cases of acute flaccid paralysis were recorded between 2001 and 2007. Our estimates of vaccine coverage and vaccine-induced immunity were based on the number of doses received by children listed in the database who had paralysis that was not caused by poliovirus.

RESULTS

The estimated efficacies per dose of monovalent type 1 oral poliovirus vaccine and trivalent oral poliovirus vaccine against type 1 paralytic poliomyelitis were 67% (95% confidence interval [CI], 39 to 82) and 16% (95% CI, 10 to 21), respectively, and the estimated efficacy per dose of trivalent oral poliovirus vaccine against type 3 paralytic poliomyelitis was 18% (95% CI, 9 to 26). In the northwestern region of Nigeria, which reported the majority of cases during the study period, coverage with at least one dose of vaccine increased from 59 to 78%. Between 2005 and 2007, vaccine-induced immunity levels among children under the age of 5 years more than doubled, to 56%.

CONCLUSIONS

The higher efficacy of monovalent type 1 oral poliovirus vaccine (four times as effective as trivalent oral poliovirus vaccine) and the moderate gains in coverage dramatically increased vaccine-induced immunity against serotype 1 in northern Nigeria. Further increases in coverage in Nigerian states with infected populations are required to achieve the levels of vaccine-induced immunity associated with the sustained elimination achieved in other parts of the country.

Uncertainty and sensitivity analyses of a decision analytic model for posteradication polio risk management

Decision analytic modeling of polio risk management policies after eradication may help inform decisionmakers about the quantitative tradeoffs implied by various options. Given the significant dynamic complexity and uncertainty involving posteradication decisions, this article aims to clarify the structure of a decision analytic model developed to help characterize the risks, costs, and benefits of various options for polio risk management after eradication of wild polioviruses and analyze the implications of different sources of uncertainty. We provide an influence diagram of the model with a description of each component, explore the impact of different assumptions about model inputs, and present probability distributions of model outputs. The results show that choices made about surveillance, response, and containment for different income groups and immunization policies play a major role in the expected final costs and polio cases. While the overall policy implications of the model remain robust to the variations of assumptions and input uncertainty we considered, the analyses suggest the need for policymakers to carefully consider tradeoffs and for further studies to address the most important knowledge gaps.

The risks, costs, and benefits of possible future global policies for managing polioviruses

OBJECTIVES

We assessed the costs, risks, and benefits of possible future major policy decisions on vaccination, surveillance, response plans, and containment following global eradication of wild polioviruses.

METHODS

We developed a decision analytic model to estimate the incremental cost-effectiveness ratios and net benefits of risk management options for polio for the 20-year period and stratified the world according to income level to capture important variability between nations.

RESULTS

For low-, lower-middle-, and upper-middle-income groups currently using oral poliovirus vaccine (OPV), we found that after successful eradication of wild polioviruses, OPV cessation would save both costs and lives when compared with continued use of OPV without supplemental immunization activities. We found cost-effectiveness ratios for switching from OPV to inactivated poliovirus vaccine to be higher (i.e., less desirable) than other health investment opportunities, depending on the actual inactivated poliovirus vaccine costs and assumptions about whether supplemental immunization activities with OPV would continue.

CONCLUSIONS

Eradication promises billions of dollars of net benefits, although global health policy leaders face difficult choices about future policies. Until successful eradication and coordination of posteradication policies, health authorities should continue routine polio vaccination and supplemental immunization activities.

Protective efficacy of a monovalent oral type 1 poliovirus vaccine: a case-control study

BACKGROUND

A high-potency monovalent oral type 1 poliovirus vaccine (mOPV1) was developed in 2005 to tackle persistent poliovirus transmission in the last remaining infected countries. Our aim was to assess the efficacy of this vaccine in India.

METHODS

We estimated the efficacy of mOPV1 used in supplementary immunisation activities from 2076 matched case-control pairs of confirmed cases of poliomyelitis caused by type 1 wild poliovirus and cases of non-polio acute flaccid paralysis in India. The effect of the introduction of mOPV1 on population immunity was calculated on the basis of estimates of vaccination coverage from data for non-polio acute flaccid paralysis.

FINDINGS

In areas of persistent poliovirus transmission in Uttar Pradesh, the protective efficacy of mOPV1 was estimated to be 30% (95% CI 19-41) per dose against type 1 paralytic disease, compared with 11% (7-14) for the trivalent oral vaccine. 76-82% of children aged 0-23 months were estimated to be protected by vaccination against type 1 poliovirus at the end of 2006, compared with 59% at the end of 2004, before the introduction of mOPV1.

INTERPRETATION

Under conditions where the efficacy of live-attenuated oral poliovirus vaccines is compromised by a high prevalence of diarrhoea and other infections, a dose of high-potency mOPV1 is almost three times more effective against type 1 poliomyelitis disease than is trivalent vaccine. Achieving high coverage with this new vaccine in areas of persistent poliovirus transmission should substantially improve the probability of rapidly eliminating transmission of the disease.

Eradicating polio: today's challenges and tomorrow's legacy

Since its launch in 1988, the Global Polio Eradication Initiative has grown into one of the largest international health efforts in history, operating in every country and area in the world. The burden of polio disease has been reduced by over 99%, and the number of countries with indigenous virus has fallen from more than 125 to just four. As importantly, a strong surveillance and laboratory infrastructure has been established for vaccine-preventable diseases (including measles, tetanus, yellow fever, rubella and Japanese encephalitis), and a massive investment has been made in the physical infrastructure and human resources needed to deliver routine immunizations and other health services in developing countries. Between 2000 and 2003, new challenges to polio eradication emerged, threatening the interruption of the transmission of wild poliovirus globally and the eventual elimination of any residual polio disease as the result of the continued use of oral polio vaccines. By the end of 2005, a range of solutions had been developed to address these late challenges, including two new monovalent oral polio vaccines, new and robust international standards for the response to polio outbreaks, and renewed political commitment across the countries that remain infected. As importantly, a comprehensive strategy had been established for managing the long-term risks of paralytic polio, centred, ironically, on the eventual elimination from routine immunizations of the vaccine that is still central to the success of the global eradication effort.

Development and consideration of global policies for managing the future risks of poliovirus outbreaks: insights and lessons learned through modeling

The success of the Global Polio Eradication Initiative promises to bring large benefits, including sustained improvements in quality of life (i.e., cases of paralytic disease and deaths avoided) and costs saved from cessation of vaccination. Obtaining and maintaining these benefits requires that policymakers manage the transition from the current massive use of oral poliovirus vaccine (OPV) to a world without OPV and free of the risks of potential future reintroductions of live polioviruses. This article describes the analytical journey that began in 2001 with a retrospective case study on polio risk management and led to development of dynamic integrated risk, economic, and decision analysis tools to inform global policies for managing the risks of polio. This analytical journey has provided several key insights and lessons learned that will be useful to future analysts involved in similar complex decision-making processes.

Risks of paralytic disease due to wild or vaccine-derived poliovirus after eradication

After the global eradication of wild polioviruses, the risk of paralytic poliomyelitis from polioviruses will still exist and require active management. Possible reintroductions of poliovirus that can spread rapidly in unprotected populations present challenges to policymakers. For example, at least one outbreak will likely occur due to circulation of a neurovirulent vaccine-derived poliovirus after discontinuation of oral poliovirus vaccine and also could possibly result from the escape of poliovirus from a laboratory or vaccine production facility or from an intentional act. In addition, continued vaccination with oral poliovirus vaccines would result in the continued occurrence of vaccine-associated paralytic poliomyelitis. The likelihood and impacts of reintroductions in the form of poliomyelitis outbreaks depend on the policy decisions and on the size and characteristics of the vulnerable population, which change over time. A plan for managing these risks must begin with an attempt to characterize and quantify them as a function of time. This article attempts to comprehensively characterize the risks, synthesize the existing data available for modeling them, and present quantitative risk estimates that can provide a starting point for informing policy decisions.

Risk management in a polio-free world

Inherent in the decision to launch the Global Polio Eradication Initiative in 1988 was the expectation for many people that immunization against poliomyelitis would eventually simply stop, as had been the case with smallpox following its eradication in 1977. However, the strategies for managing the risks associated with a "polio-free" world must be continuously refined to reflect new developments, particularly in our understanding of the live polioviruses in the oral poliovirus vaccine (OPV) and in the international approach to managing potential biohazards. The most important of these developments has been the confirmation in 2000 that vaccine-derived polioviruses (VDPVs) can circulate and cause polio outbreaks, making the use of OPV after interruption of wild poliovirus transmission incompatible with a polio-free world. A comprehensive strategy has been developed to minimize the risks associated with eventual OPV cessation, centered on appropriate long-term biocontainment of poliovirus stocks (whether for vaccine production, diagnosis, or research), the controlled reintroduction of any live poliovirus vaccine (i.e., from an OPV stockpile), and appropriate use of the inactivated poliovirus vaccine (IPV). Although some aspects of this risk management strategy are still debated, there is wide agreement that no strategy would entirely eliminate the potential risks to a polio-free world. The current strategy for risk management in a polio-free world will continue to evolve with better characterization of these risks and the development of more effective approaches both to reduce those risks and to limit their consequences should they occur.

New Strategies for the Elimination of Polio from India

The feasibility of global polio eradication is being questioned as a result of continued transmission in a few localities that act as sources for outbreaks elsewhere. Perhaps the greatest challenge is in India, where transmission has persisted in Uttar Pradesh and Bihar despite high coverage with multiple doses of vaccine. We estimate key parameters governing the seasonal epidemics in these areas and show that high population density and poor sanitation cause persistence by not only facilitating transmission of poliovirus but also severely compromising the efficacy of the trivalent vaccine. We analyze strategies to counteract this and show that switching to monovalent vaccine may finally interrupt virus transmission.

Mass vaccination: when and why

With increased demand for smallpox vaccination during the nineteenth century, vaccination days--early mass vaccination campaigns--were conducted over time-limited periods to rapidly and efficiently protect maximum numbers of susceptible persons. Two centuries later, the challenge to rapidly and efficiently protect populations by mass vaccintion continues, despite the strengthening of routine immunization services in many countries through the Expanded Programme on Immunization strategies and GAVI support. Perhaps the most widely accepted reason for mass vaccination is to rapidly increase population (herd) immunity in the setting of an existing or potential outbreak, thereby limiting the morbidity and mortality that might result, especially when there has been no routine vaccination, or because populations have been displaced and routine immunization services disrupted. A second important use of mass vaccination is to accelerate disease control to rapidly increase coverage with a new vaccine at the time of its introduction into routine immunization programmes, and to attain the herd immunity levels required to meet international targets for eradication and mortality reduction. In the twenty-first century, mass vaccination and routine immunization remain a necessary alliance for attaining both national and international goals in the control of vaccine preventable disease.

A vision of a world without polio: The OPV cessation strategy

Once the eradication of wild poliovirus has been confirmed, the public health benefits of routine immunization with OPV will no longer outweigh the burden of disease either due to paralysis caused by OPV (vaccine associated paralytic polio), or by outbreaks caused by circulating vaccine-derived polioviruses. The eventual cessation of OPV use in routine immunization programmes worldwide will become necessary to assure a lasting eradication of polio. As the world moves towards polio eradication and its certification, preparations are therefore being intensified for OPV cessation, and the risk management framework for safe OPV cessation is being put in place. The framework includes bio-containment of all known poliovirus and potentially infected substances, development of an international stockpile of oral polio vaccine, ensuring a mechanism for continued global surveillance and response for polio after eradication has been certified, and national policies if countries decide to continue vaccinating with inactivated polio vaccine (IPV). It is ironic that the vaccine on which the world has depended for polio eradication will itself become a risk to eradication once the transmission of wild poliovirus has been interrupted. Final preparations for the eventual global and simultaneous cessation of OPV will require the same level of international cooperation and coordination that has brought the world to the verge of polio eradication.

Interrupting poliovirus transmission -- new solutions to an old problem

Since the launch of the Global Polio Eradication Initiative (GPEI) in 1988, knowledge as to the nature of circulating polioviruses and the challenges to their interruption has increased tremendously, particularly during the period 2000-2005. By January 2006, however, the systematic application of the standard polio eradication strategies, combined with recent refinements, had reduced the number of countries with ongoing transmission of indigenous wild polioviruses to just four (Nigeria, India, Pakistan, and Afghanistan), the lowest ever in history. In addition, only 8 of the 22 areas that had been re-infected by wild poliovirus in 2003-2005 still required large-scale 'mop-up' activities and circulating vaccine-derived poliovirus (cVDPV) outbreaks were being readily addressed. This progress, despite new challenges late in the GPEI, was greatly facilitated by a range of solutions that included two new monovalent oral polio vaccines (mOPVs), new and robust international standards for polio outbreak response, and renewed political commitment across the remaining infected countries.

Polio eradication: interrupting transmission, towards a polio-free world

The polio eradication initiative is in its final phase and new tools have been developed to ensure its completion, including monovalent type 1 and 3 oral poliovirus vaccines (OPVs) that permit a virus-specific response to endemic and imported poliovirus. After eradication, the public health benefits of routine immunization with OPV will no longer outweigh the burden of disease, either owing to paralysis caused by OPV (vaccine-associated paralytic poliomyelitis) or outbreaks caused by reversion of OPV to neuro-virulent strains (circulating vaccine-derived polioviruses), and discontinuation of OPV use will be mandatory. Prerequisites for safe OPV cessation include biocontainment of all known poliovirus and potentially infected materials; an international stockpile of monovalent OPV and a response capacity. A mechanism is required for continued surveillance of poliomyelitis after eradication has been certified, as well as national policies if countries decide to continue vaccinating using inactivated poliovirus vaccine. It is ironic that the vaccine responsible for polio eradication will itself become a threat to eradication, once polio has been eradicated.

The human story

Eradicating human pathogens is a young science, and there is still much to learn about its role in controlling existing and emerging diseases

POLICY: OPV Cessation--the Final Step To a "Polio-Free" World

The 20-year, U.S. $4 billion-dollar international effort to eradicate wild polioviruses now includes monovalent oral poliovirus type 1 vaccine (mOPV1), which was first given to 40 million children in India in April 2005. As this vaccine is being introduced to eliminate some of the last poliovirus reservoirs, the Global Polio Eradication Initiative is planning for the eventual synchronized worldwide cessation of the routine use of all oral poliovirus vaccines (OPVs) because of their capacity to cause, rarely, outbreaks of paralytic poliomyelitis. This Policy Forum reviews the feasibility and implications of eventually stopping the use of live polio vaccines.

Can we capitalize on the virtues of vaccines? Insights from the polio eradication initiative

Twenty-five years after the eradication of smallpox, the ongoing effort to eradicate poliomyelitis has grown into the largest international health initiative ever undertaken. By 2004, however, the polio eradication effort was threatened by a challenge regularly faced by public health policymakers everywhere-misperception about the benefits and risks of vaccines. The propagation of false rumors about oral poliovirus vaccine safety led to the reinfection of 13 previously polio-free countries and the largest polio epidemic in Africa in recent years. With deft management of such challenges by local, national, and international health authorities, poliomyelitis, a disease that threatened children everywhere just 2 generations ago, could soon be relegated to history like smallpox before it.

A global call for new polio vaccines

The end is near but eradication will not be as simple as once thought.

By late 2003, poliomyelitis had been eliminated from all but six countries in the world as a result of the World Health Organization's eradication initiative. But a trio of workers based at the Geneva headquarters of the Global Polio Eradication Initiative point out in a Commentary that without new vaccines, the goal of global eradication may be unattainable.

Polio eradication: mobilizing and managing the human resources

Between 1988 and 2004, the Global Polio Eradication Initiative grew to become the largest international health effort in history, operating in every country of the world. An estimated 10 million health workers and volunteers have been engaged in implementing the necessary polio supplementary immunization activities (SIAs) on a recurring basis, and at least 35,000 well-trained workers have been conducting polio surveillance. A combination of task simplification, technological innovations and adaptation of strategies to fit local circumstances has allowed the Initiative to use a wide range of workers and volunteers, from both inside and outside the health sector, to deliver the polio vaccine during SIAs and to monitor progress in virtually every area of every country, regardless of the health infrastructure, conflict, geography and/or culture. This approach has required sustained political advocacy and mass community mobilization, together with strong management and supervisory processes. Non-monetary incentives, reimbursement of costs and substantial technical assistance have been essential. Given the unique features of eradication programmes in general, and polio eradication in particular, the implications of this approach for the broader health system must continue to be studied if it is to be replicated for the delivery and monitoring of other interventions.

Protecting investments in polio eradication: the past, present and future of surveillance for acute flaccid paralysis

In September 2003 a WHO consultation group on vaccine-derived polioviruses (VDPV) concluded that in order to prevent future generations of paralytic polio after interruption of transmission of wild poliovirus, the use of trivalent oral polio vaccine (OPV) must be stopped [1]. Another important global policy decision along the road to polio eradication thus became possible – cessation of OPV use at some time after eradication. The question now is not whether OPV must be stopped, but rather when.

Framework for evaluating the risks of paralytic poliomyelitis after global interruption of wild poliovirus transmission

With the interruption of wild poliovirus transmission globally, the need for new policies to deal with the post-certification era will rapidly arise. New policies will be required in four areas: detection and notification of circulating polioviruses; biocontainment of wild, vaccine-derived and attenuated strains of poliovirus; vaccine stockpiles and response mechanisms; and routine immunization against polioviruses. A common understanding of the potential risks of paralytic poliomyelitis in the post-certification period is essential to the development of these policies. Since 2000, there has been increasing international consensus that the risks of paralytic poliomyelitis in the post-certification era fall into two categories: those due to the continued use of the oral poliovirus vaccine (OPV) and those due to future improper handling of wild polioviruses. The specific risks within both categories have now been defined, and an understanding of the frequency and potential burden of disease associated with each is rapidly improving. This knowledge and clarity have provided a framework that is already proving valuable for identifying research priorities and discussing potential policy options with national authorities. However, this framework must be regarded as a dynamic tool, requiring regular updating as additional information on these risks becomes available through further scientific research, programmatic work, and policy decisions.

Global health goals: lessons from the worldwide effort to eradicate poliomyelitis

The Global Polio Eradication Initiative was launched in 1988. Assessment of the politics, production, financing, and economics of this international effort has suggested six lessons that might be pertinent to the pursuit of other global health goals. First, such goals should be based on technically sound strategies with proven operational feasibility in a large geographical area. Second, before launching an initiative, an informed collective decision must be negotiated and agreed in an appropriate international forum to keep to a minimum long-term risks in financing and implementation. Third, if substantial community engagement is envisaged, efficient deployment of sufficient resources at that level necessitates a defined, time-limited input by the community within a properly managed partnership. Fourth, although the so-called fair-share concept is arguably the best way to finance such goals, its limitations must be recognised early and alternative strategies developed for settings where it does not work. Fifth, international health goals must be designed and pursued within existing health systems if they are to secure and sustain broad support. Finally, countries, regions, or populations most likely to delay the achievement of a global health goal should be identified at the outset to ensure provision of sufficient resources and attention. The greatest threats to poliomyelitis eradication are a financing gap of US 210 million dollars and difficulties in strategy implementation in at most five countries.

Possible global strategies for stopping polio vaccination and how they could be harmonized

One of the challenges of the polio eradication initiative over the next few years will be the formulation of an optimal strategy for stopping poliovirus vaccination after global certification of polio eradication has been accomplished. This strategy must maximize the benefits and minimize the risks. A number of strategies are currently under consideration, including: (i) synchronized global discontinuation of use of oral poliovirus vaccine (OPV); (ii) regional or subregional coordinated OPV discontinuation; and (iii) moving from trivalent to bivalent or monovalent OPV. Other options include moving from OPV to global use of IPV for an interim period before cessation of IPV use (to eliminate circulation of vaccine-derived poliovirus, if necessary) or development of new OPV strains that are not transmissible. Each of these strategies is associated with specific advantages (financial benefits for OPV discontinuation) and disadvantages (cost of switch to IPV) and inherent uncertainties (risk of continued poliovirus circulation in certain populations or prolonged virus replication in immunodeficient persons). An ambitious research agenda addresses the remaining questions and issues. Nevertheless, several generalities are already clear. Unprecedented collaboration between countries, regions, and indeed the entire world will be required to implement a global OPV discontinuation strategy Regulatory approval will be needed for an interim bivalent OPV or for monovalent OPV in many countries. Manufacturers will need sufficient lead time to produce sufficient quantities of IPV Finally, the financial implications for any of these strategies need to be considered. Whatever strategy is followed it will be necessary to stockpile supplies of a poliovirus-containing vaccine (most probably all three types of monovalent OPV), and to develop contingency plans to respond should an outbreak of polio occur after stopping vaccination.