When an individual is confirmed to be suffering from paralytic polio, he or she is seen as the "tip of an epidemic iceberg."^source Without proper vaccine coverage and surveillance, one case of polio can escalate quickly into an epidemic. Ridding the world of polio is an ongoing struggle. Some countries have seen polio return after being designated polio-free for many years.

Polio can be prevented with a vaccine, which comes in two forms: an injection and an oral vaccine. The injected polio vaccine (IPV) is made from a version of the poliovirus that has been deactivated; it provides intestinal immunity after four shots and is the standard in high-income countries.^source The oral polio vaccine (OPV) is made from a weakened but still active strain of the virus and requires three doses for immunity; it is cheaper than IPV.^source Like other vaccines, IPV and OPV work by activating an immune response in the body so that the body builds up antibodies to protect itself from the virus.

Thanks to these vaccines, the world has seen a dramatic reduction in polio cases: from an estimated 350,000 cases in 1988 to a low of 102 cases in 2015.^source Ironically, however, the success has come with a risk: the widely used OPV can cause polio, threatening communities that have low immunity and poor vaccine coverage. This type of virus—circulating vaccine-derived poliovirus (cVDPV)—can circulate for longer than a year, easily transmitting to people who are not immune and potentially paralyzing them.^source

This was the case in 2000 in Haiti, when a two-year-old girl in the country's Northwest Department contracted paralytic polio from a circulating vaccine-derived strain of the virus. Largely due to poor sanitation, limited vaccine coverage, and weak surveillance, the outbreak caused 20 more cases across Hispaniola, a Caribbean island shared by Haiti and the Dominican Republic, over the next year.^source

The plan for eliminating wild poliovirus across North, Central and South America was realized under the leadership of Dr. Ciro de Quadros, director of the immunization program at the Pan American Health Organization (PAHO). Following a widespread OPV immunization campaign and robust country surveillance system, polio's spread was curtailed. The WHO-designated Region of the Americas was certified polio-free in 1994, after maintaining zero polio cases for three years.^source

However, countries in the Region differ politically, economically, and socially. And Haiti’s health system had been on shaky ground following a violent coup d’etat in 1991. President Jean-Bertrand Aristide had been overthrown and forced into exile, and the military proceeded to violate citizens’ human rights. The turmoil led donors to suspend their support and technical assistance to the Haitian Ministry of Public Health and Population’s programs.^source

Haiti’s weak health system degraded further under these conditions. Extremely low polio vaccine coverage and poor sanitation conditions continued to plague the country throughout the 1990s.^source Shortly after the region had been deemed polio-free, the Haitian government suspended its National Immunization Days. The result was devastating: the share of children under one-year-old receiving three doses of OPV stagnated at 30-50 percent during the 1990s.^source Combined with a lack of investment in water quality and sanitation, the conditions were conducive to a polio outbreak.

Despite these challenges, there was one piece of good news: Haiti's epidemiological surveillance system (ESS) had remained more or less intact. This proved essential in the summer of 2000, when a two-year-old girl in Haiti's Northwest Department experienced a sudden onset of paralysis, and the case was reported to the ESS. A follow-up investigation by Haiti's Ministry of Public Health and Population, PAHO’s Caribbean Epidemiology Center and the CDC confirmed the worst fears: polio was back.^source A vaccine-derived poliovirus with the ability to paralyze and rapidly spread was circulating on the island, likely infecting thousands.^source It was not long before several more cases of paralytic polio were confirmed: by 2001, 8 cases in Haiti and 14 in the Dominican Republic were confirmed.^source All of the cases were in children who were not fully vaccinated and living in areas with partial OPV coverage.^source

PAHO and the CDC jumped in to help Haiti stop polio in its tracks. They sent 16 national and international epidemiologists to conduct an active search for case of acute flaccid paralysis (AFP) in both countries.^source They also reached out to religious leaders, teachers, day care center directors, mothers, and traditional healers to locate unreported cases of AFP.^source

Meanwhile, the government of Haiti put aggressive control measures back in place. Initial action included vaccination where paralytic polio had been detected and nationwide at fixed posts such as at health facilities. In early 2001, the Ministry of Public Health and Population, with the technical help of PAHO, conducted two rounds of vaccination. But inadequate planning, logistical problems, and heavy rains undermined these activities, and immunization barely reached 40 percent of the 1.2 million Haitian children under the age of five.^source By mid-2001, the government and its partners had not managed to immunize enough of the population to guarantee protection.

This prompted minister of health, Dr. Jean Claude Voltaire, to launch a new, more ambitious strategy: nationwide school-based vaccination of all children under 10, followed by house-to-house vaccination of all remaining eligible children. By this point the international response had also intensified. PAHO, UNICEF, the World Bank, the Canadian Government, and USAID helped the health and education ministries with technical and financial support. The efforts led to two national immunization campaigns in the spring and fall of 2001.

The outbreak confirmed that health officials had to remain vigilant in Haiti. To improve surveillance, more than 100 local health care workers were trained to conduct active case searches during their community-based activities. PAHO also continued to send epidemiologists to reinforce surveillance activities, and the Ministry of Public Health and Population even offered a reward of US$100 to anyone who reported a laboratory confirmed case.^source

The government of Haiti also coordinated with the government of the Dominican Republic, sharing information across the border. By late 2001, coordination turned into collaboration. Senior health officials from the two countries' ministries of public health met at the main border area to devise a plan for ensuring that all children passing through official border crossings were vaccinated at one of the vaccine posts on either side. Furthermore, the first ladies of both countries helped inaugurate a coordinated measles-polio campaign in 2001. Both vaccines were delivered during the third national OPV immunization campaign the following year.^source

The success of the immunization campaigns and surveillance activities were hard won, as Haiti's Ministry of Public Health and Population had to overcome significant logistical problems. Gaining access to Haiti’s poorest, most vulnerable areas was a particular challenge. Most of the country's roads were in terrible condition and half of the country's health centers lacked adequate equipment to keep OPV viable. Some vaccine vials spoiled or broke, and some vaccination activities had to be rescheduled.^source

Haiti’s immunization campaigns effectively controlled the polio outbreak by blanketing the country with protection. The Expanded Program on Immunization (EPI) in the Americas reported that 2.4 million OPV doses were administered to children during the first national immunization day and house-to-house mop-up, reaching 88 percent of children under age 10. The second campaign did even better, reaching 93 percent of the target population.^source Local monitoring reports from health care workers on polio vaccine coverage and suspected AFP cases were used to validate the administrative data reported by the EPI. This confirmed about 90 percent immunization coverage and mop-up activities bumped that percentage even higher.^source

At a meeting in August 2001, one year after the first confirmed case of paralytic polio, George A.O. Alleyne, director of PAHO, called Haiti’s polio campaign a “success story”.^source The last confirmed case of paralytic polio in Haiti was documented on April 26, 2001.^source

No new cases of paralytic polio were reported after the immunization campaigns, confirming their success. Still, it remains unclear which aspects of Haiti’s efforts were the most effective and efficient, and how big of an impact the campaign had.

Stemming the paralytic poliovirus outbreak cost Haiti and its partners a total of US$4.82 million, according to the EPI.^source Of that, Haiti’s Ministry of Public Health and Population, despite its limited budget, contributed 20 percent—or about 2 percent of its total public health expenditure in 2001—to the effort. Partners contributed the rest: the CIDA, PAHO, the World Bank, the WHO, Rotary International, UNICEF, and USAID.

Notwithstanding methodological challenges, cost-benefit analyses of global polio eradication have been undertaken. The Global Polio Eradication Initiative (GPEI) estimates that eradicating polio at the global level will cost US$5.5 billion from 2013 to 2018.^source That's a lofty estimate—but failing to achieve global eradication would likely cost much more in terms of longer-term treatment costs. Eradicating polio by 2018 would result in massive savings, with some estimates showing a savings of US$40-$50 billion by 2035, and most of those savings would be in the world’s poorest countries.^source

Haiti’s polio success can be traced to several important factors: government and international agencies’ quick response, government’s renewed commitment to controlling polio, sustained and improved surveillance, and effective coordination with national and international partners.

Although the immunization program and surveillance system failed to provide an early warning of the circulating vaccine-derived polio outbreak in Hispaniola, Haiti stepped up to the plate and deployed its vaccination program and screening capabilities after a case was discovered. Notably, the activities Haiti undertook after a disappointing initial attempt mirrored international good practice. In the 1970s and 1980s, the success of polio eradication programs was based mainly on National Immunization Days, house-to-house mop-up campaigns, and robust surveillance system.^source These lessons from history proved hard to ignore.

Although Haiti’s finances were in dire straits, its renewed political commitment to eliminating polio was commendable. The Ministry of Public Health and Population's funding for one-fifth of the polio control program demonstrated that the government was serious about cutting off the polio epidemic quickly. The most visible sign of their commitment was health minister Voltaire’s and first lady Mildred Trouillot Aristide’s leadership on the combined polio-measles campaign.

Haiti’s Ministry of Public Health and Population could not have halted the outbreak on its own. Bilateral and multilateral aid agencies and foreign governments mounted a robust and sustained response to the need to detect cases of paralytic polio. Their financial support and technical expertise were indispensable. Also, coordination between the governments of Haiti and the Dominican Republic led to meetings to ensure that the two country’s efforts were in sync, which proved to be essential.

The GPEI, launched in 1988, has led the way towards an astounding 99 percent reduction in polio incidence worldwide. Still, polioviruses remain threats in a few strongholds, including in Pakistan and Afghanistan.

Whereas the outbreak in Hispaniola was restricted to two countries, the circulation of polio in other countries, such as Nigeria, spread to neighboring countries. Polio’s ability to cross borders underscores the urgency of closing the gaps in immunity and bringing an end to polio. But polio elimination programs face budget shortfalls, and it has become increasingly difficult to maintain high levels of vaccine coverage in countries already certified polio-free.^source

There is also debate about the best strategies for polio control. Some claim polio vaccination campaigns now deliver fewer returns than when the burden of disease was higher, and that those resources may be better spent on water and sanitation infrastructure, for example.^source However, NIDs remain a proven tool. This has been evident in India and Nigeria, where canceling NIDs resulted in a spike in polio cases, and the return of NIDs curbed the epidemics.

Another debate is whether to continue towards GPEI’s goal of global eradication of polio or to keep the disease tightly controlled via continued vaccination. To achieve eradication, leaders of the GPEI and affected country governments must agree on an “end game” strategy. This will require the elimination of OPV to prevent a vaccine-derived polio outbreak.

There are two potential strategies for eliminating the use of OPV. The first is a transition from OPV to IPV. However, there are questions about the efficacy of IPV in preventing poliovirus circulation, and IPV is more expensive strategy is a carefully coordinated termination of OPV without replacing it with IPV. However, this could cause new outbreaks of vaccine-derived virus if not coordinated on a global scale 

An alternative option is to continue to rely on OPV to keep the disease under tight control. However, as Haiti demonstrates, the vaccine itself may cause the next outbreak. Indefinite use of OPV can be lethal if not coupled with sensitive surveillance, vaccine stockpile maintenance, and investments in water and sanitation. Most health economists and advocacy groups argue for a carefully planned strategy for worldwide cessation of OPV use.