Poliomyelitis eradication

Main article: Poliomyelitis

  The global eradication of poliomyelitis is a public health effort to eliminate all cases of poliomyelitis infection. The global effort, begun in 1988 and led by the World Health Organization, UNICEF and The Rotary Foundation, has reduced the number of annual diagnosed cases from the hundreds of thousands to around a thousand. Should the effort be successful, the eradication of polio will represent only the second time mankind has ever completely eliminated a disease. The first such disease was smallpox, which was officially eradicated in 1979.^[1]

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Factors influencing eradication

Eradication of polio has been defined in various ways -- as elimination of the occurrence of a poliomyelitis even in the absence of human intervention,^[2] as extinction of poliovirus, such that the infectious agent no longer exists in nature or in the laboratory,^[3] as control of an infection to the point at which transmission of the disease ceased within a specified area,^[2] and as reduction of the worldwide incidence of poliomyelitis to zero as a result of deliberate efforts, and requiring no further control measures.^[4]

In theory, if the right tools were available, it would be possible to eradicate all infectious diseases. In reality there are distinct biological features of the organisms and technical factors of dealing with them that make their potential eradicability more or less likely.^[5] Three indicators however, are considered of primary importance in determining the likelihood of successful eradication: That effective interventional tools are available to interrupt transmission of the agent, such as a vaccine. That diagnostic tools, with sufficient sensitivity and specificity, be available to detect infections that can lead to transmission of the disease, and that humans are required for the life-cycle of the agent, which has no other vertebrate reservoir and cannot amplify in the environment.

Strategy

The most important step in eradication of polio is interruption of endemic transmission of poliovirus. Stopping polio transmission has been pursued through a combination of routine immunization, supplementary immunization campaigns and surveillance of possible outbreaks. The four key strategies outlined by the World Health Organization for stopping polio transmission are:^[6]

1. High infant immunization coverage with four doses of oral polio vaccine (OPV) in the first year of life in developing and endemic countries, and routine immunization with OPV and/or IPV elsewhere.
2. Organization of “National immunization days” to provide supplementary doses of oral polio vaccine to all children less than five years of age.
3. Active surveillance for wild poliovirus through reporting and laboratory testing of all cases of acute flaccid paralysis^[7] among children less than fifteen years of age.
4. Targeted "mop-up" campaigns once wild poliovirus transmission is limited to a specific focal area.

Vaccination

Among the factors that have bolstered efforts to eradicate polio is that the oral polio vaccine is both highly effective and cheap (about US$0.10 per dose, or US$0.30 per child^[8]); vaccination generally provides lifelong immunity to the virus.^[9] A study carried out in an isolated Eskimo village showed that antibodies produced from subclinical wild virus infection persisted for at least 40 years.^[10] Because the immune response to oral polio vaccine is very similar to natural polio infection, it is expected that oral polio vaccination provides similar long term immunity.^[11]

Contact immunity to polio can occur when attenuated poliovirus derived from the oral polio vaccine is excreted, and infects and indirectly vaccinates unvaccinated individuals.^[12]

Herd immunity

Polio vaccination is also important in the development of herd immunity.^[13] For polio to occur in a population, there needs to be an infecting organism (poliovirus), a susceptible human population, and a cycle of transmission. Poliovirus is transmitted only through person-to-person contact and the transmission cycle of polio is from one infected person to another person susceptible to the disease, and so on.^[12] If the vast majority of the population is immune to a particular agent, the ability of that pathogen to infect another host is reduced; the cycle of transmission is interrupted, and the pathogen cannot reproduce and dies out. This concept, called community immunity or herd immunity, is important to disease eradication because if the number of susceptible individuals can be reduced to a small number through vaccination, the pathogen itself can also be eliminated.

When many hosts are vaccinated, especially simultaneously, the transmission of wild virus is blocked, and the virus is unable to find another susceptible individual to infect. Because poliovirus can only survive for a short time in the environment (a few weeks at room temperature, and a few months between 0–8° Celsius (32–46° Fahrenheit)) without a human host the virus dies out.^[14]

Herd immunity is an important supplement to vaccination. Among those individuals who receive oral polio vaccine, only 95 percent will develop immunity.^[15] That means 5 of every 100 given the vaccine won’t develop any immunity and will be susceptible to developing polio. According to the concepts of herd immunity this population whom the vaccine fails, are still protected by the immunity of those around them. Herd immunity can only be achieved when vaccination levels are high.^[13] It is estimated that 80-86 percent of individuals in a population must be immune to polio for the susceptible individuals to be protected by herd immunity.^[13] If routine immunization were stopped, the number of unvaccinated, susceptible individuals would soon exceed the capability of herd immunity to protect them.^[16]

Obstacles

Among the greatest obstacles to global polio eradication are the lack of basic health infrastructure, which limits vaccine distribution, the crippling effects of civil war and internal strife, and a philosophical objection to vaccination based on religious reasons in the remaining polio endemic countries. Another challenge has been maintaining the potency of live (attenuated) vaccines in extremely hot or remote areas. The oral polio vaccine must be kept at 2-8° Celsius for vaccination to be successful.^[9]

Progress

Following the widespread use of poliovirus vaccine in the mid-1950s, the incidence of poliomyelitis declined rapidly in many industrialized countries.^[15] Czechoslovakia became the first country in the world to scientifically demonstrate nationwide eradication of poliomyelitis in 1960.^[22] In 1962 — just one year after Sabin's oral polio vaccine (OPV) was licensed in most industrialized countries — Cuba began using the oral vaccine in a series of nationwide polio campaigns. The early success of these mass vaccination campaigns suggested that polioviruses could be globally eradicated.^[23] The Pan American Health Organization (PAHO), under the leadership of Ciro de Quadros, launched an initiative to eradicate polio from the Americas in 1985.^[24]

1988–1991

In 1988, the World Health Organization, together with Rotary International, UNICEF, and the U.S. Centers for Disease Control and Prevention passed the Global Polio Eradication Initiative, with the goal of eradicating polio by the year 2000. The Initiative was inspired by Rotary International's 1985 pledge to raise $120 million toward immunising all of the world's children against the disease.^[24] The last case of wild poliovirus poliomyelitis in the Americas was reported in Peru, August 1991.^[24]

1994–2000

On 20 August 1994 the Americas were certified as polio-free.^[25] This achievement was a milestone in efforts to eradicate the disease.

In 1994 the Indian Government launched the Pulse Polio Campaign to eliminate polio. The current campaign involves annual vaccination of all children under age five.^[26] Most families have allowed their children to take the vaccine.

In 1995 Operation Mecacar (Mediterranean, Caucasus, Central Asian Republics and Russia) were launched; National Immunization Days were coordinated in 19 European and Mediterranean countries.^[27] In 1998, Melik Minas of Turkey became the last case of polio reported in Europe.^[28] In 1997 Mum Chanty of Cambodia became the last person to contract polio in the Indo-West Pacific region.^[29] In 2000 the Western Pacific Region (including China) was certified Polio-free.^[29]

In October 1999, the last isolation of type 2 poliovirus occurred in India. This type of poliovirus appears to have been eradicated.^[15]

2001–2004

By 2001, 575 million children (almost one-tenth the world's population) had received some 2 billion doses of oral polio vaccine.^[18] The World Health Organization announced that Europe was polio-free on June 21, 2002 in the Copenhagen Glyptotek.^[30]

In 2002, an outbreak of polio in India occurred after the number of planned polio vaccination campaigns was reduced and the state of Uttar Pradesh accounted for nearly two-thirds of total worldwide cases reported.^[31] (See Global polio incidence map (2002).)

In the Kano province in Northern Nigeria, which operates under Sharia (Muslim religious law), the immunization campaign was suspended in September 2003 when prominent Muslim leaders claimed vaccines supplied by Western donors were adulterated to reduce fertility and spread HIV as part of a U.S.-led drive against Islam. On June 30, 2004, after a 10-month ban on polio vaccinations, the WHO announced that Kano had pledged to restart the campaign in early July. During the ban the virus spread across Nigeria and into 12 neighboring countries that had previously been polio-free.^[24] By 2006, this ban would be blamed for 1,500 children being paralyzed and having caused $450 million for emergency activities. In addition to the rumors of sterility and the ban by Nigeria's Kano state, civil war and internal strife in the Sudan and Ivory Coast have complicated WHO's polio eradication goal. In 2004, almost two-thirds all the polio cases in the world occurred in Nigeria (760 out of 1170 total).

2005

There were 1,979 cases of wild poliovirus (excludes vaccine derived polio viruses) in 2005.^[19] Most remaining polio infections were located in two areas: the Indian subcontinent and Nigeria.

Nigeria experienced a drop in the number of polio cases of nearly a half from last year, according to the World Health Organization. Officials credit the drop in new infections to improved political control in the southern states and resumed immunisation in the north, where Muslim clerics led a boycott of vaccination in late 2003.^[32]

Eradication efforts in the Indian sub-continent have met with a large measure of success. Using the Pulse Polio campaign to increase polio immunization rates, India recorded just 66 cases in 2005; down from 135 cases reported in 2004, 225 in 2003, and 1,600 in 2002.^[20]

Yemen, Indonesia and Sudan, countries which had been declared polio-free since before 2000, each reported hundreds of cases - probably imported from Nigeria.^[32] On May 5 2005, news reports broke that a new case of polio was diagnosed in Java, Indonesia and the virus strain was suspected to be the same as the one that has caused outbreaks in Nigeria. New public fears over the safety, which were unfounded, impeded vaccination efforts in Indonesia. In summer 2005 the WHO, UNICEF and the Indonesian government made new efforts to lay the fears to rest, recruiting celebrities and religious leaders in a publicity campaign to promote vaccination.^[33]

The first case of the polio outbreak in Sudan was detected in May 2004. The reemergence of polio led to stepped up vaccination campaigns. In the city of Darfur; 78,654 children were immunized and 20,432 more in southern Sudan (Yirol and Chelkou).^[34]

In the United States it was reported that "on September 29, 2005 the Minnesota Department of Health (MDH) identified poliovirus type 1 in an unvaccinated, immunocompromised infant girl aged 7 months (the index patient) in an Amish community whose members predominantly were unvaccinated for polio. The patient has no paralysis; the source of the patient's infection is unknown. Subsequently, poliovirus infections in three other children within the index patient's community have been documented."^[35]

2006–present

In 2006 only four countries in the world (Nigeria, India, Pakistan, and Afghanistan) were reported to have endemic poliomyelitis. Cases in other countries are attributed to importation. A total of 1997 cases worldwide were reported in 2006, of these 1869 cases occurred in countries with endemic polio.^[19] Nigeria accounted for the majority of cases (1122 cases) but India has reported more than ten times the number of cases this year as it had in 2005 (676 cases, or 30% of worldwide cases). Pakistan and Afghanistan reported 40 and 31 cases respectively in 2006. Polio re-surfaced in Bangladesh after nearly six years of absence with 18 new cases reported. "Our country is not safe, as neighbours India and Pakistan are not polio free", declared Health Minister ASM Matiur Rahman.^[36] (See: Map of reported polio cases in 2006)

In 2007 Pakistan continued to battle polio. In tribal areas, immunization campaigns are unpopular as Muslim clerics have claimed that the immunizations are part of an American conspiracy designed to sterilize the local Muslim population.^[37] In February 2007, physician Abdul Ghani, who was in charge of polio immunizations in a key area of disease occurrence in northern Pakistan, was killed in a terrorist bombing.^[38] In July 2007, a student traveling from Pakistan imported the first polio case to Australia in over 20 years.^[39]

See also

• Eradication of smallpox
• Population health
• Mathematical modelling in epidemiology
• Transmission risks and rates

References