5 Jun

Polioviruses occur  as  three serotypes. They  are  highly conta- gious and  paralysis can occur. Infection of one person may lead to infection of other household members or others in close con- tact in 73–96%  of the  cases, depending on the  contactee’s age. The  virus is spread by the  fecal-oral route with some  trans- missions being   oral-oral. The  virus first replicates in  the mucosal membranes  of the  pharynx and   gastrointestinal tract, then, 3–35  days  later, in  the  blood  stream and, to  a lesser degree, in  the  central nervous system. Polio  was  the dreaded summer disease up until the  1960s. While  up to 95% of cases are  subclinical, paralytic poliomyelitis accounted for 2%  of the  cases. From 2–5%  of young  children and   from 15–30% of adults who acquired paralytic polio died. Before the development of preventive vaccines, treatment  of paralytic polio  consisted of medications, iron  lungs, limb  and   back braces, and  rehabilitation therapy.

The  inactivated  poliovirus vaccine (IPV)  was  developed by Jonas Salk in the  early 1950s  and  was introduced for use in the  United States in 1955.  Although this vaccine was  shown to be safe  and  efficacious, its  use  quickly declined after intro- duction of the oral poliovirus vaccine (OPV) in the early 1960s.

An enhanced version of the  inactivated vaccine was developed in 1978  (182) and  later licensed in the  United States in 1987. This  more  potent formulation results in  improved immunity in children and  adults (183). In children given  the  three dose regimen, 99–100% developed antibody responses to all  three types of poliovirus two months after the second dose (184). Sig- nificant increases in  antibody concentrations were  observed after administration of the  third dose.  In  separate clinical studies, 99–100% of subjects developed protective antibodies after three doses  (185,186). The use  of IPV results in less  gas- trointestinal immunity than OPV  (163),  although the  newer enhanced formulation induces a significant degree of mucosal immunity that has  been  demonstrated to  produce effective protection  (185,188). The  duration of immunity induced by IPV is unknown, but  is thought to be long-term (Table 4.5). A study in  Sweden using 4 doses  of less  potent IPV  indicated that over  90% of vaccine recipients had  persistent antibodies after 25 years (189).

The  oral  polio  vaccine was  first licensed in  the  United States in  1963  and  consists of live  attenuated strains of the three serotypes of poliovirus, all grown in monkey kidney cell culture. In the 1960s, OPV quickly became the favored vaccine because of its  ease  of oral  administration, consistent produc- tion  of gastrointestinal  immunity, expected long-lasting immunity, and  spread of the  vaccine virus to  unvaccinated contacts (190). After three doses of OPV, over 95% of recipients produce immunity to  all  three serotypes of poliovirus (184). This  immunity is considered to be long-lasting, and  likely  life- long. Because of fecal shedding of the  vaccine virus after OPV administration, this vaccine can  immunize unvaccinated con- tacts (191). However, viral shedding of mutated virus may also lead  to  vaccine-associated paralytic  poliomyelitis (VAPP)  in unvaccinated contacts, particularly the  immunosuppressed.

Since  the  introduction and  widespread use  of the  two polio vaccines, the  number of poliovirus infections and  compli- cations has  dramatically decreased. In  1994,  the  Western Hemisphere was certified to be free of indigenous wild poliovi- rus  (192). The  last case  of indigenously acquired wild poliovi- rus  infection in  the  United States occurred in  1979  (193).

a   Immunogenicity appears to  be  variable for  OPV,  particularly in  underdeveloped areas. Factors associated with 1) maternal antibody level;  2) season; 3) diarrhea at time of vaccination; 4) exposure to other recipients; and  5) breastfeeding.

b   Estimated.

Since that time, an average of 8–9 cases of paralytic polio have been  reported each  year in the  United States due to the  use of the  oral,  live  attenuated polio  vaccine (OPV)  (194).  VAPP occurs  in one case  per  2.4 million doses,  but  is more  common after the  first vaccine dose  (one  case  per  750,000 first OPV doses)  (194).

The World  Health Organization developed a strategy for global  eradication of poliomyelitis by the  end of the  year 2000, which unfortunately  was  not  met.  Significant progress has been  achieved toward that  goal,  with a  90%  reduction of

poliomyelitis cases between 1988  and  1996  (195).  In  1988, poliovirus was  found  on  every  continent  except Australia. However, in  1998,  only  three major foci of disease remained including the  regions of South Asia, West  Africa,  and  Central Africa  (196).  Worldwide, there were  2979  new  cases in  2000 and  537 in 2001  (197). Global  eradication continues to be elu- sive  as  local  customs and  fear  affect  vaccination rates. As an example, parents in  Nigeria recently were  refusing to  allow children to be vaccinated for fear  that the  vaccine might con- tain HIV or make their children infertile. The vaccine is viewed as  a Western ploy  to curb  population growth. Nigeria is one country where polio eradication is in danger of failure (198).

In  2002,  India had  over  85%  of the  new  cases of polio worldwide despite national immunization days  and  house-to- house visits to administer OPV (199).

Adverse Effects

Vaccine-associated paralytic poliomyelitis (VAPP).

Between 1980  and  1994,  125  cases of VAPP  were  re- ported in  the   United States: 49  cases occurred in healthy  vaccine recipients; 40  cases developed in healthy contacts of the  vaccine recipient; 23 cases oc- curred in immunodeficient vaccines; 7 cases developed in immunodeficient contacts of vaccine recipients; and the  remaining 6 cases developed in  community con- tacts (194). VAPP more  frequently occurs  in adults, im- munodeficient persons, and  those receiving the  first dose  of OPV  (187). Because of the  diminished risk for wild  poliovirus disease in  the  United States, the  risk for VAPP is now considered to be less  acceptable (194). It is now  recommended that, to eliminate the  risk for VAPP,  an  all-IPV schedule be  used for  routine child- hood  vaccination in  the  United States. All  children should receive four  doses  of IPV: at age  2 months, age 4 months, between ages  6 and  18 months, and  between ages  4 and  6 years.

Inactivated polio vaccine (IPV). No serious adverse ef- fects have been  reported with IPV. This  vaccine contains trace amounts of polymyxin B, neomycin, and  strepto- mycin and  may cause hypersensitivity reactions in per- sons  allergic to these substances.

Oral  poliovirus vaccine (OPV). OPV  has  no  serious adverse effects  other than VAPP.

Guillain-Barré syndrome. Evidence indicates that nei- ther OPV nor IPV increases the  risk for Guillain-Barré syndrome (194).

Special Considerations

Oral  poliovirus vaccine (OPV). If available, OPV may be used only for the  following special circumstances:

Mass   vaccination campaigns to  control outbreaks  of paralytic polio.

Unvaccinated children who will be traveling within 4 weeks to areas where polio is endemic or epidemic. Children of  parents who  do  not   accept  the   recom-

mended number of vaccine injections; these children may  receive OPV only for the  third or fourth dose or both. In this situation, health-care providers should administer OPV  only  after discussing the  risk for VAPP with parents or caregivers.

As a result, OPV supplies are  expected to be very  lim- ited  in the  United States after inventories are  depleted (200).

Monkeyvirus SV40  and rare cancer (pleural me- sothelioma). High  levels  of SV40 cause tumors when injected in  rodents. There is  no  association between prior childhood vaccination of strains of SV40  in polio vaccine and  pleural mesothelioma. Persons least likely to have been  immunized (age >75 yrs) were  those with increased rates of plural mesothelioma (201).

Pleconaril. Pleconaril was used to treat three acute flaccid paralysis cases: two  vaccine-mediated and  one  wild- type  with a good clinical and  virological response (202). Immunocompromised. Live  vaccines are  contraindi- cated in people  who  are  infected with HIV  because of the  risk of infection from  attenuated microorganisms. Benefits of OPV outweigh the  risks and  should contin- ue  to  be  used in  countries where HIV  infections are endemic.

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