 |
Immunity against polio can be stimulated in two ways: - through immunization, or
- following natural infection with poliovirus.
Poliovirus infection provides lifelong immunity against the disease, but this protection is limited to the particular type of poliovirus involved (Type 1, 2, or 3). Unfortunately, infection with one type does not protect an individual against infection with the other two types. The development of effective vaccines to prevent paralytic polio was one of the major medical breakthroughs of the 20th century. Two different kinds of vaccine are available:- A live attenuated (weakened) oral polio vaccine (OPV) developed by Dr. Albert Sabin in 1961. OPV is given orally.
- An inactivated (killed) polio vaccine (IPV), developed in 1955 by Dr. Jonas Salk; unlike OPV, IPV has to be injected by a trained health worker.
Both vaccines are highly effective against all three types of poliovirus.
There are, however, significant differences in the way each vaccine
works.
ORAL POLIO VACCINE (OPV)The action of oral polio vaccine (OPV) is two-pronged: OPV produces antibodies in the blood ('humoral' or serum immunity) to all three types of poliovirus. In the event of infection, this will protect the individual against polio paralysis by preventing the spread of poliovirus to the nervous system. OPV also produces a local immune response in the lining ('mucous membrane') of the intestines - the primary site for poliovirus multiplication. The antibodies limit the multiplication of 'wild' (naturally occurring) virus inside the gut, preventing effective infection. This intestinal immune response to OPV is probably the main reason why mass campaigns with OPV can rapidly stop person-to-person transmission of wild poliovirus.
Advantages of Oral Polio VaccineOPV is an orally applicable vaccine. It does not have to be administered by a trained health worker, can be given by volunteers, and - unlike most other vaccines - does not require sterile injection equipment. The vaccine is relatively inexpensive (current price for public health programmes in developing countries is 8 US cents a dose) - a major consideration when governments have to purchase massive quantities of vaccine for use during National Immunization Days.The short-term shedding of vaccine virus in the stools of recently immunized children means that in areas where hygiene and sanitation are poor - and the incidence of polio is likely to be highest - immunization with OPV can result in the 'passive' immunization of persons within close contact. As discussed above, the unique ability of OPV to induce intestinal, local immunity is probably responsible for the the extraordinary effect of OPV mass campaigns in interrupting wild poliovirus transmission. Due to these advantages, OPV remains the vaccine of choice for the eradication of polio, which would not be feasible with inactivated poliovaccine (IPV).
Disadvantages of Oral Polio Vaccine
Although OPV is safe and effective, in extremely
rare cases (approx. 1 in every 2.5 million doses of the vaccine) the
live attenuated vaccine virus in OPV can cause paralysis - either
in the vaccinated child, or in a close contact. Immune deficiency
of the recipient may be among the causes. This - extremely low - risk
of vaccine-associated polio (VAPP) is well known to, and accepted
by most public health programmes in the world because without OPV,
hundreds of thousands of children would be crippled every year. Immunization
programmes in countries where the risk of wild-virus caused polio
has come down to zero are now considering combined immunization schedules
using both OPV and IPV.
Rarely, a strain of poliovirus in OPV may
genetically change and circulate among a population. These are known
as vaccine-derived polioviruses (VDPV) and knowledge on them is
growing.
INACTIVATED POLIO VACCINE (IPV)Inactivated polio vaccine (IPV) needs to be injected and works by producing protective antibodies in the blood (serum immunity) - thus preventing the spread of poliovirus to the central nervous system. However, it induces only very low levels of immunity to polivirus locally, inside the gut. As a result, it provides individual protection against polio paralysis but, unlike OPV, cannot prevent the spread of wild polio virus.For an unusual, colorful narration of the story of polio vaccine development, turn to this link.
Advantages of inactivated Polio VaccineIPV is not a 'live' vaccine - the polio virus is inactivated - and immunization with IPV carries no risk of vaccine-associated polio paralysis. Immunization with IPV triggers an excellent response of the immune system in most IPV recipients.
Disadvantages of inactivated Polio VaccineUnlike the oral vaccine, IPV confers only very little immunity in the intestinal tract. When a person immunized with IPV is infected with wild poliovirus, virus can still multiply inside the intestines and be shed in stools -- risking continued circulation. For this reason, OPV is the vaccine of choice wherever a polio outbreak needs to be contained, even in countries which rely exclusively on IPV for their routine immunization programme (polio outbreak in the Netherlands in 1992).Other disadvantages of IPV include the price (over five times that of OPV), the cost of the syringe, and the need for trained health workers to administer the vaccine using sterile injection procedures.
|
|
 |
