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Polio Highlight

FAQ #5 What is the risk for countries if they do not introduce IPV?

What is the risk for countries if they do not introduce IPV?

Two main risks are associated with OPV type 2 withdrawal:

  • immediate time-limited risk of cVDPV2 emergence; and
  • medium and long-term risks of poliovirus re-introduction from a vaccine manufacturing site, research facility or diagnostic laboratory. 

All countries face a time-limited (1-2 years) risk of cVDPV2 outbreak during OPV type 2 withdrawal if they do not introduce a dose of IPV.


Why can’t OPV withdrawal occur immediately and all countries switch entirely to IPV use instead of 1 dose in routine immunization?

Until polio transmission is interrupted globally, OPV will be a critical component of the eradication strategy. OPV is the appropriate polio vaccine for achieving the eradication of wild polioviruses worldwide because it is inexpensive, easy to administer and offers good oral and intestinal immunity, which is needed to interrupt person-to-person spread of the virus, particularly in settings of high population density and poor sanitation.

24 October - World Polio Day

World Polio Day provides governments, civil society and communities worldwide an opportunity to intensify efforts to eradicate the disease once and for all. Countries are uniting now for the common goal of global polio eradication; thanks to vaccination campaigns that have protected millions, a disease that once paralyzed 1,000 children each day is now almost history. The Americas was the first region of the world to be declared polio-free in 1994.

FAQ #4 What schedule should countries be using for IPV, and how many doses are recommended?

What schedule should countries be using for IPV, and how many doses are recommended?

In April of 2014, the Pan American Health Organization’s (PAHO) Technical Advisory Group on Vaccine-preventable Diseases (TAG) issued the following recommendations for the Region of the Americas:

  • When introducing IPV, countries should consider sequential schedules. Ideally, countries should consider two IPV doses followed by two OPV doses. However, if a country is considering only one IPV dose, this should be with the first DTP dose and followed by three OPV doses.
  • Countries should not consider moving directly to an IPV only schedule at this time, unless they meet the criteria previously recommended by TAG and WHO (low risk of transmission and importation, high homogeneous coverage, and good sanitation).

Vaccination Schedule recommended for the introduction of inactivated poliovirus vaccine (IPV) in combination with the oral poliovirus vaccine (OPV)









First option






Alternate option







This schedule, in addition to preparing the countries for the switch from tOPV to bOPV, has the additional advantage of lowering the incidence of VAPP cases, considering that in our Region, around 50% of VAPP cases are associated with the first dose of OPV.


What is the difference between IPV and OPV?

IPV and OPV evoke different immune responses and therefore have distinct advantages and disadvantages. To complete eradication and get the benefits of both, they should be used together.

Figure 1: A comparison of advantages and disadvantages for OPV and IPV



Oral Polio Vaccine (OPV)

    Humoral (antibodies in the blood) immunity.

    Gut/intestinal immunity.

    Easy to administer via drops.


    Vaccine-associated paralytic poliomyelitis (VAPP) globally occurs in rare cases (2-4 cases per 1 million children).

    Rarely, through circulation in poorly immunized populations, the vaccine viruses mutate to circulating vaccine-derived polioviruses (cVDPVs) and can cause outbreaks of paralytic polio.

Inactivated Polio Vaccine (IPV)

    Very good humoral immunity.

    Equivalent to OPV in inducing immunity in the oral cavity thus is as effective as OPV in stopping oral – oral transmission of virus.

    Insufficient to prevent wild polio virus (WPV) replication in guts of infected person and consequently poliovirus can still be transmitted by excretion in stool.

    Requires injection.

    More expensive than OPV.

FAQ #3 When do countries need to introduce IPV and switch to bOPV?

When do countries need to introduce IPV and switch to bOPV?

OPV type 2 withdrawal would be achieved by switching from trivalent OPV (tOPV) to bivalent OPV (bOPV) (containing only types 1 and 3 poliovirus) in routine immunization programs. The World Health Organization’s (WHO) Strategic Advisory Group of Experts on immunization (SAGE) has called for a global withdrawal of type 2-containing OPV during 2016. This sets the stage for ending bOPV use entirely in 2019-2020. As a risk mitigation measure, SAGE recommends that prior to the ‘tOPV-bOPV switch’ all countries that currently use only OPV in their routine immunization programs introduce at least 1 dose of IPV into their routine schedules (i.e., by the end of 2015).


Why should countries introduce IPV prior to the tOPV-bOPV switch?

The withdrawal of OPV type 2 would leave a gap in population immunity against type 2 poliovirus. Thus, immediately following global withdrawal of OPV type 2, countries that have not introduced IPV would be at an increased risk of outbreaks in the case of reintroduction of a type 2 virus. A reintroduction or emergence of circulating vaccine-derived poliovirus type 2 (cVDPV2) could potentially result in a substantial polio outbreak or even re-establishment of global transmission.  Such an outbreak could be rapidly interrupted through mOPV type 2. Vaccinating the population with IPV through routine immunization would lessen the risk that reintroduction would lead to sustained transmission.  If reintroduction of type 2 polioviruses does occur post-eradication, having a population that has received IPV would also facilitate rapid control through targeted use of mOPV type 2.

FAQ #2 Why will countries need to switch from tOPV to bOPV?

Why will countries need to switch from tOPV to bOPV?

There are three types of wild poliovirus (WPV) - type 1, 2 and 3 - each of which is targeted by a different component of the trivalent oral polio vaccine (tOPV).

Live attenuated vaccines are very effective against the wild virus, but in very rare cases can lead to paralysis. There are two ways this can occur:

  • Vaccine Associated Paralytic Poliomyelitis (VAPP): At a global level for every birth cohort of 1 million children in OPV-only using countries, there are 2-4 cases of VAPP.  This translates to an estimated 250 – 500 VAPP cases globally per year.  Of these, about 40% are caused by OPV’s type 2 component. In the Region of the Americas, the VAPP risk is 1 case per 7.68 million doses administered.
  • Circulating Vaccine Derived Poliovirus (cVDPV) outbreaks: these rare outbreaks occur when a vaccine-related virus is passed from person-to-person, mutating over time and acquiring wild virus transmissibility and neurovirulence characteristics.  Almost all cVDPV outbreaks in recent years have been caused by a type 2 vaccine-derived virus. 

Although wild poliovirus type 2 appears to have been eradicated globally in 1999, vaccine-related type 2 viruses continue to cause the majority of cVDPV outbreaks and many VAPP cases. Therefore, OPV type 2 now carries more risk than benefit and undermines global polio eradication efforts. Thus, tOPV will be replaced with bivalent OPV (bOPV), which will continue to target the remaining polio types 1 and 3. Once these types are eradicated, bOPV will also be withdrawn.


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