When the wild polio virus (WPV1) reappeared in northern Nigeria last summer, international aid agencies launched a massive campaign to immunize millions of children throughout the region. It had been about two years since the last case of polio in Africa and had looked like Nigeria was well on its way to becoming the final African country to boast eradication.
To help prevent such polio outbreaks, ongoing and universal vaccination remain essential, of course. But challenges remain, especially as the traditional oral polio vaccine (OPV) is increasingly replaced by injectable inactivated polio vaccine (IPV).
Developed in the 1950s, OPV is a live weakened virus that is generally safe, easy to administer, effective and inexpensive to manufacture. OPV also exposes children’s siblings to the vaccine’s benefits, which helps provide much-desired herd protection. That means fewer children may have to be vaccinated.
OPV can be deployed more quickly than IPV, making it more useful during outbreaks. OPV is also shed in fecal matter and typically spreads the vaccine virus to family members and neighbors, which is normally a good thing. Yet vaccine virus has the potential to revert back to wild-type polio virus, and, in rare cases, has been shown to cause paralysis in both the person vaccinated and contacts.
Given such concerns, demand for IPV, an inactivated polio vaccine, is on the rise around the world. A switch from OPV to IPV is part of the WHO’s global plan to stop the virus and end vaccine-related paralysis. But IPV has its own challenges – this polio vaccine is commonly combined with other vaccinations. Several multiple-component vaccines are available, including Pentacel (DTap, HIB, Polio) and Pediarix (DTap, Hep B, Polio). Depending on their mix, these vaccines protect against polio as well as diphtheria, tetanus and pertussis (DTap), Haemophilus influenza type b (HIB) and hepatitis B.
Combination vaccines are a good idea unless the children in need of a polio vaccine have already received vaccinations for these other diseases. In those communities where healthcare records are incomplete or unavailable, a child’s vaccination history may be unknown – administering a combined vaccine could then interfere with established schedules and expose children to costly and unnecessary vaccinations. In addition, such multiple-component vaccines can be very expensive to develop and manufacture.
The solution, therefore, is to develop a stand-alone IPV that is safe and inexpensive to manufacture. With the potential for more polio outbreaks in the developing world, and with the longtime focus on eradication, global agencies such as WHO, UNICEF and the Bill & Melinda Gates Foundation are urgently seeking ways to develop such a stand-alone IPV. Developing countries need the flexibility that a stand-alone vaccine can offer, and want to reduce the expense of administering superfluous vaccines.
While IPV does not provide family-level protection because the virus is inactivated, and therefore requires that everyone be vaccinated, it does provide the best hope for safe global eradication. In the future, a stand-alone IPV should bring significant benefits to the global eradication plan and reduce the current risks associated with OPV.