The number of cases of polio last year climbed – and spread into countries where it was previously wiped out — despite an aggressive worldwide effort to eradicate the potentially paralyzing virus in the nations still battling it half a century after the introduction of the polio vaccine.

An estimated 350,000 cases of the disease occurred annually in 1988, when world leaders launched the Global Polio Eradication Initiative to eliminate the virus in the 125-plus countries where it still existed. By 2006, polio remained endemic to just four countries – Nigeria, Afghanistan, India and Pakistan – and by 2007, the number of reported cases was down to 1,315. But over the next year, infections rose by 26 percent, to 1,655 cases, according to the Morbidity and Mortality Weekly Report (MMWR).

The biggest jump was in Nigeria, where there were 801 cases last year, up from 285 in 2007, according to the MMWR. As we noted in January, the northern part of the country is plagued by flawed vaccination campaigns that miss up to half of the children that require them. (The oral vaccine is given at birth and in three subsequent doses over a child’s first year.) The report says these lapses contributed to the disease’s spread to six countries — Benin, Burkina Faso, Cote d'Ivoire, Ghana, Mali and Togo — where polio had been eradicated as well as to Niger and Chad, which had been battling resurgences of the virus.

The situation in Nigeria is "a reflection not so much of something getting worse but of things not getting better," Steven Wassilak, a medical epidemiologist at the Centers for Disease Control and Prevention (CDC) tells Vaccine campaigns are "not well enough planned or executed so they miss 40 percent of children in the very core of the infectious area. So as children are born who don’t get vaccine or enough doses, you get a buildup of [susceptible] children entering the population."

Politics also plays a role in polio's persistence. Some Nigerian states banned the vaccine between 2003 and 2005 amid unfounded rumors that it was contaminated with HIV or would cause sterility. "It's hard to undo rumors completely, even if you have more and more leaders talking about the safety of the vaccine," Wassilak says.

There were 559 cases of polio in India last year, down from 874 the year before; 118 in Pakistan, compared to 32 the previous year; and 31 in Afghanistan, versus 17 in 2007. Polio persisted in India because children living in crowded areas and already weakened by diarrhea and malnutrition may not have been able to mount a strong immune response, according to the report. The virus continued to spread in Afghanistan because anti-government activity in the South prevented children from receiving the vaccine, CDC virologist Mark Pallansche tells, and in parts of Pakistan, he says, organizational problems and lawlessness in tribal areas interfered with kids being inoculated. 

In January, the Bill & Melinda Gates Foundation, Rotary International and the British and German governments committed to $630 million to eradicate polio.

Image of child receiving polio vaccine/USAID via Wikimedia Commons