Have Polio-Free Countries Lost Sight of Need to Keep Vaccination Rates High?

A recent alarm warning against complacency in polio vaccination has come from an unlikely place: Canada.

Infectious poliomyelitis, eradicated in much of the world during the past 2 decades, remains endemic only in Nigeria, India, Afghanistan, and Pakistan. But a new outbreak last spring in Tajikistan has resulted in 452 laboratory-confirmed cases of wild poliovirus type 1 and 20 deaths. At least 7 related cases have been reported in the Russian Federation.

The outbreak is the first to strike a World Health Organization (WHO)–certified polio-free region. Tajikistan is in the WHO European Region, which was certified polio-free in 2002.

Tajikistan's outbreak, imported from northern India, prompted a bluntly worded editorial in the Canadian Medical Association Journal (CMAJ). The Journal 's public health section editor, Noni MacDonald, MD, and its editor-in-chief, Paul Hébert, MD, wrote that the Tajikistan outbreak “should be clanging alarm bells” throughout polio-free regions, including developed countries in North America and Europe (MacDonald N and Hébert P. CMAJ. 2010;182[10]:1013).

“The threat now is more than theoretical,” Hébert said in an interview. “The risk is, including in the United States, largely because we seem to have let our guard down” in maintaining high vaccination rates.

The editorial noted that the percentage of children in Ontario who have been immunized against polio during the past decade has ranged between the high 70s and low 80s. The WHO recommends vaccination coverage of at least 90% to prevent transmission. The agency reported that polio vaccine coverage in Tajikistan was 83% in 2005, but increased to 93% in 2009.

School requirements for polio vaccination help maintain high coverage rates in the United States. Even so, the National Immunization Survey in 2007-2008 reported that 9 states had polio immunization rates below 90%. Idaho's 86.5% was the lowest (http://www2a.cdc.gov/nip/coverage/nis/nis_iap.asp?fmt=r&rpt=tab21_pol_race_iap&qtr=Q3/2007-Q2/2008).

Samuel Katz, MD, chair of the WHO Polio Research Committee and chair emeritus of pediatrics at Duke University School of Medicine in Durham, NC, called it “disappointing” that so many states' vaccination rates were below the 90% threshold. Katz noted that because most young parents and health professionals have never seen polio unless they worked overseas, immunization may not be as high a priority as it should.

“Parents have concerns if [vaccination] is safe, or even necessary, and physicians may be a little less convincing that it is necessary to get kids immunized if they’ve never seen it,” he said. The last indigenous case of wild poliovirus transmission in the United States was in 1979, according to the US Centers for Disease Control and Prevention (CDC). The United States and Canada are included in the WHO Region of the Americas, which was certified polio-free in 1994.

Until polio is eradicated, Katz and others said, outbreaks in developed countries are possible but unlikely to spread.

“Most countries, really all countries, in the developed world are protected by herd immunity given by vaccination, so it's unlikely that the introduction of a wild polio virus would cause a major epidemic in those countries,” said Stanley Plotkin, MD, professor emeritus of pediatrics at the University of Pennsylvania School of Medicine in Philadelphia.

Steven Wassilak, MD, a medical epidemiologist with CDC's Global Immunization Division, agreed with the need for vigilance in maintaining high polio immunization rates. “Several countries have allowed their vaccination rates to slip substantially,” he said.

But Wassilak rejected the CMAJ editorial's contention that the WHO should be more proactive in warning countries of the dangers of low immunization rates than simply posting case numbers at http://www.polioeradication.org. He noted that a new strategic plan from the WHO and its polio eradication partners calls for local governments in high-risk countries to become more involved and accountable in maintaining high immunization rates. “Those plans already have derived some benefits,” he said.

An example is Nigeria, where polio is endemic. As of August 2, the WHO reported that Nigeria had 6 cases of polio compared with 365 at the same time last year. India, also an endemic country, had 25 cases compared with 163 at the same time last year. Yet Pakistan, another endemic country, has 34 cases compared with 25 last year.

“We can't see progress in Pakistan to date,” Wassilak said. “But we have made progress in the global fight.”