Wild poliovirus in China

7 September 2011

The World Health Organization has confirmed that cases of poliomyelitis in China were caused by wild poliovirus type 1:

1 SEPTEMBER 2011 – The Ministry of Health, China, has informed WHO that wild poliovirus type 1 (WPV1) has been isolated from four young children, aged between four months and two years, with onset of paralysis between 3 and 27 July 2011. All four cases are from Hetian prefecture, Xinjiang Uygur autonomous region, China. Genetic sequencing of the isolated viruses indicates they are genetically-related to viruses currently circulating in Pakistan. The last WPV case in China was reported in 1999, due to an importation from India. The last indigenous polio case occurred in China in 1994.

Remember that only one in 100 poliovirus infections lead to paralysis. For the four paralytic cases reported in China, there are likely 400 individuals who were infected with the virus but did not display obvious symptoms. This suggests a lapse in immunization coverage in this region. Consequently a polio vaccination campaign is planned for early September, the target being 3.8 million children <15 years of age in the outbreak area.

Pakistan, which shares a border with Hetian prefecture, appears to be the origin of the poliovirus strain causing the outbreak. Wild poliovirus type 1 continues to circulate in Pakistan, causing 76 reported cases of paralytic disease so far this year. This incident emphasizes the need for continued high immunization coverage in all countries until eradication of the virus is achieved. It is estimated that China has achieved >99.5% coverage for the third dose of poliovirus vaccine. In a country of 1.3 billion people, that leaves a large number of susceptible individuals.