Two infectious disease experts warn that a new polio outbreak in Syria caused by wild-type polio virus 1 (WPV1) and asymptomatic cases in Israel might endanger Europe and other neighboring regions, according to correspondence published online November 8 in the Lancet.

“[The World Health Organization] has confirmed an outbreak of at least ten cases of polio in Syria, where vaccination coverage has dramatically decreased during the civil war,” write Professor Martin Eichner, from the Institute of Clinical Epidemiology and Applied Biometry, University of Tübingen, and Stefan Brockmann, from the Department for Infection Control, Reutlingen Regional Public Health Office, Germany. “Furthermore, [WPV1] has been isolated from sewage and faeces from asymptomatic carriers in Israel since February, 2013.

“Moreover, hundreds of thousands of people are fleeing Syria and seek refuge in neighbouring countries and Europe,” they continue. During the Hajj in Saudi Arabia last month, visitors from countries with known polio transmission were vaccinated, but Syria was not included with those countries.

“The potential risk of transmission to [the European Union (EU)] and elsewhere documents the need for strong ongoing global efforts to eradicate this disease,” CDC Director Tom Frieden, MD, MPH, told Medscape Medical News. “Polio anywhere is a threat of polio everywhere.”

The situation in the Middle East, combined with the vaccination approach used in Europe, is concerning, according to the authors. Most EU countries currently use inactivated polio vaccine (IPV) rather than oral polio vaccination (OPV). Similar to many other regions, most EU states discontinued use of OPV because of rare reports that it caused acute flaccid paralysis (AFP), even though OPV offers high protection against acquiring and transmitting the infection. Only some EU member states still permit OPV use, and none has a stockpile of it.

In contrast, the more widely used IPV is highly effective in preventing AFP and active polio disease, but is only partially effective in preventing infection with polio virus. For decades, Europe has been free of circulating polio viruses and, therefore, IPV has been sufficient.

However, IPV will only continue to be effective in preventing transmission if vaccination coverage continues to be very high, if hygienic standards are good throughout the population, and if there is low crowding. These conditions could easily be disrupted by the present situation of large numbers of refugees fleeing from Syria to Europe and other neighboring countries.

If the polio virus is reintroduced into the community, herd immunity may be insufficient to prevent sustained transmission in European regions where vaccination coverage is low, such as Bosnia and Herzegovina (87%), Ukraine (74%), and Austria (IPV coverage rate 83%).

For every 200 WPV1 infections, only 1 results in symptomatic polio. Therefore, hundreds of individuals could be infected and the virus could circulate for nearly a year before an outbreak could be identified from a single case of AFP.

“Vaccinating only Syrian refugees — as has been recommended by the European Centre for Disease Prevention and Control — must be judged as insufficient; more comprehensive measures should be taken into consideration;” the authors conclude. “Routine screening of sewage for poliovirus has not been done in most European countries, but this intensified surveillance measure should be considered for settlements with large numbers of Syrian refugees.”

Implications for the United States

In the United States, routine vaccination against polio currently uses IPV, which may create concerns similar to those now affecting Europe.

“The IPV vaccine is effective at preventing disease, but [OPV] is more effective at preventing even asymptomatic infection,” Jennifer L. Lyons, MD, from the Division of Neurological Infections, Department of Neurology, Brigham and Women’s Hospital in Boston, Massachusetts, told Medscape Medical News. “However, the OPV has been associated with a low but known risk of vaccine-related infection and, as such, is no longer routinely used.”

“It is difficult to project the effect that this scenario will have on the US, but vigilance is always good practice,” Dr. Lyons said. “Adherence to vaccination guidelines and maintenance of proper hygiene are likely the best preventive measures to take.”

Dr. Frieden told Medscape Medical News that IPV is used in Israel, as well as in the EU and the United States. “There have been no cases of indigenous polio in these regions, an especially significant fact considering that poliovirus has been found in environmental sewage samples in Israel,” Dr. Frieden said. “IPV is effective in protecting individuals against polio. The US has been free of indigenous polio since 1979.”

He also noted that the risk of importations of any infectious disease, including polio, into the United States is always a concern and highlights the importance of being vaccinated and of working to control infectious diseases wherever they are spreading.

“CDC works to minimize the risk for polio in the US through its traveler’s health and global migration program and global health program,” Dr. Frieden concluded. “We collaborate closely with international organizations and other countries to implement international and US guidance on vaccination for immigrants. In response to the polio cases in the Middle East and Horn of Africa, CDC has issued new recommendations for polio vaccine use among high-risk refugee populations and is working with international partners to implement them. Through the Global Polio Eradication Initiative, CDC works intensively with international health partners and Rotary International to eradicate polio at its source.”