In April 2003, a vaccine-derived poliovirus Sabin 2 (VDPV) was isolated from sewage water in Vrakuna, a Bratislava city district in western Slovakia. The mutated strain was 87% genetically identical to the vaccine strain and differed from the wild poliovirus strain by 1.6%, and it was assumed that the strain isolated in sewage had already gained wild poliovirus neurovirulence and infectivity. Investigators hypothesised that this divergence could have been caused by immunodeficient people excreting vaccinal poliovirus Sabin 2. There were concerns that this modified virus could cause paralytic disease in people.

After active environmental surveillance was launched in October 2003, two more genetically mutated polioviruses from the vaccine strain Sabin 2 were isolated from sewage water samples in Skalica, a town 150 km west of Bratislava. These isolates also showed high divergence from the vaccine strain, and in both localities the strains had almost identical differences to the vaccine strain.

VDPV occurs as a result of excretion by immunodeficient patients, but also by healthy vaccinated people. Tests on 556 sewage water samples from eight different sewage system branch collectors were carried out, and 72 mutated strains of Sabin 2 derived from the oral polio vaccine were found. Isolated viruses were highly divergent: the level of genetic difference of the most recent ones from the wild poliovirus was 0.05%. Two of these isolates were less than 85% (84.9% and 84.4%) similar to the vaccine strain, which is the threshold theoretically beyond which a mutated strain gains the neurovirulent and infective characteristics of a wild poliovirus [1].

Investigation of the VDPV source
Additional samples from sewage water and stool samples from immunodeficient people in the Skalica area were tested to locate the VDPV excretors. The investigation focused on this local area because it had a relatively small sewage system serving fewer residents.

Test on samples from immunodeficient children were all negative. Testing of samples from immunodeficient people with B lymphocyte disorders, and from people who had moved to the area since 2001 is ongoing. All results have so far been negative.

Polio surveillance in Slovakia
Polio has been under surveillance in Slovakia for more than 50 years. In the past, polio epidemics occurred in 5 year cycles, the last being in 1953 before the start of mass vaccination with the inactive Salk vaccine in 1957.The live attenuated (weakened) oral polio Sabin vaccine was introduced in 1960, the year of the last reported poliomyelitis cases in Slovakia.

Monitoring the circulation of polioviruses by examining sewage waters began in Slovakia in 1970. Samples of polioviruses and other enteroviruses are collected throughout the year in selected drainage inlets to municipal sewage plants throughout Slovakia. To reinforce surveillance of high-risk population groups, sewage waters from all refugee centres providing accommodation and basic services for refugees coming to Slovakia mostly from Asia (e.g., Pakistan, Afghanistan or Chechnya) are regularly taken for examination. Wild poliovirus was last isolated from sewage water in 1972, and vaccinal polioviruses have been sporadically detected since 1972.

Vaccination and polio immunity
Inactivated (dead) parenteral polio vaccine has been used nationally since 2005. Vaccination coverage in children has been very high for the past 20 years, reaching almost 98%. The effectiveness of the vaccination policy has been regularly monitored by seroprevalence studies, the most recent being in 1997. The results showed that immunity of the Slovak population to all polio types was high. The proportion of individuals with poliovirus type 1 antibodies was 94%; with poliovirus type 2 antibodies, 97%; and with poliovirus type 3 antibodies, 97%. In children aged between 0 and 15 years, the proportion ranged from 98% to 100%.

Surveillance of acute flaccid paralysis in Slovakia
Reporting, examination and analysis of acute flaccid paralysis (AFP) has been done in Slovakia since 1970. No cases of AFP compatible with poliomyelitis were reported in Slovakia in 2004 or in previous years. The last case of paralytic poliomyelitis was recorded in 1960. No cases of postvaccinal paralytic poliomyelitis were reported in 2004 or in previous years. The temporary circulation of vaccinal polioviruses in the population as a result of short-term vaccination campaigns and vaccination of immunodeficient people with inactive polio vaccine have probably contributed to this situation.

Conclusions
A special epidemiological situation has occurred in Slovakia, where a poliovirus contained in a live polio vaccine used in Slovakia since 1960 has changed its genetic characteristics and differs only minimally from the wild poliovirus.

VDPV can cause serious neurological disease, particularly in immunodeficient people, and in groups of susceptible unvaccinated or incompletely vaccinated people. This is why monitoring of polio vaccination coverage in children as well as vaccination of susceptible people in places with incidence of VDPV is carried out. In Skalica, the live vaccine was immediately replaced with an inactivated (dead) vaccine, and since 1 January 2005, inactivated vaccine has been used nationwide.

Repeated VDPV isolation within a relatively short time period presents a problem for experts, particularly in regard to the Certification of Poliomyelitis Elimination in Slovakia as a part of the polio-free WHO European Region and other planned global activities to eradicate poliomyelitis in the world [2,3].

Acknowledgements
World Health Organization, for their assistance in carrying out the investigations.