Investigations of clinical isolations of oral poliovirus vaccine strains between 2000 and 2005 in southern Taiwan
Introduction
Live-attenuated oral poliovirus vaccine (OPV) has played an important role in the eradication of wild-type polioviruses in many parts of the world since the 1960s.1, 2 OPV replicates in the gastrointestinal (G.I.) tract after vaccination and can infect unvaccinated hosts. This increases the population immunity in areas with suboptimal vaccine coverage.1, 2 Vaccinees can shed viruses up to 4 months or longer; this period can be decades if recipients are immunodeficient.3, 4, 5 The excreted viruses may rarely revert to neurovirulent strains and produce vaccine-associated paralytic poliomyelitis (VAPP) in the recipients and their contacts.1, 5 Sequential infections and outbreaks caused by vaccine-derived polioviruses (VDPV) have been reported where immunization rates were not high.6
The poliovirus contains a 7.5 kb single-stranded RNA genome that has a high rate of genetic evolution through nucleotide substitutions and recombination with other polioviruses or human enteroviruses C (HEV-C).7, 8 The nucleotide substitution rate is around 1% per year in the VP1 gene. Sequence divergence in VP1 from Sabin strains is used by the World Health Organization (WHO) to categorize isolates into: (1) <1%, OPV-like; (2) 1–15%, VDPVs; and (3) >15%, wild-type.1, 9, 10 Recombination can occur early and frequently in OPV recipients.11 Previous studies showed that recombinant strains were mostly OPV-like isolates, but also could be VDPVs or cause VAPP.12, 13, 14, 15, 16 Recombinants containing HEV-C sequences have been isolated from cases of acute flaccid paralysis (AFP).16, 17, 18, 19
As the goal of global eradication of poliomyelitis approaches, the occurrence of diseases related to OPV and genetic variability of Sabin strains from clinical cases should be investigated. By linking laboratory data and vaccination records and further reviewing of patients’ charts from hospitals, we examined these issues in southern Taiwan during 2000–2005.
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Surveillance system and patient source
The Centers for Disease Control (CDC), Taiwan since 1999 has maintained a laboratory-based infectious disease surveillance program that utilizes over 750 sentinel physicians to identify and collect specimens from patients with flu-like syndromes, hand–foot–mouth disease, herpangina or acute gastroenteritis.20, 21 Depending on patients’ initial symptoms, throat swab, rectal swab, nasopharyngeal aspirate, cerebrospinal fluid or other specimens were collected.21 Comprehensive bacterial and viral
Patient series
Among 12,236 enteroviruses isolated in the surveillance during 2000–2005, 1.5% was polioviruses.21 All polioviruses were confirmed as OPV-like, except VDPVs from an immunodeficiency patient in northern Taiwan that was reported elsewhere and not included here.29 Higher percentages of polioviruses, 3.8% in 2001 and 1.6% in 2002 were noticed, as compared to 0.84–0.88% in 2000 and 2003–2005 (p < 0.05). This increased detection of OPV in 2001–2002 prompted us to investigate the extent of illnesses
Discussion
This is the first clinical and virologic investigation in Taiwan of a large series of children with poliovirus isolation. OPV-like strains were the only pathogen found in 208 immunocompetent patients with LRI, AGE, and even fatal myocarditis. In 53 patients that had received at least 3 doses of tOPV, the time of poliovirus isolation was >10 weeks after the vaccination, suggesting that they likely were re-infected from the environment. Similarly, 6 patients acquired Sabin strains from the
Acknowledgements
The study was supported by the intramural funding of the National Health Research Institutes, Taiwan. Authors wish to thank members of “Virology Reference Laboratories Network” and “Sentinel Physician Surveillance of Infectious Diseases” by Centers for Disease Control, Taiwan.
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Chia-Yu Chi and Fan-Chen Tseng contributed equally to this work.