Background: During late 2012 and early 2013 several outbreaks of diphtheria were notified in the North of the Lao People's Democratic Republic. The aim of this study was to determine whether the re-emergence of this vaccine-preventable disease was due to insufficient vaccination coverage or reduction of vaccine effectiveness within the affected regions.
Methods: A serosurvey was conducted in the Huaphan Province on a cluster sampling of 132 children aged 12-59 months. Serum samples, socio-demographic data, nutritional status and vaccination history were collected when available. Anti-diphtheria and anti-tetanus IgG antibody levels were measured by ELISA.
Results: Overall, 63.6% of participants had detectable diphtheria antibodies and 71.2% tetanus antibodies. Factors independently associated with non-vaccination against diphtheria were the distance from the health centre (OR: 6.35 [95% CI: 1.4-28.8], p = 0.01), the Lao Theung ethnicity (OR: 12.2 [95% CI:1,74-85, 4], p = 0.01) and the lack of advice on vaccination given at birth (OR: 9.8 [95% CI: 1.5-63.8], (p = 0.01) while the level of maternal edu-cation was a protective factor (OR: 0.08 [95% CI: 0.008-0.81], p = 0.03). Most respondents claimed financial difficulties as the main reason for non-vaccination. Out of 55 children whose vaccination certificates stated that they were given all 3 doses of diphtheria-containing vaccine, 83.6% had diphtheria antibodies and 92.7% had tetanus antibodies. Furthermore, despite a high prevalence of stunted and underweight children (53% and 25.8%, respectively), the low levels of anti-diphtheria antibodies were not correlated to the nutritional status.
Conclusions: Our data highlight a significant deficit in both the vaccination coverage and diphtheria vaccine effectiveness within the Huaphan Province. Technical deficiencies in the methods of storage and distribution of vaccines as well as unreliability of vaccination cards are discussed. Several hypotheses are advanced to explain such a decline in immunity against diphtheria and recommendations are provided to prevent future outbreaks.