Diphtheria and tetanus seroepidemiology among children in Ukraine, 2017.

Background: The drastic decline of Ukraine's immunization coverage since 2009 led to concerns about potential resurgence diphtheria and tetanus, along with other vaccine-preventable diseases.

Methods: To assess population immunity against diphtheria and tetanus, we tested specimens from the serosurvey conducted in 2017 among children born in 2006-2015, the birth cohorts targeted by the nationwide outbreak response immunization following a circulating vaccine-derived poliovirus type 1 outbreak in Zakarpattya province in 2015. We surveyed four regions of Ukraine, using cluster sampling in Zakarpattya, Sumy, and Odessa provinces and simple random sampling in Kyiv City. We tested serum specimens for IgG antibodies against diphtheria and tetanus, using microbead assays (MBA). We estimated seroprevalence and calculated 95% confidence intervals. We also obtained information on the immunization status of surveyed children.

Results: Seroprevalence of ≥0.1 IU/mL diphtheria antibodies was <80% in all survey sites (50.0%-79.2%). Seroprevalence of ≥0.1 IU/mL tetanus antibodies was ≥80% in Sumy, Kyiv City, and Odessa (80.2%-89.1%) and 61.6% in Zakarpattya. Across the sites, the proportion of children vaccinated age-appropriately with diphtheria-tetanus-containing vaccines (DTCV) was 28.5%-57.4% among children born in 2006-2010 and 34.1%-54.3% among children born in 2011-2015. The proportion of recipients of <3 DTCV doses increased from 7.1%-16.7% among children born in 2006-2010 to 19.8%-38.6% among children born in 2011-2015, as did the proportion of recipients of zero DTCV doses (2.6%-8.8% versus 8.0%-14.0%, respectively).

Conclusions: Protection against diphtheria among children born in 2006-2015 was suboptimal (<80%), particularly in Zakarpattya. Protection against tetanus was adequate (≥80%) except in Zakarpattya. Diphtheria-tetanus immunization status was suboptimal across all sites. Catch-up vaccination of unvaccinated/under-vaccinated children and other efforts to increase immunization coverage would close these immunity gaps and prevent the resurgence of diphtheria and tetanus in Ukraine, particularly in Zakarpattya.