An Investigation of a Measles Outbreak in Bishkek City, Kyrgyzstan: Epidemiologic Characteristics and Estimated Measles Vaccination Coverage, 2015

  • Vaccine preventable diseases
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Background:
During Dec 2014-April 2015, 17783 measles cases, affecting individuals under 50 years of age, were reported in Kyrgyzstan. According to 2005-2013 official statistics, >97% of the one-year and six-year old children, targeted for measles vaccination, were vaccinated. This outbreak, however, raised concerns that the actual measles-containing vaccine’s coverage rate (VCR) was less than the official statistics and is not optimal for measles elimination goals. In 2015, we carried out an investigation in Bishkek city, the capital of Kyrgyzstan, to describe the outbreak and verify the official coverage rates.

Methods:
For descriptive purposes, we used the city surveillance data for the outbreak period (6874 notification) and the population census statistics. We limited the VCR study to measles case-children aged one to five years (n=948); 186 of those were lab-confirmed and 762 were diagnosed on clinical and epidemiologic grounds. Using a mathematical derivation from the screening method equation for estimating vaccine effectiveness (VE), we expressed VCR as a function of the proportion measles case-children vaccinated (P): VCR=P/(1-VE+VE*P). For one vaccine dose, we assumed an optimal 95% VE, and a less optimal 70% VE (possible vaccine failure in field conditions); and that children were similarly exposed to infection.

Results:
The highest measles attack rate, 59/1000 (1347/22800), was among the under-one-year old. The lowest rate, 4.4/1000 (275/62500), was among children aged 6-9 years. In the VCR study, the proportion case-children vaccinated was 23.5% (223/948); this yields a VCR of 86%. For a possible lower VE (70%), the VCR is even less (51%).

Conclusion:
The actual VCR in Bishkek is likely less than the official rates. A vaccination campaign was conducted for those 1-20 years old. Additionally, we recommended that VCR should be regularly monitored using sound methodology. Vaccination of children less than one year of age needs to be considered when outbreaks occur.

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