Impact of climate on varicella distribution in Bulgaria (2009-2018).

Objectives: Temperature is the most important environmental variable associated with the varicella frequency across the world. The present study compares the incidence of varicella in the districts of Bulgaria against some climatic factors and tries to find environmental variables which account for the differences in the varicella distribution observed among the Bulgarian districts.

Methods: The 28 Bulgarian districts were used as units of observation and their average 10-year varicella incidence (2009-2018) was tested for correlation with the standard bioclimatic variables of WorldClim, version 2.

Results: The WorldClim estimates for the annual mean temperature, the maximal temperature of the warmest month, the minimal temperature of the coldest month, the mean temperature of the coldest quarter, and the solar radiation inversely and not significantly correlated with the average 10-year varicella frequency. The precipitation of the warmest quarter and the wind speed correlated positively and also not significantly. Only the mean temperature of the driest quarter correlates significantly with the incidence at district level (Spearman's rank correlation coefficient of -0.45, p = 0.02). The mean of average 10-year varicella incidence rates among districts with driest quarter during the winter (January, February, March) was 387.6 ± 114.1, while among districts with driest quarter during the summer/autumn (July, August, September or August, September, October) 283.3 ± 102.1 (p = 0.02, ANOVA test).

Conclusions: Dry winter and/or wet summer appear as significant determinants for the fluctuant spread of varicella infection in Bulgaria.