Case Study: Contribution of Extended Sequencing and Phylogeographic Analysis in the Investigation of Measles Outbreaks in Tunisia in 2019

Despite the availability of an effective vaccine for several decades, the measles virus continues to spread worldwide. From 2018 to 2019, several countries experienced large measles outbreaks with genotype B3, including Tunisia. We analyzed 66 samples collected from serologically confirmed measles cases during this outbreak. Fifty-five percent were aged less than 12 months and had not received a measles vaccine. Phylogenetic analysis using the 450 nucleoprotein (N450) window revealed that all strains belonged to genotype B3, with five different variants identified. The N450 sequence of the predominant one, which circulated all through the epidemic period, was identical to the named strain MVs/Kabul.AFG/20.14/3. For better molecular discrimination, the amplification and sequencing of 1018 nucleotides in the non-coding region between the M and F genes (MF-NCRs) revealed higher variability with at least nine clusters. A phylogeographic study using Bayesian methods suggested the Governorate of Kasserine (on the borders of Algeria) as the introduction point with a TMRCA (Time to Most Recent Common Ancestor) for the 2019 sequences estimated around October 2018. These findings emphasize the crucial role of advanced molecular investigations in tracing measles transmission pathways which, together with good vaccine coverage, will help the final success of the global measles elimination program.