A systematic review on the factors driving inter-regional transmission of measles to and from Asia

Abstract

Background Currently, the measles transmission patterns to and from Asia are not well understood. Studying phylogenetic relationships between viral gene sequences sampled from different localities might generate additional insights onto the disease transmission patterns. Such genetic analysis may be included in local practices, and may provide clues to help optimize public health policies to minimize the geographical spread of measles.

Objective The objective of this systematic review and phylogenetic cross-referencing analysis was to uncover Asia-specific inter-regional transmission patterns of measles.

Methods The preferred reporting items for systematic reviews and meta-analyses (PRISMA) was used for conducting this systematic review on Medline and EMBASE. Pure opinion papers and those not related to imported/exported inter-regional measles cases were excluded. Phylogenetic trees built on the reported partial nucleocapsid gene sequences from the MeaNS and GenBank databases were visualized on FigTree for cross-referencing and analysed for their relationships with other virus strains published elsewhere.

Results After screening for eligibility, 29 papers were included for the systematic review. There were 13 studies describing cases related to air travel, 10 related to travellers, 2 related to adopted children, 4 related to imported nosocomial cases, 2 related to close contacts, 6 related to family, 2 related to migrants, 1 related to military, 2 related to neighbours, 2 related to school, and 7 with unknown modes of transmission. A total of 374 imported cases were described where 49 primary and 107 secondary cases were specified. Of the cases reported, 67 were non-vaccinated, 13 had been vaccinated with 1 dose of MCV, 3 had at least 1 dose of MCV, 27 had been vaccinated with 2 doses of MCV, 11 recalled having been vaccinated but no records were provided, and 189 had unknown vaccination status. Phylogenetic analysis revealed that 18% of the sequences belonged to the lineage of H1 genotype, 6% were D9, 22% were B3, 19% were D8, 1% were D3, 5% were D5, <1% were G3, 18% were D4. Phylogenetic analysis confirmed the epidemiological data described in most papers, and provided suggestions on the potential origin of cases where epidemiological data was incomplete.

Conclusions This systematic review shows that the reported inter-regional transmissions of measles into and out of Asia were associated with mainly with air-travel and travellers, and majority of such transmissions involve unvaccinated individuals. Notably, diagnosis of measles is often delayed and vaccination records are often not available in the reports in the literature. In addition, some vaccinated individuals are still infected with measles, which calls for further considerations in the efficacy of measles vaccination and research on subclinical cases. Although phylogenetic analysis was able to trace the origin of measles virus transmission related to Asia, in some occasions the inference was ambiguous due to the insufficient or missing genetic data. Finally, complete detailed reporting and genome sequencing for all documented cases should be encouraged. This will facilitate more comprehensive and meaningful phylogenetic studies for measles transmission.