Characterization of large mumps outbreak among vaccinated Palestinian refugees

Abstract

During a large mumps virus (MuV) outbreak which occurred in the Palestinian refugee camps of the West Bank, 68.1% (2,636/3,871) of the cases were vaccinated with one dose of trivalent measles, mumps, and rubella (MMR) vaccine. Attack rates by camp ranged from less than 1 case per 1,000 people in the population to 43/1,000 (overall, 11/1,000). The outbreak lasted from December 2003 to June 2005, with two peaks, one from April to May 2004 and the other from March to April 2005. To control the outbreak, a mass MMR vaccination campaign was conducted in May 2005. Evaluation of the immune status of cases (n=59) and healthy controls (n=51) revealed high levels of mumps immunoglobulin G (IgG) and a low MuV-specific IgM in clinical cases indicative of a booster immune response. This suggested a secondary rather than a primary infection due to the insufficient protection conferred by the single vaccine dose included in the vaccination program. This prediction was further confirmed by the low seroprevalence (68.6%) found in the healthy control group, which was below the threshold level required for MuV herd immunity. Mumps diagnosis was established mainly by reverse transcription-PCR in clinical samples obtained within 48 h from the onset of disease. Of the parotid fluids and nasopharyngeal aspirates analyzed, 92% were positive for MuV RNA, while only 33% of the urine samples were positive. Phylogenetic analysis of the MuV SH gene identified the outbreak strain as the H genotype, which has been in circulation worldwide at least since 1989.

FIG. 1.

Monthly distribution of patients clinically diagnosed with mumps. Mumps cases that occurred within 1 month post the mass MMR vaccination (15 May to 15 June 2005) were considered vaccine complication cases and were not included in the final case count.

FIG. 2.

Age distribution of patients clinically diagnosed with mumps.

FIG. 3.

Phylogenetic analysis of MuV SH gene nucleotides detected in samples from patients residing in the refugee camps of the West Bank. The ClustalX nearest neighbor-joining method (number of bootstraps, 1,000) was used to compare a 316-nucleotide sequence of the MuV SH gene with MuV reference sequences from the EMBL/GenBank database.