Introduction

The 2-dose regime of measles vaccination has greatly reduced the morbidity and mortality of measles [1,2]. Measles vaccination has been offered to all children in Sweden since 1971, and a 2-dose regimen of measles-mumps-rubella (MMR) vaccination was introduced in the Swedish national child vaccination programme in 1982. The first MMR vaccination is currently scheduled at 18 months and the second at 6–8 years of age. The average vaccine coverage has been estimated to exceed 95% in children 2 years or older and in a seroprevalence study from 2007, protective levels of antibodies against measles were found in 98% of participants [3,4].

There have been reports of measles in previously immunised individuals, especially in healthcare workers (HCW) [510]. In areas with high vaccination coverage, it has been estimated that the majority of cases in an outbreak will be breakthrough infections (i.e. infections in individuals with a history of vaccination) [11,12]. Transmission of infection from individuals with breakthrough infections seems to be rare [10,1315], and the recently published national measles guidelines by Public Health England recommend only limited contact tracing around such cases [16]. The current Swedish guidelines recommend extensive contact tracing around all confirmed cases of measles, leading to substantial contact tracing efforts [4]. Breakthrough infections cannot be distinguished from naïve (i.e. primary) infections based on clinical presentation alone [13]. Measles-specific IgG antibody titres are usually high in acute-phase serum but IgM may be undetectable, making analysis of measles virus RNA by real-time PCR the preferred diagnostic method for breakthrough infections [17,18]. Analysis of measles IgG antibody avidity or measles neutralising antibodies by plaque reduction neutralisation assays (PRN) in acute serum samples are used to confirm breakthrough infections [10,15,19,20].

The aim of this study was to report an outbreak of measles, with focus on the real-time PCR results in nasopharyngeal, urine and serum samples in individuals without pre-existing immunity in comparison with subjects with breakthrough infections. Based on the observations, we propose a fast provisional classification of breakthrough infections that may guide decisions regarding contact tracing and infection control during an outbreak.

 

Credit: CDC

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Citation style for this article: Sundell NicklasDotevall LeifSansone MartinaAndersson MariaLindh MagnusWahlberg ThomasTyrberg TobiasWestin Johan,Liljeqvist Jan-ÅkeBergström TomasStudahl MarieAndersson Lars-Magnus. Measles outbreak in Gothenburg urban area, Sweden, 2017 to 2018: low viral load in breakthrough infections. Euro Surveill. 2019;24(17):pii=1900114. https://doi.org/10.2807/1560-7917.ES.2019.24.17.1900114