There have been reports of measles in previously immunised individuals, especially in healthcare workers (HCW) [5–10]. 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 [10,13–15], and the recently published national measles guidelines by Public Health England recommend only limited contact tracing around such cases . The current Swedish guidelines recommend extensive contact tracing around all confirmed cases of measles, leading to substantial contact tracing efforts . Breakthrough infections cannot be distinguished from naïve (i.e. primary) infections based on clinical presentation alone . 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.
Citation style for this article: Euro Surveill. 2019;24(17):pii=1900114. https://doi.org/10.2807/1560-7917.ES.2019.24.17.1900114. Measles outbreak in Gothenburg urban area, Sweden, 2017 to 2018: low viral load in breakthrough infections.