Measles outbreak in Gothenburg urban area, Sweden, 2017 to 2018: low viral load in breakthrough infections.

In an outbreak of measles in Gothenburg, Sweden, breakthrough infections (i.e. infections in individuals with a history of vaccination) were common. The objective of this study was to compare measles RNA levels between naive (i.e. primary) and breakthrough infections. We also propose a fast provisional classification of breakthrough infections. Medical records were reviewed and real-time PCR-positive samples geno-typed. Cases were classified as naive, break-through or vaccine infections. We compared clinical symptoms and measles RNA cycle threshold (Ct) values between breakthrough and naive infections. Sixteen of 28 confirmed cases of measles in this outbreak were breakthrough infections. A fast provisional classification, based on previous history of measles vaccination and detectable levels of measles IgG in acute serum, correctly identified 14 of the 16 breakthrough infections, confirmed by IgG avidity testing. Measles viral load was significantly lower in nasopharyngeal samples from individuals with breakthrough compared with naive infections (median Ct-values: 32 and 19, respectively, p < 0.0001). No onward transmission from break-through infections was identified. Our results indicate that a high risk of onward transmission is limited to naive infections. We propose a fast provisional classification of breakthrough measles that can guide contact tracing in outbreak settings.