Measles outbreak in the context of post-elimination phase in individuals with prior evidence of immunity. Secondary immune response or re-infections?

Abstract

Background: Several outbreaks of imported and imported-related cases were recorded in Argentina after measles elimination. Secondary vaccine failure (measles in vaccinated individuals with confirmed seroconversion) can occur. Confirmation of measles in vaccinated individuals and negative IgM suggests a secondary immune response. Re-infection is a confirmed case in a person with high-avidity IgG. The best method for confirmation is RT-PCR. Methods & Materials: Descriptive study of an imported measles outbreak. Results: In April 2017 a suspected measles case was notified in a traveler who returned from Australia and Indonesia, subsequently confirmed by IgM and RNA detection. Two secondary cases were detected. None could recall evidence of measles immunity. High-avidity IgG was detected in samples from all the cases. D8 genotypes was confirmed, revealing a single transmission chain. Outrbreak investigation and control actions encompassed 9 argentine provinces and 11 countries.Tabled 1Age#Serum sample (days after rash)IgMIgG (mIU/mL)IgG Avidity (%)RT-PCR NPS*NPS nasopharyngeal swab (CT¶CT value: RT-PCR average cycle threshold value)RT-PCR Urine (CT Value)GenotypeCase125Serum 1 (1)Positive1548High (87%)Positive (27)Positive (25)D8Serum 2 (14)Positive5519High (81%)Case230Serum 1 (4)Positive25867High (93%)Positive (32)Positive (36)D8Case336Serum 1 (2)Negative3226High (78%)Positive (34)Positive (39)D8Serum 2 (48)Negative26575High (98%)* NPS nasopharyngeal swab¶ CT value: RT-PCR average cycle threshold Open table in a new tab Conclusion: This travel-related limited measles outbreak in individuals with prior immunity reflects the high protection in the population of Argentina, unlike what happens in other regional countries where the outbreak magnitude is higher due to the presence of non-immune population and heterogeneous vaccine coverage. Generally, secondary immune response cases present with low viral load and have limited trasnmission. However, case #1 had high viral load and infected the other cases. The abscence of circulating virus and the periodic boosting that may have provided additional protection from infection may alter the paradigm of lifelong immunity. The diagnosis of measles in the context of elimination is a challenge. Supplementary test are needed besides specific IgM determination, only possible to perform in highly-specialized laboratories