Dengue outbreaks: Report of short heterologous protection in Reunion Island

Abstract

Background: Reunion Island is facing a dengue outbreak with a first epidemic wave in 2018 and a second one in 2019. During this outbreak, almost 25 000 confirmed cases and more than 75 000 clinically compatible cases were notified. While only DENV2 was isolated among autochthonous cases in 2018, DENV1 locally emerged in 2019. This occurrence of DENV1 from April was most probably from a transmission chain initiated around an imported case that travelled in Southeast Asia and in which DENV1 was isolated. While infection confers lifelong immunity to the same serotype, heterologous protection is shorter. Even though the length of this protection is not precisely known, it is often reported that the complete heterologous protection would last 2 months and that partial protection would extend to 2 years. Case description: Here we report on 11 patients in which 2 positive PCRs for dengue were declared during the 2 epidemic waves. All these patients suffered from symptomatic infections and when signs and symptoms were collected (in 6 of them), back pain, myalgia, loss of appetite and asthenia were the most frequent followed by fever, arthralgia and vomiting. Noticeably, none experienced a severe episode nor required to be hospitalized. At this stage, typing using type-specific RT-PCR assays of the two positive samples was performed in one patient and confirmed the secondary infection. DENV1 was isolated in 3 samples from 2019 from 3 patients also diagnosed in 2018. Because of an IgG positive serology, it strongly suggests the heterologous infection despite no typing in 2018. Due to technical issues, 3 samples could not be typed. Four typing results are pending. The median length between the 2 infections was 13 months [min-max: 4.5–18 months]. Discussion: These preliminary findings showed that two symptomatic dengue infections can occur even if infections are only distanced of 4 months and suggests that heterologous protection is shorter than expected. Conclusion: These results contrast with the 24 months protection usually reported. Of note and as previously described, it seems that the risk of developing a severe form during secondary infection is increasing with the time interval between the 2 infections.