Plague is caused by the bacterium
Yersinia pestis, and is a zoonotic disease mainly affecting rodents. Humans are occasionally infected through fleabites, thereby causing bubonic plague (BP) or by inhalation of infectious droplets leading to pneumonic plague (PP); the rarest but most contagious form. Plague was introduced to Madagascar in 1898 via steam ships at the east port of Toamasina. It reached the central highlands in 1921 and since then remained endemic in two traditional foci located in the central and northern highlands above 800 m of elevation [
1]. Plague re-emerged in the west port city of Mahajanga in 1991. Presently, plague continues to persist in 5 countries in the world and Madagascar is the worst affected [
2]. A large outbreak of PP in Madagascar in 2017 [
3] attracted extensive attention and raised the possibility of an outbreak evolution in an endemic region. Given the significant public health risk of re-emergence of PP in an urban area and its potential for rapid spread, and the threat of weaponization of
Y. pestis, there is an urgent need for a vaccine to provide enduring protection and for clinically proven effective antibiotic therapy. Further, the emergence of antibiotic-resistant
Y. pestis strains has previously been documented in Madagascar [
4] and to date there is no readily available licensed vaccine for plague.
Y. pestis expresses a specific capsule-like surface antigen, the fraction 1 protein or F1 antigen which is synthesized in vivo in large quantities at 37 °C [
5]. F1 antigen is highly immunogenic and was reported to confer anti phagocytic properties [
6]. Anti-F1 antibodies have been widely used for serological diagnosis of plague infection [
7‐
9]. They are known to be among the protective antibodies against
Y. pestis infection [
10]. Reports on short and long-term persistence of antibodies against
Y. pestis among plague recovered patients are scarce [
9,
11]. Knowledge of the humoral immune response from confirmed plague patients would be valuable to improve the development of an effective vaccine. Also, understanding the antibody kinetics has become of increasing importance for the potential use of serology as diagnostic tool. Indeed, due to various constraints, the confirmation rate during the last 2017 PP outbreak was very low [
3], serology would have helped to confirm more cases.