The goal of this clinical trial is to learn if administration of plasma, from a whole blood donation from an individual who has declared a Chikungunya infection for less than 6 months , to a newborn, whose mother has a peripartum chikungunya infection, will have an impact on the proportion of newborns surviving without encephalitis/encephalopathy (EE) within the first 5 days of life. Researchers will compare results to an observational study of 30 newborns who couldn't have been proposed to participate at the clinical trial, because of delay of diagnosis or delay of transfer to hospital which doesn't allow transfusion or parents not accepting plasma transfusion to the newborn.

Participants of the clinical trial will:

• receive a transfusion,
• visit the clinic and undergo biological tests every day until day 7 and once between 1 and 3 months.

Participant of observational study as part of their regular medical care, and biological data will be reused for the research from the mother's diagnosis until the newborn reaches 3 months of age

Inclusion Criteria:

• Treatment group

Newborn:

• whose mother has a clinical and/or laboratory diagnosis of Chikungunya (CHIK) infection occurring between 2 days before and 2 days after birth.

• Hospitalized in a Level III neonatology unit in Mayotte or at the University Hospital of La Réunion.

• Eligible for experimental treatment (called PC-CHIK-V):

  • If the maternal infection was diagnosed between2 days before and the day of delivery, treatment can be administered within the first 12 hours of the newborn's life.
  • If the maternal infection was diagnosed between 1 day and 2 days after birth, treatment can be administered within 12 hours of diagnosis.

• Whose holders of parental authority have given free, informed, and written consent prior to any examination required for the research. Parallel Cohort of

Untreated Newborns (parallel observational cohort)

Newborns:

• Whose mother has a clinical and/or laboratory diagnosis of CHIK-V infection occurring between 2 days before and 2 days after birth.
• Hospitalized in a Level III neonatology unit in Mayotte or at the University Hospital of La Réunion.
• Not treated with PC-CHIK-V:
• Monitored in one of the units of the Neonatology Department for at least 5 days from the day of birth.
• Whose holders of parental authority (or the holder if the mother is the sole holder of parental authority) have been informed of the research and have not objected to the use of data collected during hospitalization for research purposes.

Exclusion Criteria:

• Newborns who present at birth with clinical criteria for ahypoxic-ischemic encephalopathy (HIE) that require therapeutic hypothermia according to the national protocol for "neonatal therapeutic hypothermia"
• Newborns who require phototherapy treatment at the time of enrollment using devices with a maximum energy wavelength less than 425 nm and/or with an emission bandwidth with a lower limit of <375 nm.