The overall objective of Tarsila Study is to estimate the real-world vaccine effectiveness of maternal RSVpreF immunization during pregnancy in Brazil against medically attended (inpatient and outpatient) respiratory syncytial virus-associated acute respiratory illness in infants. To achieve this, two multicenter, prospective, test-negative case-control studies will be conducted among infants with ARI: the TARSILA Inpatient Study, evaluating hospitalized infants and the TARSILA Outpatient Study, evaluating infants managed in outpatient settings.

Tarsila comprises two multicenter, prospective, case-control studies using a test-negative design in Southern Brazil (states of Rio Grande do Sul, Santa Catarina, and Paraná). The Tarsila Inpatient study will enroll infants hospitalized due to acute respiratory illness and will be conducted at referral hospitals. The Tarsila Outpatient study will enroll infants with acute respiratory illness managed in outpatient settings and will be conducted at outpatient healthcare facilities. Both studies are aligned with the nationwide introduction of maternal RSVpreF vaccination in December 2025, with enrollment occurring from April, 2026 to September, 2028. In both studies, the same clinical case definition will be applied to cases and controls. Reverse transcription quantitative polymerase chain reaction (RT-qPCR) for RSV will be prospectively performed to classify case infants (RT-qPCR positive for RSV) and control infants (RT-qPCR negative for RSV). Maternal RSVpreF vaccination status and potential confounders will be assessed through structured interviews and review of medical records.

Tarsila Inpatient Study

Inclusion Criteria

• Infant born in Brazil.
• Mother resident in Brazil during the related pregnancy.
• Infant date of birth 14 days or more after start of the first national RSVpreF vaccination campaign to ensure potential to have been born to an RSVpreF-vaccinated mother.
• Infant ≤360 days of age at time of RSV specimen collection.
• Infant born at ≥37 weeks of gestational age.
• Infant hospital admission for at least 24 hours with signs and symptoms meeting the case definition of ARI, defined as the presence of any of the following: nasal secretion for ≥24 hours; respiratory distress, labored breathing, or tachypnea (respiratory rate ≥60 breaths/min for infants <2 months or ≥50 breaths/min for infants 2-12 months); cough; inability to feed for any period due to respiratory symptoms; apnea; or other relevant respiratory symptoms (examples include but are not limited to wheezing, crackles, and nasal flaring).

Exclusion Criteria:

• Maternal or infant receipt of any other licensed or investigational RSV preventive product.
• Infant receipt of blood transfusion or other blood products containing antibodies since birth.
• Infant previously enrolled in this study.
• Lack of informed consent from the mother or the infant's legal guardian.
• Absent or inconclusive RT-qPCR result for RSV.
• Birth to a mother whose RSVpreF vaccination status could not be confirmed.

Tarsila Outpatient Study

Inclusion Criteria:

• Infant born in Brazil.
• Mother resident in Brazil during the related pregnancy.
• Infant date of birth 14 days or more after start of the first national RSVpreF vaccination campaign to ensure potential to have been born to an RSVpreF-vaccinated mother.
• Infant ≤360 days of age at time of RSV specimen collection.
• Infant born at ≥37 weeks of gestational age.
• Infant presentation to an outpatient healthcare facility with signs and symptoms meeting the case definition of ARI, defined as the presence of any of the following: nasal secretion for ≥24 hours; respiratory distress, labored breathing, or tachypnea (respiratory rate ≥60 breaths/min for infants <2 months or ≥50 breaths/min for infants 2-12 months); cough; inability to feed for any period due to respiratory symptoms; apnea; or other relevant respiratory symptoms (examples include but are not limited to wheezing, crackles, and nasal flaring).

Exclusion Criteria:

• Mother or infant receipt of any other licensed or investigational RSV preventive product.
• Infant receipt of blood transfusion or other blood products containing antibodies since birth.
• Lack of informed consent from the mother or the infant's legal guardian.
• Absent or inconclusive RT-qPCR result for RSV.
• Birth to a mother whose RSVpreF vaccination status could not be confirmed.
• Enrollment in the study within the previous 30 days. If the interval exceeds 30 days, infants will be excluded if any previous episode tested positive for RSV.

Infants with acute respiratory illness (ARI)