Elisa Correcher-Martínez

Background/Objectives: Guillain–Barré syndrome (GBS) is a rare but serious immune-mediated neurological disorder monitored as an adverse event of special interest during the COVID-19 pandemic. This study aimed to estimate the incidence of GBS following COVID-19 vaccination and SARS-CoV-2 infection and to compare the risk by vaccine platform. Methods: We conducted a population-based retrospective cohort study using data from the Valencia Health System Integrated Database (Spain) between January 2018 and March 2022. Two cohorts were defined: individuals receiving COVID-19 vaccines (mRNA-based vaccines (BNT162b2 [Pfizer–BioNTech] and mRNA-1273 [Moderna]) or non-virus-vectored (NVV) adenoviral vector-based vaccines (ChAdOx1-S [AstraZeneca] and Ad26.COV2.S [Janssen])) and individuals with laboratory-confirmed SARS-CoV-2 infection. Incident GBS cases were identified within a predefined 42-day risk window following vaccination or infection. Incidence rates were calculated per 100,000 vaccine doses administered or SARS-CoV-2 infections. Results: Among 5,109,919 individuals, 4,270,610 received at least one COVID-19 vaccine dose and 920,643 experienced at least one SARS-CoV-2 infection. A total of 69 GBS cases occurred within 42 days following vaccination (incidence: 0.67 per 100,000 doses), whereas 21 cases occurred after infection (incidence: 2.20 per 100,000 infections). Incidence was lower after mRNA-based vaccines (0.55 per 100,000 doses) than after NVV vaccines (1.57 per 100,000 doses). Conclusions: This study confirms that GBS occurrence following vaccination is rare. The incidence is lower among individuals who received mRNA vaccines compared to those who received NVV vaccines. Moreover, GBS appears to be more frequent after a COVID-19 infection than after vaccination. These findings highlight the importance of integrating pharmacoepidemiological analyses with pharmacovigilance data to contextualize rare but serious adverse events.

Background: Respiratory syncytial virus (RSV) poses a significant health burden in children, being the major cause of lower respiratory tract infection (LRTI), including bronchiolitis. During the 2023–2024 RSV season, Spain introduced nirsevimab, a monoclonal antibody for universal RSV prophylaxis in infants. This study reviews the safety of nirsevimab through post-marketing surveillance. Material and Methods: A descriptive pharmacovigilance study was made based on spontaneous reporting data of suspected adverse events (SAEs) from the Spanish Pharmacovigilance System for Medicinal Products for Human Use (SEFV-H) and industry reports. SAEs reported between September 2023 and May 2024 were extracted from the Spanish Pharmacovigilance Adverse Reactions Data (FEDRA) database. Cases were analysed by sex, age, severity, and SAEs classification using the Preferred Terms (PT) level of the Medical Dictionary for Regulatory Activities (MedDRA). Reporting rates were estimated based on immunization coverage and birth data. Results: Sixty-seven cases reported 141 SAEs, yielding an overall rate of 23.1 cases per 100,000 doses. Common events included rash (8.51%), drug ineffectiveness (7.09%), and pyrexia (7.09%). Serious events constituted 53.70% of reports, including two fatalities (3.00%). No new safety signals or unexpected risks, such as antibody-dependent enhancement (ADE), were identified. Discussion: SAEs reported peaked early in the campaign, reflecting heightened reporting in new immunization programs. The safety profile aligns with clinical trial findings and regulatory expectations, confirming nirsevimab’s benefit–risk balance. Continued pharmacovigilance is critical for maintaining public trust in RSV prophylaxis. Nirsevimab demonstrated a favorable safety profile during Spain’s initial universal RSV immunization campaign in infants, supporting its continued use in reducing RSV-related morbidity.

We test the robustness of the self-controlled risk interval (SCRI) design in a setting where time between doses may introduce time-varying confounding, using both negative control outcomes (NCOs) and quantitative bias analysis (QBA). All vaccinated cases identified from 5 European databases between 1 September 2020 and end of data availability were included. Exposures were doses 1-3 of the Pfizer, Moderna, AstraZeneca, and Janssen COVID-19 vaccines; outcomes were myocarditis and otitis externa (NCO). The SCRI used a 60-day control window and dose-specific 28-day risk windows, stratified by vaccine brand and adjusted for calendar time. The QBA included two scenarios: (i) baseline probability of the confounder was higher in the control window and (ii) vice versa. The NCO was not associated with any of the COVID-19 vaccine types or doses except Moderna dose 1 (IRR = 1.09, 95%CI 1.01-1.09). The QBA suggested even the strongest literature-reported confounder (COVID-19; RR_myocarditis = 18.3) could only explain away part of the observed effect from IRR = 3 to IRR = 1.40. The SCRI seems robust to unmeasured confounding in the COVID-19 setting, although a strong unmeasured confounder could bias the observed effect upward. Replication of our findings for other safety signals would strengthen this conclusion.