Clinical Phenotype and Contagiousness of Early Breakthrough SARS-CoV-2 Infections after BNT162b2 COVID-19 mRNA Vaccine: A Parallel Cohort Study in Healthcare Workers

Abstract

We evaluated the clinical protection of BNT162b2 mRNA vaccine in healthcare workers (HCWs) and how COVID-19 manifestations and contagiousness change as the time since first dose increases. A matched (1:2 ratio) parallel cohort study was performed. During the first three months of vaccination campaign, HCWs of the entire health district ASL Città di Torino (Turin, Italy) were classified according to SARS-CoV-2-positivity in respect of the vaccination schedule: post-first-dose (fHCWs, n = 110), no difference was observed in fHCWs, while PHCWs and THCWs showed higher prevalence of asymptomatic infections, fewer signs/symptoms with a milder systemic involvement, and significantly higher Ct values (PHCWs 30.3 (24.1–35.5) vs. 22.3 (19.6–30.6), p = 0.023; THCWs 35.0 (31.3–35.9) vs. 22.5 (18.2–30.6), p = 0.024). Duration of symptoms was also shorter in THCWs (5 days (3–6) vs. 9 (7–14), p = 0.028). A linear increase of 3.81 points in Ct values was observed across the groups by vaccination status (p = 0.001) after adjusting for age, sex, comorbidities, and time between COVID-19 onset and swab collection. BNT162b2 decreased the risk of PCR-confirmed infections and severe disease, and was associated with a virologic picture of lesser epidemiologic concern as soon as 12 days after the first vaccine dose.

Keywords

• COVID-19
• vaccine
• contagiousness
• cycle threshold
• symptoms
• protection