Safety of Pfizer-BioNTech vaccine in a cohort of healthcare providers: Differences between naïve and previously infected by SARS-CoV-2.

Introduction: The Spanish Society of Immunology recently warned that a history of past COVID-19 could result in a higher incidence of adverse events (AEs) related to vaccination. We set out to analyze whether there were any differences in AEs between healthcare workers vaccinated for COVID-19 (either after the first or second dose) who had had a prior diagnosis SARS-CoV-2 infection at any time compared to those who had not had COVID-19 before vaccination.

Methods: This was a retrospective cohort study in a population of healthcare workers. AEs related to the first and second doses of the Pfizer vaccine were recorded. We compared the incidence of AEs and compared individuals with 0-3 different AEs to those with 4 or more AEs. The relative risks (RR) and their 95% confidence intervals were calculated.

Results: Past infection was associated with having more AEs after the first dose (p < 0.001), but not the second one (p = 0.476), as well as a higher incidence of AEs (p < 0.001). Common AEs that were statistically associated with past COVID infection included arthralgia, asthenia, fever, chills, headache, and myalgia (p ≤ 0.001). The RR for having an increased absolute number of different AEs was 1.18 (95 %CI [1.05, 1.33]) after the first dose and 1.05 (95 %CI [0.96, 1.14]) after the second dose. The maximum number of days between past infection and vaccination was 306.

Conclusions: Our results showed that the incidence of AEs was higher in individuals with a history of prior COVID-19 infection.