The effectiveness of single and two-dose Pfizer-BioNTech vaccine against SARS-COV-2: A real-world evidence from Saudi Arabia

Background: Several studies proved the effectiveness of Severe Acute Respiratory Syndrome Corona Virus (SARS-CoV-2) vaccines; however, the number of doses and the period between doses that warrant the highest protection remain unclear. This study aims to assess the effectiveness of the Pfizer-BioNTech vaccine and to evaluate the effectiveness of early and delayed second-dose administration of the vaccine.Methods: This is a retrospective cohort study that was conducted using the data from March 1st, 2021, to August 31st, 2021. Data regarding vaccination coverage and confirmed SARS-CoV-2 infection were obtained using academic hospitals databases and Health Electronic Surveillance Network (HESN) platform. The vaccination status of the participants were categorized as: unvaccinated, vaccinated 1st dose, and vaccinated 2nd dose of Pfizer-BioNTech vaccine. The outcome of interest was positive polymerase chain reaction test for SARS-CoV-2. Generalized linear model with a Poisson distribution was used to estimate the incidence of the infection.Findings: Among 66,775 participants included, 2615 SARS-CoV-2 infections were observed. The sample was relatively young with median age of 22 years and 43% female. A single dose of Pfizer-BioNTech vaccine had 40 % effectiveness. The effectiveness of the vaccine was doubled after the second dose of Pfizer-BioNTech (80 %). The time between the first and the second dose appears to be crucial after observing 75 %, 90 % and 85 % effectiveness with early vaccination, on-time vaccination, and delayed vaccination, respectively. Conclusion: For Pfizer-BioNTech vaccine recipients in Saudi Arabia, particularly among a predominantly young population, higher effectiveness against SARS-CoV-2 was observed with two doses of the vaccine. The timing of the second dose appears crucial for the extent of protection against SARS-CoV-2. However, potential residual confounding cannot be discounted, and further studies are needed to validate these findings and improve generalizability.(c) 2023 The Authors. Published by Elsevier Ltd on behalf of King Saud Bin Abdulaziz University for Health Sciences. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).