COVID-19 pandemic led to a dramatic loss of human life worldwide and presented an unprecedented challenge to public health. One of the solutions in addressing the problem was mass vaccination in order to attain herd immunity. However, most people were hesitant to be vaccinated particularly due to their fear of the adverse events; hence the goal of this study was to determine the possible adverse events (AEs) experienced during administration of CoronaVac^TM COVID-19 Vaccine.

The objective of the study was to monitorthe occurrence of adverse events within one hour, two to 24 hours, and 25 to 72 hours after administration of the first and second dose of CoronaVac^TM COVID-19 Vaccine in a tertiary government hospital in Pangasinan.

This cross-sectional study was done from February to April 2021 at Region 1 Medical Center. Employees who received the two doses of Sinovac-CoronaVac^TM vaccine and gave their informed consent were included in the study. Participants were monitored closely for adverse events within 30 minutes following administration of the vaccine and were instructed to report any local and systemic AEs to the Hospital Epidemiology Center Office. Individuals were also followed up through telephone to investigate the occurrence of any complaints after 24 to 72 hours.

A total of 353 participants who received the complete doses of CoronaVac^TM were included in the study. The incidence of AEs was higher after the first dose compared with the second dose. After the first dose of CoronaVac^TM 12.5% reported AEs after the 1st hour, 14.2% two to 24 hours after the vaccine, and 1.4% on the 25^th hour up to the 72^nd hour. Pain on injection site was the most common adverse event during the first hour (8.2%). On the 2nd hour up to 24 hours, headache, and myalgia (14.2%) were more evident. Headache was reported in 1.1% of the participants after 25-72 hours of vaccination. After the second dose, only one participant reported multiple AEs such as fatigue, headache, rash and retroorbital pain (1, 0.7%).

In this study, 28.1% of the participants experienced adverse events afterthe first dose of CoronaVac^TM vaccine. AEs were higher after the first dose (28.1%) compared with the second dose (0.3%). Injection site pain was the most common adverse event during the first hour, then headache and myalgia during the 2^nd hour up to 24 hours, and headache on the 25^th – 72^nd hour after vaccination. Only one participant reported several AEs after the 2^nd dose of the vaccine.