People who got Pfizer's Covid-19 vaccine showed "substantially higher" levels of antibodies compared to those who received China's Sinovac shots, according to a new study.The study, led by University of Hong Kong (HKU), aims to estimate the incidence of natural infections over time and level of population immunity due to infections and vaccinations, the South China Morning Post reported on Saturday.The presence of antibodies is a sign that the vaccine is working to protect an individual, although the quantity of the proteins generated by the body's immune system to identify and neutralise the coronavirus does not directly correlate to the level of immunity, the report said.Some who received the Sinovac vaccine might need a third booster shot as well, lead researcher Professor Benjamin Cowling, an epidemiologist with HKU was quoted as saying by the TRTWorld.The study involved tracking the antibody responses of 1,000 people who received either vaccine.On June 1, the World Health Organization (WHO) authorised the Sinovac-CoronaVac Covid-19 vaccine for emergency use.The two-dose Sinovac-CoronaVac product is an inactivated vaccine. Its easy storage requirements make it very manageable and particularly suitable for low-resource settings.Based on available evidence, the WHO recommended the vaccine for use in adults 18 years and older, in a two-dose schedule with a spacing of two to four weeks.Vaccine efficacy results showed that the vaccine prevented symptomatic disease in 51 per cent of those vaccinated and prevented severe Covid-19 and hospitalisation in 100 per cent of the studied population.On the other hand, the Pfizer-BioNTech Covid vaccine is based on mRNA technology and has been authorised for emergency use by the US Food and Drug Administration.Studies have shown that the two-dose shot is effective against Covid-19, as well as its variants - Delta, Gamma and Beta.