hospitalization and death of the inoculated medical workers by 96% and 98% respectively. Uruguay released real-world data based on 795,684 people who have received both doses of CoronaVac for more than 14 days as of 1 June 2021. In this group, 8,298 tested positive, 45 were admitted to the ICU, and 35 died of COVID-19. This indicated 64.52% and 61.47% effectiveness in reducing COVID-19 cases for people aged 18–49 and 50 and older respectively. The vaccine was 94.95% and 92.18% effective in reducing ICU admissions, and 95.35% and 95.2% effective to prevent deaths. Among fully-vaccinated healthcare professionals, the vaccine was 66% effective in preventing cases, and 100% effective in preventing ICU admissions and deaths. Preliminary results from a test-negative case-control study from 19 January to 13 April 2021 in Manaus suggest that, after just one dose, the vaccine is effective against asymptomatic infections and against symptomatic disease. The one-dose group consisted of 53,176 healthcare professionals. During the study, 66% of the samples were of the Gamma variant. The same study found paradoxically reduced effectiveness against symptomatic disease in another group that received two doses, an effect attributed by the authors to unmeasured confounding leading to downward bias in the efficacy estimate. In Chile, the effectiveness against symptomatic disease dropped from 67% between February–April 2021 to 58.5% in early July. Because of this, on 11 August, Chile began giving an additional dose of the Oxford–AstraZeneca vaccine to people over 55 who were fully vaccinated with CoronaVac before 31 March. In October 2021, a large scale study in Chile with 2,017,878 participants reported that, with a CoronaVac booster, effectiveness against hospitalization increased from 84% to 88% and effectiveness against symptomatic disease increased from 56% to 80%. The greatest increases were seen with a heterologous Oxford–AstraZeneca vaccine booster, resulting in 96% effectiveness against hospitalization and 93% against symptomatic disease. A study by the University of Hong Kong in December 2021 found that a third dose of CoronaVac does not provide sufficient protection against the Omicron variant, directly contradicting the vaccine manufacturer's claim and a study by the Pontifical Catholic University of Chile that it does. In April 2022, a Singapore study by infectious diseases experts from the National Centre for Infectious Diseases and the Ministry of Health found that CoronaVac recipients were 4.59 times more likely to have severe COVID-19 as well as 2.37 times more likely to be infected, compared to those