dose of the Comirnaty vaccine given to those who received two doses of either Comirnaty or CoronaVac provides protective levels of measured antibodies against the Omicron variant of SARS-CoV-2. Three doses of CoronaVac, however, did not provide adequate levels of protective antibodies by the same measure, in direct contradiction to claims made by the vaccine manufacturer. In October 2022, a Hong Kong study found that two doses of CoronaVac gave a protection of only 64% to 75% for older adults. However an extra booster or a third dose of CoronaVac was able to raise the level of protection against COVID-19 to about 98%. Medical uses The vaccine is given by intramuscular injection into the deltoid muscle. The initial course consists of two doses. The World Health Organization (WHO) recommends an interval of 4 weeks between doses, with data from Chile suggesting that a longer interval provides more robust immunity. Early evidence suggested that immunity wanes rapidly, so a booster dose after the initial course might be needed. Effectiveness In Brazil, a study was conducted in Serrana, population 45,000, where authorities attempted to vaccinate the entire adult population with CoronaVac. After 75% of the adult population received the vaccine, preliminary results show deaths fell by 95%, hospitalizations by 86% and symptomatic cases by 80%. According to Ricardo Palacios, a director at São Paulo's Instituto Butantan, "The most important result was understanding that we can control the pandemic even without vaccinating the entire population." In July 2021, researchers from the Chilean Ministry of Health published a paper containing real-world data for their vaccination campaign. The study was conducted between 2 February and 1 May 2021, consisting of 10.2 million people. The effectiveness of CoronaVac vaccine after the second dose was in preventing symptomatic disease, for hospitalization, for ICU admission, and in preventing COVID-19 related deaths. The effectiveness after the first dose was against symptomatic disease, against hospitalization, against ICU admission, and against deaths. The study concluded that the vaccine was effective in preventing COVID-19. In May 2021, real-world data from Indonesia showed the vaccine was highly effective, with 94% of inoculated health care workers protected against symptomatic infection by the vaccine, besting results of clinical trials. In this study of 128,290 health workers in Jakarta, less than 1% of the vaccinated healthcare workers contracted symptomatic COVID-19, compared to more than 8% among the unvaccinated healthcare workers. The vaccine reduced the risk of