Recently, the National Health Commission released the Notice on Printing and Distributing the Implementation Plan for the Second Dose of Enhanced Immunization of COVID-19 Vaccine on its website. Some citizens wondered why the second dose of intensive immunization should be started? Can patients with basic diseases be vaccinated? Sun Limei, director of the Institute of Immunization of the Guangdong Center for Disease Control and Prevention, answered this question. Why should the second dose of booster immunization be started? Sun Limei: The elderly are high-risk people with severe diseases. It has been 6 months since some elderly people in our province received the enhanced immunization. The protection provided by the first dose of enhanced immunization has been weakening over time. The second dose of enhanced immunization will help the elderly people quickly improve their antibody level and increase the effect of severe disease prevention. A number of literature studies showed that two weeks after the second dose of booster vaccine, the titers of IgG antibody and neutralizing antibody increased 9-10 times, which could restore the antibody level in the body to that after the first dose of booster vaccine. The study also showed that the second dose of booster inoculation was safe. The adverse reactions were mainly pain at the vaccination site, skin itching, dizziness, rash, fever, etc. There were no related serious adverse reactions, and the safety was similar to that of the first dose of intensive vaccination. According to incomplete statistics, as of November 2022, more than 20 countries/regions, including Israel, the United States and the Hong Kong Special Administrative Region, have launched the second dose of COVID-19 vaccine for key populations, mainly targeting the elderly, people with low immunity and medical personnel. At present, there are many kinds of COVID-19 vaccine on the market. How should citizens choose? Sun Limei: According to the progress of vaccine research and development, all vaccines approved for conditional marketing or emergency use can be used for the second dose of booster immunization. However, it is suggested that priority should be given to sequential booster immunization, or the second dose of booster immunization should be given to vaccines containing or having good cross immunity to Omicron virus strains. Sequential immunization is a vaccination strategy that uses different technical routes or different antigen types of vaccines, according to a certain interval and vaccination times, in order to obtain better prevention effect and safety. With the increasing evidence of COVID-19 vaccination in China, many studies have shown that sequential booster immunization can significantly improve the antibody level compared with homologous booster immunization. At present, can all varieties be reserved in the outpatient department? Sun Limei: During vaccination, due to different vaccination sites and different periods of time, various types of vaccines may be deficient. Please pay attention to the relevant outpatient information after making an appointment, and the outpatient will publicize the current vaccine varieties through various channels (Guangdong Miao APP, WeChat official account, etc.). Before going to the clinic for vaccination, consult and inquire whether the clinic has the required vaccine varieties, complete the vaccination as soon as possible, and obtain protection as soon as possible. In addition, it is recommended that before making an appointment for vaccination, you check the outpatient information on Guangdong Disease Control WeChat official account or Guangdong Miao APP to confirm whether the outpatient service is open. Can patients with basic diseases be vaccinated? Sun Limei: According to the data of the COVID-19 report in China, the case fatality rate of patients with basic diseases is 5-10 times higher than that of patients without basic diseases. Therefore, it is recommended to complete the COVID-19 as soon as possible according to the procedure when patients with basic diseases can be vaccinated after assessment