Recently, sporadic cases of COVID-19 infection have occurred in many places, and some of them are secondary infections. The State Council Joint Prevention and Control Mechanism organizes experts to interpret topics related to secondary infections. At present, the main epidemic strain in China is the Omicron XBB series variant strain. Chen Cao, a researcher at the Institute of Viral Diseases of the Chinese Center for Disease Control and Prevention, introduced that the XBB series variant is a recombinant strain of the Omicron variant. From the data monitored by the China Center for Disease Control and Prevention, the proportion of XBB variant strains in imported cases remains high, accounting for 95.2% of the collected strains from May 15th to 21st; Among local cases, the XBB variant accounted for 91.9% of the collected strains from May 15th to 21st, while other Omicron variants accounted for less than 10%. At the beginning of this year, the proportion of the main popular Omicron BA. 5.2 and BF. 7 in China was already extremely low. From our monitoring data in the past four weeks, it can be seen that the top three sub branches of XBB series variant strains in China are XBB1.9.1, XBB1.16, and XBB1.5. "Chen Cao said, this proportion is consistent with the main epidemic strains in the world and surrounding countries and regions of China during the same period. With the continuous increase of imported variant strains and the continuous decline of antibody levels in the Chinese population, the number of cases infected with XBB variant strains may increase in the near future. There is no significant change in pathogenicity between the XBB series variant and the various sub branches of the early popular Omicron variant, so there is no need to panic. Chen Cao stated that the China Center for Disease Control and Prevention will continue to carry out mutation strain monitoring for confirmed cases, especially severe and fatal cases, and special populations in sentinel hospitals. It will strengthen the monitoring of early cases and key cases of clustered outbreaks of variant strains, and work with customs to monitor imported variant strains. Based on a multi-channel monitoring and early warning system, if risk signals are found, disease control departments in various regions will promptly warn, take prevention and control measures, and respond to social concerns in a timely manner. The epidemic monitoring data shows that after mid May, the epidemic in China was in a low-level wave like epidemic situation. "Wang Liping, a researcher at the Communication and Prevention Department of the China Center for Disease Control and Prevention, introduced that after a stable transition in February, the epidemic in China entered a sporadic and sporadic situation, with a slight increase starting from late April. From the data of fever outpatient visits, although there has been an increase in the number of patients seeking treatment since late April, it is far lower than the number of visits during the peak of the previous wave of the epidemic, with the majority of patients seeking treatment being mild. Based on comprehensive analysis, the epidemic caused by the XBB series mutant strain will continue for some time, but the national epidemic prevention and control situation is stable and controllable, with little impact on normal medical treatment and social operation, "said Wang Liping. What are the symptoms of a secondary infection? Li Dong, the chief physician of the Department of Infectious Diseases at Beijing You'an Hospital affiliated with Capital Medical University, once introduced that the symptoms of infected individuals mainly include fever, sore throat, cough, nasal congestion and runny nose, as well as some people experiencing headaches, muscle soreness, joint pain, and individual symptoms of vomiting and diarrhea. Observations have found that the main population of secondary infections is young adults, who are between the ages of 18 and 60. Overall, the symptoms of these individuals are significantly lighter than those of the first infection, such as severe headaches, "razor throat", and severe symptoms