Researchers at the University of Hong Kongs AIDS Institute and the State Key Laboratory of Emerging Infectious Diseases conducted a study on immune responses in mRNA-vaccinees after infection by the Omicron variant. The study found that mRNA-vaccinees had low levels of vaccine-induced neutralizing antibodies and T cell responses at the time of infection, but mounted rapid broadly reactive neutralizing antibodies and T cell responses upon breakthrough infection. The research suggests that complete mRNA vaccinations are important for protection against SARS-CoV-2 variants, and the development of Omicron-targeted vaccines as booster shots is urgently needed.