The study by D. Hobson and colleagues investigates the protective effect of serum haemagglutinin-inhibiting (HI) antibody against influenza A2 and B viruses in a large group of adult volunteers. The research uses intranasal inoculation with living partially attenuated strains of influenza viruses to control the timing and dosage of infection, allowing for precise scoring of infections and higher infection rates compared to natural epidemics. The results show a consistent inverse relationship between HI antibody titre and the likelihood of infection, with a 50% protective dose (PD50) of 1/18–1/36. Notably, volunteers with no detectable pre-challenge antibody were less susceptible to infection than those with lowtitre antibody. In a group challenged with influenza B, pre-challenge antibody titres against viral neuraminidase did not significantly protect against infection. The study suggests that serum HI antibody confers protection, but further research is needed to confirm the role of other antibodies, such as neuraminidase-inhibiting (NI) antibody, in protection against influenza. The findings also indicate that very high HI titres may not be necessary for short-term protection, and that naturally acquired HI antibody may differ from vaccine-induced antibody.The study by D. Hobson and colleagues investigates the protective effect of serum haemagglutinin-inhibiting (HI) antibody against influenza A2 and B viruses in a large group of adult volunteers. The research uses intranasal inoculation with living partially attenuated strains of influenza viruses to control the timing and dosage of infection, allowing for precise scoring of infections and higher infection rates compared to natural epidemics. The results show a consistent inverse relationship between HI antibody titre and the likelihood of infection, with a 50% protective dose (PD50) of 1/18–1/36. Notably, volunteers with no detectable pre-challenge antibody were less susceptible to infection than those with lowtitre antibody. In a group challenged with influenza B, pre-challenge antibody titres against viral neuraminidase did not significantly protect against infection. The study suggests that serum HI antibody confers protection, but further research is needed to confirm the role of other antibodies, such as neuraminidase-inhibiting (NI) antibody, in protection against influenza. The findings also indicate that very high HI titres may not be necessary for short-term protection, and that naturally acquired HI antibody may differ from vaccine-induced antibody.