Pre-existing immunity protects against airborne spread of flu viruses

Influenza viruses pose a major public health threat through both seasonal epidemics and sporadic pandemics. Every influenza season is different, with one virus subtype (H3N2 or H1N1) typically dominating. In the 2017-2018 H3N2 influenza season, 40% of the cases were in the elderly, in contrast to the 2009 H1N1 pandemic, in which the highest burden of infection was found in individuals 5-24 years of age. This age-based discrepancy suggests that pre-existing immunity could impact susceptibility to infection, since people of different age groups are exposed to different strains in early childhood. But the precise role of immunity from previous infections on susceptibility to airborne infection has been unclear. This is an important question to answer because airborne transmission is critical for the emergence of pandemic viruses such as H1N1 and SARS-CoV-2, which is responsible for COVID-19.

Using the ferret model, Lakdawala and colleagues examined the roles of exposure duration and immunity on influenza transmission. The results showed that a 48-hour exposure is sufficient for efficient transmission of H1N1 and H3N2 viruses. Moreover, pre-existing immunity against the 2009 H1N1 pandemic virus protected animals from airborne transmission of a seasonal H3N2 influenza virus. This protection did not depend on cross-neutralizing antibodies, so understanding the underlying immunological mechanism will require further studies. According to the authors, the findings may have important implications for the current COVID-19 pandemic.

The authors add, "Prior immunity to influenza viruses can provide a barrier to airborne transmission of seasonal viruses. This may help to explain the heterogeneity in seasonal influenza transmission and susceptibility of different aged populations to circulating strains."