Zoonotic and pandemic influenza A viruses: lessons from H5N1 and H2N2

Influenza A viruses can be directly transmitted from animals to humans (zoonotic viruses), cause worldwide outbreaks (pandemic viruses) or infect people annually (seasonal viruses). Influenza A viruses that are transmitted efficiently from one person to another have in common that they can spread via the air. We have studied zoonotic H5N1 and pandemic H2N2 influenza viruses with respect to their evolution over time, their potential to transmit via the air, and the virus properties that are required to be efficiently transmitted via the air.
From 1997 onwards, H5N1 influenza viruses have occasionally infected humans resulting in severe illness and several deaths. To investigate if this new zoonotic virus had the capacity to spread via the air, we adapted this virus to ferrets, an animal model often used to study airborne transmission of influenza A viruses. We have shown that as little as five mutations in the genetic material of the virus, changing three virus properties, are sufficient for the virus to transmit via the air.
Since influenza A viruses evolve constantly, our immune system cannot protect us from infections with new influenza viruses that might emerge in the future. H2N2 influenza virus has caused a pandemic in 1957 and caused seasonal outbreaks in humans until 1968. We have shown that the pandemic potential of H2H2 influenza viruses circulating in birds is low. Furthermore, we have investigated the genetic diversity of H2N2 influenza viruses over time and have determined the mutations that had an effect on recognition by the immune system.