In contrast to previously published reports, we have detected sustained infectivity of aerosolized influenza viruses in respiratory mucus over a wide-range of relative humidity conditions, indicating a risk of airborne transmission in a broad range of environments.

Immunity to influenza viruses can be long-lived, but reinfections with antigenically distinct viral strains and subtypes are common. Reinfections can boost antibody responses against viral strains first encountered in childhood through a process termed “original antigenic sin.” It is unknown how initial childhood exposures affect the induction of antibodies against the hemagglutinin (HA) stalk domain of influenza viruses. This is an important consideration since broadly reactive HA stalk antibodies can protect against infection, and universal vaccine platforms are being developed to induce these antibodies. Here we show that experimentally infected ferrets and naturally infected humans establish strong “immunological imprints” against HA stalk antigens first encountered during primary influenza virus infections. We found that HA stalk antibodies are surprisingly boosted upon subsequent infections with antigenically distinct influenza A virus subtypes. Paradoxically, these heterosubtypic-boosted HA stalk antibodies do not bind efficiently to the boosting influenza virus strain. Our results demonstrate that an individual’s HA stalk antibody response is dependent on the specific subtype of influenza virus that they first encounter early in life. We propose that humans are susceptible to heterosubtypic influenza virus infections later in life since these viruses boost HA stalk antibodies that do not bind efficiently to the boosting antigen.

Highly transmissible influenza viruses (IV) must remain stable and infectious under a wide range of environmental conditions following release from the respiratory tract into the air. Understanding how expelled IV persist in the environment is critical to limiting the spread of these viruses. Little is known about how the stability of different IV in expelled aerosols is impacted by exposure to environmental stressors, such as relative humidity (RH). Given that not all IV are equally capable of efficient airborne transmission in people, we anticipated that not all IV would respond uniformly to ambient RH. Therefore, we have examined the stability of human-pathogenic seasonal and avian IV in suspended aerosols and stationary droplets under a range of RH conditions. H3N2 and influenza B virus (IBV) isolates are resistant to RH-dependent decay in aerosols in the presence of human airway surface liquid, but we observed strain-dependent variations in the longevities of H1N1, H3N2, and IBV in droplets. Surprisingly, low-pathogenicity avian influenza H6N1 and H9N2 viruses, which cause sporadic infections in humans but are unable to transmit person to person, demonstrated a trend toward increased sensitivity at midrange to high-range RH. Taken together, our observations suggest that the levels of vulnerability to decay at midrange RH differ with virus type and host origin. IMPORTANCE The rapid spread of influenza viruses (IV) from person to person during seasonal epidemics causes acute respiratory infections that can lead to hospitalizations and life-threatening illness. Atmospheric conditions such as relative humidity (RH) can impact the viability of IV released into the air. To understand how different IV are affected by their environment, we compared the levels of stability of human-pathogenic seasonal and avian IV under a range of RH conditions and found that highly transmissible seasonal IV were less sensitive to decay under midrange RH conditions in droplets. We observed that certain RH conditions can support the persistence of infectious viruses on surfaces and in the air for extended periods of time. Together, our findings will facilitate understanding of factors affecting the persistence and spread of IV in our environment.

Human-to-human transmission of influenza viruses is a serious public health threat, yet the precise role of immunity from previous infections on the susceptibility to airborne infection is still unknown. Using the ferret model, we examined the roles of exposure duration and heterosubtypic immunity on influenza transmission. We demonstrate that a 48 hour exposure is sufficient for efficient transmission of H1N1 and H3N2 viruses. To test pre-existing immunity, a gap of 8–12 weeks between primary and secondary infections was imposed to reduce innate responses and ensure robust infection of donor animals with heterosubtypic viruses. We found that pre-existing H3N2 immunity did not significantly block transmission of the 2009 H1N1pandemic (H1N1pdm09) virus to immune animals. Surprisingly, airborne transmission of seasonal H3N2 influenza strains was abrogated in recipient animals with H1N1pdm09 pre-existing immunity. This protection from natural infection with H3N2 virus was independent of neutralizing antibodies. Pre-existing immunity with influenza B virus did not block H3N2 virus transmission, indicating that the protection was likely driven by the adaptive immune response. We demonstrate that pre-existing immunity can impact susceptibility to heterologous influenza virus strains, and implicate a novel correlate of protection that can limit the spread of respiratory pathogens through the air.