Conventional parenteral delivery of flu vaccines is limited in its ability to induce locally-produced immune responses in the respiratory tract as well as its capacity for efficient widespread distribution [1
]. Recent studies evaluated intranasal delivery of recombinant vector-based influenza vaccines as an alternative route of delivery that may enhance safety, efficacy, and ease of administration [3
]. Vaccination in the respiratory tract may enhance protection against respiratory diseases such as influenza, tuberculosis, and measles, and may provide more generalized protection by inducing long-lasting mucosal immune responses [7
]. Studies have also shown that mucosal immunity induced via intranasal delivery provides cross-protection against heterologous strains [9
], and enhances heterosubtypic immunity for protection against multiple influenza A subtypes [9
]. Other logistical advantages of an intranasal vaccine include the reduced risk of infection and contamination due to the non-use of needles and syringes, and avoiding the need for disposal strategies of sharps after mass vaccination campaigns [18
The currently licensed intranasal vaccine FluMist™ is a live-attenuated virus administered using a Becton-Dickenson AccuSpray™ device which generates a high-speed spray of large vaccine particles, with a mass median aerosol diameter (MMAD) > 70 μm. With particle size and speed being key factors determining aerosol deposition in the airway, these high-speed, large particle sprays are often trapped in the external nares and do not navigate to the internal airways which are the primary target of vaccination. Furthermore, droplets deposited in the nose can drip out or roll back toward the pharynx causing unpleasant sensations, diminishing acceptability of the vaccine [7
]. In contrast, controlled aerosolization helps to minimize vaccine particle size variability and ensures delivery to the lower respiratory tract and internal target airways [21
]. In animals, aerosol vaccination is currently used globally to immunize poultry against Newcastle disease and shows promise of successful immunization in fowls and pigs against a variety of diseases including fowlpox, infectious bronchitis, hog cholera, pseudorabies, erysipelas, gastroenteritis, pasteurellosis, and mycoplasmosis [19
]. Notably, aerosol measles vaccination of 4 million Mexican schoolchildren in 1989-90 demonstrated a seroconversion rate of 52-64% (similar to subcutaneous administration) and an overall efficacy of 96%, with excellent public acceptance and fewer side effects than subcutaneous vaccination [22
]. However, while aerosol vaccination shows advantages in eliciting protective immune responses as well as cost-efficacy of administration, more studies are needed to further characterize the method and ensure that it is a safe and practical alternative.
Here, we elucidate the dynamics of aerosolization by analyzing particle size, vector viability, and actual delivered dose of an aerosolized adenoviral vector. This vector has been previously used alone or in combination with DNA prime immunizations to protect against lethal influenza challenges in mice and ferrets [23
]. In addition, we compare the efficacy of aerosol vaccination to intramuscular (IM) injection of this recombinant adenoviral (Ad) vaccine encoding seasonal H1N1 immunogens against homologous challenge in ferrets.
Results indicate that vaccine concentration influences aerosol size, viability, and actual delivered dose, and should be considered in designing an optimal aerosol vaccination regimen. Results from the influenza challenge study indicate that aerosol vaccination elicits humoral immune responses and protects against H1N1 influenza challenge. Furthermore, the use of aerosol as the modality of vaccination has previously shown minimal to no lasting pathology in the lung, and is comparable to IM injection in immunogenicity and protection.