Selection of the Most Efficacious of Twenty-Two Inactivated Sendai Virus Nasal Vaccines by Determination of the Protection Index in Mice

Background and Purpose: Sendai virus nasal vaccines inactivated with various chemicals induce complete protection against contact-challenge exposure with the Nagoya strain. The study reported here was to reevaluate the efficacy of the inactivants by determining the protective index (PI) in mice, using the more virulent MN strain.

Methods: Mice were given each of 22 inactivated vaccines intranasally three times. After challenge exposure with 10^-2 to 10⁶ MID₅₀ of virus, infection of cells of the respiratory tract was determined by immunofluorescence.

Results: Twelve vaccines induced PI ≥2.0 in the nasal mucosa and were classified as group 1. The first half of the preceding vaccines that induced PI ≥3.2 in the larynx were classified subgroup a, and the rest were classified subgroup b. Of the other 10 vaccines, 6 that induced PI ≤2.0 in the larynx and 4 that induced intermediate PI in the nasal mucosa and larynx were ranked as groups 3 and 2, respectively; PI of the trachea decreased by numeric order of groups. Serum hemagglutination inhibition titer induced by intranasal vaccination was low in general.

Conclusion: On the basis of PI values, 6 of the 22 nasal vaccines provided the strongest defense in the respiratory tract.