Serological responses and results of natural infectious challenge of recipients of zonal ultracentrifuged influenza A2-Aichi-2-68 vaccine.

In an epidemic of Hong Kong influenza in a penal institution, acute respiratory illness occurred in 5.6% of 196 inmates given single subcutaneous doses of 332, 535, or 1 265 CCA of zonal centrifuged, inactivated vaccine and in 11.7% of 755 nonvaccinated inmates (P<0.03). The frequency of illness among inmates who received single doses of 54 or 137 CCA of vaccine was not significantly different from that among the nonimmunized inmates (chi(2)=0.93, not significant). The vaccine was prepared with the A2/Aichi/2/68 strain of influenza and administered 7 weeks before the outbreak. Virus shedding was significantly lower in vaccinated inmates with respiratory disease (11.1%) than in nonvaccinated inmates (73.7%, P<0.04).Serological studies showed an increasing mean titre and increasing frequency of 4-fold or greater haemagglutination inhibition and neutralizing antibody responses with increasing dosage of vaccine.