Some studies on the selection and efficiency of live influenza vaccine viruses.

Live influenza virus vaccination has potential advantages and disadvantages compared with killed virus vaccine given by intramuscular injection.Attempts have been made to achieve complete attenuation of influenza A2 and B strains by up to 30 serial passages in the allantoic cavity of eggs at 33 degrees C. No attenuation occurred. The method of passing viruses in the presence of an increasing concentration of horse serum, thus selecting inhibitor-resistant variants, has also been tested. It was shown that serial passage induced an increasing degree of inhibitor resistance in the virus and an increasing degree of attenuation when tested in volunteers. This was observed with both an Asian strain and a Hong Kong strain. The Hong Kong virus was also passed serially at 25 degrees C in eggs but did not seem much further attenuated as a result; however, a virus strain supplied by H. F. Maassab with a much higher degree of adaptation to low temperature is being tested.Partly or completely attenuated strains have been used to induce or detect protection. Influenza B strains gave partial protection after 1 dose and almost complete protection after 2 doses and protection persisted for 7 months. A comparative study showed that there was protection after vaccination with killed virus by the parenteral route, and with live virus by the nasal route.