New findings challenge the traditional policy of replacing old strains in existing human influenza vaccines with recent variants with only minimal changes.
Influenza viruses are constantly changing their hemagglutinin (HA) protein, which is the major component of flu vaccines. Current flu vaccines are only effective against virus strains that match the vaccine strains. Consequently, the strains in the human flu vaccines are updated every few years, based on recommendations by the World Health Organization. The situation is even more complex in swine, because of multiple co-circulating and geographically separated swine flu strains. None of the available swine flu vaccines can protect against all these strains.
Pigs and humans need several doses of killed influenza vaccine in order to provide immunity: a "priming" dose is followed by booster vaccinations with the same or very similar vaccine strains. Kristien Van Reeth, of Belgium's Ghent University, and collaborators are comparing different vaccines and vaccination strategies in swine, which are used as a model for humans. They obtained promising results by priming and boosting pigs with virus strains with very distinct HA proteins. This strategy was shown to elicit antibodies that protect against both strains, negating the need for two doses of both strains. The swine vaccinated in Van Reeth's study also had increased immunity against variant strains that were not included in the vaccine.
These results are in line with similar research in the mouse and ferret model of influenza. But this is the first study in swine, with swine influenza viruses that closely resemble "H3N2" influenza viruses circulating in humans and that occasionally transmit from swine to humans. The researchers now aim at finding an optimal combination of vaccine strains, which would protect against H3N2 viruses from both humans and swine and control (reverse) zoonotic transmission of influenza viruses. Their findings also challenge the traditional policy of replacing old strains in existing human influenza vaccines with recent variants with only minimal changes.