And many elderly people may also have some immunity from previous infections with similar influenza viruses or previous vaccinations. (sciencemag.org)
Until now, it was widely believed that in order for seasonal flu to escape the immunity individuals acquire from previous infections or vaccinations, it would take at least four amino acid substitutions. (sciencedaily.com)
At the same time, it removes toxins created by previous infections. (healthcare-online.org)